Hidden features of Python

Question

What are the lesser-known but useful features of the Python programming language   Try to limit answers to Python core  One feature per answer  Give an example and short description of the feature  not just a link to documentation  Label the feature using a title as the first line   Quick links to answers   Argument Unpacking Braces Chaining Comparison Operators Decorators Default Argument Gotchas   Dangers of Mutable Default arguments Descriptors Dictionary default  get value Docstring Tests Ellipsis Slicing Syntax Enumeration For else Function as iter   argument Generator expressions import this In Place Value Swapping List stepping   missing   items Multi-line Regex Named string formatting Nested list generator comprehensions New types at runtime  pth files ROT13 Encoding Regex Debugging Sending to Generators Tab Completion in Interactive Interpreter Ternary Expression try except else Unpacking print   function with statement

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The simplicity of     gt  gt  gt   str  in  string  True  gt  gt  gt   no  in  yes  False  gt  gt  gt     is something i love about Python  I have seen a lot of not very pythonic idiom like that instead    if  yes  find  no      -1      pass

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Decorators  Decorators allow to wrap a function or method in another function that can add functionality  modify arguments or results  etc  You write decorators one line above the function definition  beginning with an  at  sign       Example shows a print args decorator that prints the decorated function s arguments before calling it    gt  gt  gt  def print args function    gt  gt  gt      def wrapper  args    kwargs    gt  gt  gt          print  Arguments    args  kwargs  gt  gt  gt          return function  args    kwargs   gt  gt  gt      return wrapper   gt  gt  gt   print args  gt  gt  gt  def write text    gt  gt  gt      print text   gt  gt  gt  write  foo   Arguments    foo       foo

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Objects in boolean context  Empty tuples  lists  dicts  strings and many other objects are equivalent to False in boolean context  and non-empty are equivalent to True    empty tuple      empty list      empty dict      empty string      empty set   set   if empty tuple or empty list or empty dict or empty string or empty set    print  Never happens     This allows logical operations to return one of it s operands instead of True False  which is useful in some situations   s   t or  Default value    s will be assigned  Default value                             if t is false empty none

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This answer has been moved into the question itself  as requested by many people

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Descriptors  They re the magic behind a whole bunch of core Python features    When you use dotted access to look up a member  eg  x y   Python first looks for the member in the instance dictionary  If it s not found  it looks for it in the class dictionary  If it finds it in the class dictionary  and the object implements the descriptor protocol  instead of just returning it  Python executes it  A descriptor is any class that implements the   get      set    or   delete   methods   Here s how you d implement your own  read-only  version of property using descriptors   class Property object       def   init   self  fget           self fget   fget      def   get   self  obj  type           if obj is None              return self         return self fget obj    and you d use it just like the built-in property     class MyClass object        Property     def foo self           return  Foo     Descriptors are used in Python to implement properties  bound methods  static methods  class methods and slots  amongst other things  Understanding them makes it easy to see why a lot of things that previously looked like Python  quirks  are the way they are   Raymond Hettinger has an excellent tutorial that does a much better job of describing them than I do

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Interactive Interpreter Tab Completion  try      import readline except ImportError      print  Unable to load readline module   else      import rlcompleter     readline parse and bind  tab  complete      gt  gt  gt  class myclass         def function self             print  my function        gt  gt  gt  class instance   myclass    gt  gt  gt  class instance  lt TAB gt  class instance   class     class instance   module   class instance   doc       class instance function  gt  gt  gt  class instance f lt TAB gt unction     You will also have to set a PYTHONSTARTUP environment variable

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Operator overloading for the set builtin    gt  gt  gt  a   set  1 2 3 4    gt  gt  gt  b   set  3 4 5 6    gt  gt  gt  a   b   Union  1  2  3  4  5  6   gt  gt  gt  a  amp  b   Intersection  3  4   gt  gt  gt  a  lt  b   Subset False  gt  gt  gt  a - b   Difference  1  2   gt  gt  gt  a   b   Symmetric Difference  1  2  5  6    More detail from the standard library reference  Set Types

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inspect module is also a cool feature

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that dict get   has a default value of None  thereby avoiding KeyErrors   In  1   test     1    a     In  2   test 2  ---------------------------------------------------------------------------  lt type  exceptions KeyError  gt               Traceback  most recent call last    amp lt ipython console amp gt  in  lt module gt      lt type  exceptions KeyError  gt   2  In  3   test get  2    In  4   test get  1   Out 4    a   In  5   test get  2      None Out 5   True   and even to specify this  at the scene    In  6   test get  2   Some        Some  Out 6   True   And you can use setdefault   to have a value set and returned if it doesn t exist    gt  gt  gt  a       gt  gt  gt  b   a setdefault  foo    bar    gt  gt  gt  a   foo    bar    gt  gt  gt  b  bar

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Functional support   Generators and generator expressions  specifically   Ruby made this mainstream again  but Python can do it just as well   Not as ubiquitous in the libraries as in Ruby  which is too bad  but I like the syntax better  it s simpler   Because they re not as ubiquitous  I don t see as many examples out there on why they re useful  but they ve allowed me to write cleaner  more efficient code

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The fact that EVERYTHING is an object  and as such is extensible   I can add member variables as metadata to a function that I define    gt  gt  gt  def addInts x y           return x   y  gt  gt  gt  addInts params     integer   integer    gt  gt  gt  addInts returnType    integer    This can be very useful for writing dynamic unit tests  e g

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Nice treatment of infinite recursion in dictionaries    gt  gt  gt  a       gt  gt  gt  b       gt  gt  gt  a  b     b  gt  gt  gt  b  a     a  gt  gt  gt  print a   b     a

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gt  gt  gt  from functools import partial  gt  gt  gt  bound func   partial range  0  10   gt  gt  gt  bound func    0  1  2  3  4  5  6  7  8  9   gt  gt  gt  bound func 2   0  2  4  6  8    not really a hidden feature but partial is extremely useful for having late evaluation of functions   you can bind as many or as few parameters in the initial call to partial as you want  and call it with any remaining parameters later  in this example i ve bound the begin end args to range  but call it the second time with a step arg   See the documentation

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unzip un-needed in Python  Someone blogged about Python not having an unzip function to go with zip    unzip is straight-forward to calculate because    gt  gt  gt  t1    0 1 2 3   gt  gt  gt  t2    7 6 5 4   gt  gt  gt   t1 t2     zip  zip t1 t2   True   On reflection though  I d rather have an explicit unzip

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Referencing a list comprehension as it is being built    You can reference a list comprehension as it is being built by the symbol    1     For example  the following function unique-ifies a list of elements without changing their order by referencing its list comprehension  def unique my list       return  x for x in my list if x not in locals      1

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The reversed   builtin  It makes iterating much cleaner in many cases   quick example   for i in reversed  1  2  3        print i    produces   3 2 1   However  reversed   also works with arbitrary iterators  such as lines in a file  or generator expressions

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Slices as lvalues  This Sieve of Eratosthenes produces a list that has either the prime number or 0  Elements are 0 d out with the slice assignment in the loop   def eras n       last   n   1     sieve    0 0    list range 2  last       sqn   int round n    0 5       it    i for i in xrange 2  sqn   1  if sieve i       for i in it          sieve i i last i     0     n  i - i   1      return filter None  sieve    To work  the slice on the left must be assigned a list on the right of the same length

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Ternary operator   gt  gt  gt   ham  if True else  spam   ham   gt  gt  gt   ham  if False else  spam   spam    This was added in 2 5  prior to that you could use    gt  gt  gt  True and  ham  or  spam   ham   gt  gt  gt  False and  ham  or  spam   spam    However  if the values you want to work with would be considered false  there is a difference    gt  gt  gt     if True else  spam      gt  gt  gt  True and    or  spam   spam

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You can construct a functions kwargs on demand   kwargs      kwargs str   s  icontains    field     some value some function   kwargs    The str   call is somehow needed  since python complains otherwise that it is no string  Don t know why    I use this for a dynamic filters within Djangos object model   result   model class objects filter   kwargs

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With a minute amount of work  the threading module becomes amazingly easy to use  This decorator changes a function so that it runs in its own thread  returning a placeholder class instance instead of its regular result  You can probe for the answer by checking placeolder result or wait for it by calling placeholder awaitResult    def threadify function               exceptionally simple threading decorator  Just       gt  gt  gt   threadify         def longOperation result               time sleep 3              return result      gt  gt  gt  A  longOperation  A has finished        gt  gt  gt  B  longOperation  B has finished        A doesn t have a result yet       gt  gt  gt  print A result     None      until we wait for it       gt  gt  gt  print A awaitResult       A has finished      we could also wait manually - half a second more should be enough for B       gt  gt  gt  time sleep 0 5   print B result     B has finished             class thr  threading Thread object           def   init   self   args    kwargs               threading Thread   init     self                 self args  self kwargs   args  kwargs             self result   None             self start           def awaitResult self               self join               return self result                 def run self               self result function  self args    self kwargs      return thr

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some cool features with reduce and operator    gt  gt  gt  from operator import add mul  gt  gt  gt  reduce add  1 2 3 4   10  gt  gt  gt  reduce mul  1 2 3 4   24  gt  gt  gt  reduce add   1 2 3 4   1 2 3 4     1  2  3  4  1  2  3  4   gt  gt  gt  reduce add  1 2 3 4   10  gt  gt  gt  reduce mul  1 2 3 4   24

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Regarding Nick Johnson s implementation of a Property class  just a demonstration of descriptors  of course  not a replacement for the built-in   I d include a setter that raises an AttributeError    class Property object       def   init   self  fget           self fget   fget      def   get   self  obj  type           if obj is None              return self         return self fget obj       def   set   self  obj  value          raise AttributeError   Read-only attribute    Including the setter makes this a data descriptor as opposed to a method non-data descriptor  A data descriptor has precedence over instance dictionaries  Now an instance can t have a different object assigned to the property name  and attempts to assign to the property will raise an attribute error

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gt  gt  gt  float  infinity   inf  gt  gt  gt  float  NaN   nan   More info    http   docs python org library functions html float http   www python org dev peps pep-0754  python nan and inf values

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Creating dictionary of two sequences that have related data  In  15   t1    1  2  3   In  16   t2    4  5  6   In  17   dict  zip t1 t2   Out 17    1  4  2  5  3  6

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Context managers and the  with  Statement  Introduced in PEP 343  a context manager is an object that acts as a run-time context for a suite of statements   Since the feature makes use of new keywords  it is introduced gradually  it is available in Python 2 5 via the   future   directive  Python 2 6 and above  including Python 3  has it available by default   I have used the  with  statement a lot because I think it s a very useful construct  here is a quick demo   from   future   import with statement  with open  foo txt    w   as f      f write  hello      What s happening here behind the scenes  is that the  with  statement calls the special   enter   and   exit   methods on the file object  Exception details are also passed to   exit   if any exception was raised from the with statement body  allowing for exception handling to happen there   What this does for you in this particular case is that it guarantees that the file is closed when execution falls out of scope of the with suite  regardless if that occurs normally or whether an exception was thrown  It is basically a way of abstracting away common exception-handling code   Other common use cases for this include locking with threads and database transactions

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The Zen of Python   gt  gt  gt  import this The Zen of Python  by Tim Peters  Beautiful is better than ugly  Explicit is better than implicit  Simple is better than complex  Complex is better than complicated  Flat is better than nested  Sparse is better than dense  Readability counts  Special cases aren t special enough to break the rules  Although practicality beats purity  Errors should never pass silently  Unless explicitly silenced  In the face of ambiguity  refuse the temptation to guess  There should be one-- and preferably only one --obvious way to do it  Although that way may not be obvious at first unless you re Dutch  Now is better than never  Although never is often better than  right  now  If the implementation is hard to explain  it s a bad idea  If the implementation is easy to explain  it may be a good idea  Namespaces are one honking great idea -- let s do more of those

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Backslashes inside raw strings can still escape quotes  See this    gt  gt  gt  print repr r aaa  bbb    aaa   bbb    Note that both the backslash and the double-quote are present in the final string   As consequence  you can t end a raw string with a backslash    gt  gt  gt  print repr r C     SyntaxError  EOL while scanning string literal  gt  gt  gt  print repr r C       C        This happens because raw strings were implemented to help writing regular expressions  and not to write Windows paths  Read a long discussion about this at Gotcha     backslashes in Windows filenames

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The for   else syntax  see http   docs python org ref for html    for i in foo      if i    0          break else      print  i was never 0     The  else  block will be normally executed at the end of the for loop  unless the break is called   The above code could be emulated as follows   found   False for i in foo      if i    0          found   True         break if not found       print  i was never 0

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If you ve renamed a class in your application where you re loading user-saved files via Pickle  and one of the renamed classes are stored in a user s old save  you will not be able to load in that pickled file   However  simply add in a reference to your class definition and everything s good   e g   before   class Bleh      pass   now    class Blah      pass   so  your user s pickled saved file contains a reference to Bleh  which doesn t exist due to the rename  The fix   Bleh   Blah   simple

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Print multiline strings one screenful at a time  Not really useful feature hidden in the site  Printer class  whose the license object is an instance  The latter  when called  prints the Python license  One can create another object of the same type  passing a string -- e g  the content of a file -- as the second argument  and call it   type license  0 open  textfile txt   read   0      That would print the file content splitted by a certain number of lines at a time       file row 21 file row 22 file row 23  Hit Return for more  or q  and Return  to quit

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tuple unpacking in python 3  in python 3  you can use a syntax identical to optional arguments in function definition for tuple unpacking    gt  gt  gt  first second  rest    1 2 3 4 5 6 7 8   gt  gt  gt  first 1  gt  gt  gt  second 2  gt  gt  gt  rest  3  4  5  6  7  8    but a feature less known and more powerful allows you to have an unknown number of elements in the middle of the list    gt  gt  gt  first  rest last    1 2 3 4 5 6 7 8   gt  gt  gt  first 1  gt  gt  gt  rest  2  3  4  5  6  7   gt  gt  gt  last 8

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Using keyword arguments as assignments  Sometimes one wants to build a range of functions depending on one or more parameters  However this might easily lead to closures all referring to the same object and value   funcs       for k in range 10        funcs append  lambda  k    gt  gt  gt  funcs 0    9  gt  gt  gt  funcs 7    9   This behaviour can be avoided by turning the lambda expression into a function depending only on its arguments  A keyword parameter stores the current value that is bound to it  The function call doesn t have to be altered   funcs       for k in range 10        funcs append  lambda k   k  k    gt  gt  gt  funcs 0    0  gt  gt  gt  funcs 7    7

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Using keyword arguments as assignments  Sometimes one wants to build a range of functions depending on one or more parameters  However this might easily lead to closures all referring to the same object and value   funcs       for k in range 10        funcs append  lambda  k    gt  gt  gt  funcs 0    9  gt  gt  gt  funcs 7    9   This behaviour can be avoided by turning the lambda expression into a function depending only on its arguments  A keyword parameter stores the current value that is bound to it  The function call doesn t have to be altered   funcs       for k in range 10        funcs append  lambda k   k  k    gt  gt  gt  funcs 0    0  gt  gt  gt  funcs 7    7

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Nested Function Parameter Re-binding  def create printers n       for i in xrange n           def printer i i     Doesn t work without the i i             print i         yield printer

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The getattr built-in function     gt  gt  gt  class C        def getMontys self           self montys     Cleese   Palin   Idle   Gilliam   Jones   Chapman           return self montys    gt  gt  gt  c   C    gt  gt  gt  getattr c  getMontys       Cleese    Palin    Idle    Gilliam    Jones    Chapman    gt  gt  gt     Useful if you want to dispatch function depending on the context  See examples in Dive Into Python  Here

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Functional support   Generators and generator expressions  specifically   Ruby made this mainstream again  but Python can do it just as well   Not as ubiquitous in the libraries as in Ruby  which is too bad  but I like the syntax better  it s simpler   Because they re not as ubiquitous  I don t see as many examples out there on why they re useful  but they ve allowed me to write cleaner  more efficient code

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Simple way to test if a key is in a dict    gt  gt  gt   key  in    key    1   True   gt  gt  gt  d   dict key 1  key2 2   gt  gt  gt  if  key  in d          print  Yup       Yup

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Classes as first-class objects  shown through a dynamic class definition   Note the use of the closure as well  If this particular example looks like a  right  approach to a problem  carefully reconsider     several times     def makeMeANewClass parent  value     class IAmAnObjectToo parent       def theValue self         return value   return IAmAnObjectToo  Klass   makeMeANewClass str   fred   o   Klass   print isinstance o  str       gt  True print o theValue              gt  fred

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Python s advanced slicing operation has a barely known syntax element  the ellipsis    gt  gt  gt  class C object        def   getitem   self  item         return item       gt  gt  gt  C   1 2       3   slice 1  2  None   Ellipsis  3    Unfortunately it s barely useful as the ellipsis is only supported if tuples are involved

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You can easily transpose an array with zip   a     1 2    3 4    5 6   zip  a      1  3  5    2  4  6

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import antigravity

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getattr takes a third parameter  getattr obj  attribute name  default  is like   try      return obj attribute except AttributeError      return default   except that attribute name can be any string   This can be really useful for duck typing  Maybe you have something like   class MyThing      pass class MyOtherThing      pass if isinstance obj   MyThing  MyOtherThing        process obj     btw  isinstance obj   a b   means isinstance obj  a  or isinstance obj  b     When you make a new kind of thing  you d need to add it to that tuple everywhere it occurs   That construction also causes problems when reloading modules or importing the same file under two names  It happens more than people like to admit   But instead you could say   class MyThing      processable   True class MyOtherThing      processable   True if getattr obj   processable   False       process obj    Add inheritance and it gets even better  all of your examples of processable objects can inherit from  class Processable      processable   True   but you don t have to convince everybody to inherit from your base class  just to set an attribute

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Manipulating sys modules  You can manipulate the modules cache directly  making modules available or unavailable as you wish    gt  gt  gt  import sys  gt  gt  gt  import ham Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  ImportError  No module named ham    Make the  ham  module available -- as a non-module object even   gt  gt  gt  sys modules  ham      ham  eggs  saussages and spam    gt  gt  gt  import ham  gt  gt  gt  ham  ham  eggs  saussages and spam      Now remove it again   gt  gt  gt  sys modules  ham     None  gt  gt  gt  import ham Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  ImportError  No module named ham   This works even for modules that are available  and to some extent for modules that already are imported    gt  gt  gt  import os   Stop future imports of  os    gt  gt  gt  sys modules  os     None  gt  gt  gt  import os Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  ImportError  No module named os   Our old imported module is still available   gt  gt  gt  os  lt module  os  from   usr lib python2 5 os pyc  gt    As the last line shows  changing sys modules only affects future import statements  not past ones  so if you want to affect other modules it s important to make these changes before you give them a chance to try and import the modules -- so before you import them  typically  None is a special value in sys modules  used for negative caching  indicating the module was not found the first time  so there s no point in looking again   Any other value will be the result of the import operation -- even when it is not a module object  You can use this to replace modules with objects that behave exactly like you want  Deleting the entry from sys modules entirely causes the next import to do a normal search for the module  even if it was already imported before

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iter   can take a callable argument  For instance   def seek next line f       for c in iter lambda  f read 1    n            pass   The iter callable  until value  function repeatedly calls callable and yields its result until until value is returned

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Zero-argument and variable-argument lambdas  Lambda functions are usually used for a quick transformation of one value into another  but they can also be used to wrap a value in a function    gt  gt  gt  f   lambda   foo   gt  gt  gt  f    foo    They can also accept the usual  args and   kwargs syntax    gt  gt  gt  g   lambda  args    kwargs  args 0   kwargs  thing    gt  gt  gt  g 1  2  3  thing  stuff    1   stuff

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string-escape and unicode-escape encodings  Lets say you have a string from outer source  that contains  n   t and so on  How to transform them into new-line or tab  Just decode string using string-escape encoding     gt  gt  gt  print s Hello nStack toverflow  gt  gt  gt  print s decode  string-escape   Hello Stack   overflow   Another problem  You have normal string with unicode literals like  u01245  How to make it work  Just decode string using unicode-escape encoding    gt  gt  gt  s     u041f u0440 u0438 u0432 u0456 u0442   u0441 u0432 u0456 u0442    gt  gt  gt  print s  u041f u0440 u0438 u0432 u0456 u0442   u0441 u0432 u0456 u0442   gt  gt  gt  print unicode s   u041f u0440 u0438 u0432 u0456 u0442   u0441 u0432 u0456 u0442   gt  gt  gt  print unicode s   unicode-escape

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List comprehensions  list comprehensions  Compare the more traditional  without list comprehension    foo      for x in xrange 10     if x   2    0       foo append x    to   foo    x for x in xrange 10  if x   2    0

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Combine unpacking with the print function     in 2 6  lt   python  lt  3 0  3 0   the print function is native from   future   import print function   mylist     foo    bar    some other value   1 2 3 4    print  mylist

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You can decorate functions with classes - replacing the function with a class instance   class countCalls object           decorator replaces a function with a  countCalls  instance     which behaves like the original function  but keeps track of calls       gt  gt  gt   countCalls         def doNothing                pass      gt  gt  gt  doNothing        gt  gt  gt  doNothing        gt  gt  gt  print doNothing timesCalled     2             def   init    self  functionToTrack           self functionToTrack   functionToTrack         self timesCalled   0     def   call    self   args    kwargs           self timesCalled    1         return self functionToTrack  args    kwargs

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Generators  I think that a lot of beginning Python developers pass over generators without really grasping what they re for or getting any sense of their power   It wasn t until I read David M  Beazley s PyCon presentation on generators  it s available here  that I realized how useful  essential  really  they are   That presentation illuminated what was for me an entirely new way of programming  and I recommend it to anyone who doesn t have a deep understanding of generators

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Dynamically added attributes   This might be useful if you think about adding some attributes to your classes just by calling them  This can be done by overriding the   getattribute   member function which is called when the dot operand is used  So  let s see a dummy class for example   class Dummy object       def   getattribute   self  name           f   lambda   Hello with  s  name         return f   When you instantiate a Dummy object and do a method call you   ll get the following    gt  gt  gt  d   Dummy    gt  gt  gt  d b    Hello with b    Finally  you can even set the attribute to your class so it can be dynamically defined  This could be useful if you work with Python web frameworks and want to do queries by parsing the attribute s name   I have a gist at github with this simple code and its equivalent on Ruby made by a friend   Take care

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commands getoutput  If you want to get the output of a function which outputs directly to stdout or stderr as is the case with os system  commands getoutput comes to the rescue  The whole module is just made of awesome    gt  gt  gt  print commands getoutput  ls   myFile1 txt    myFile2 txt    myFile3 txt    myFile4 txt    myFile5 txt myFile6 txt    myFile7 txt    myFile8 txt    myFile9 txt    myFile10 txt myFile11 txt   myFile12 txt   myFile13 txt   myFile14 txt   module py

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Doctest  documentation and unit-testing at the same time   Example extracted from the Python documentation   def factorial n          Return the factorial of n  an exact integer  gt   0       If the result is small enough to fit in an int  return an int      Else return a long        gt  gt  gt   factorial n  for n in range 6        1  1  2  6  24  120       gt  gt  gt  factorial -1      Traceback  most recent call last                   ValueError  n must be  gt   0      Factorials of floats are OK  but the float must be an exact integer               import math     if not n  gt   0          raise ValueError  n must be  gt   0       if math floor n     n          raise ValueError  n must be exact integer       if n 1    n     catch a value like 1e300         raise OverflowError  n too large       result   1     factor   2     while factor  lt   n          result    factor         factor    1     return result  def  test        import doctest     doctest testmod        if   name         main          test

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infinite recursion in list   gt  gt  gt  a    1 2   gt  gt  gt  a append a   gt  gt  gt  a  1  2          gt  gt  gt  a 2   1  2          gt  gt  gt  a 2  2  2  2  2  2  2  2  2     a True

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You can override the mro of a class with a metaclass   gt  gt  gt  class A object           def a method self               print  A         gt  gt  gt  class B object           def b method self               print  B         gt  gt  gt  class MROMagicMeta type           def mro cls               return  cls  B  object        gt  gt  gt  class C A  metaclass MROMagicMeta           def c method self               print  C         gt  gt  gt  cls   C    gt  gt  gt  cls c method   C  gt  gt  gt  cls a method   Traceback  most recent call last    File   lt stdin gt    line 1  in  lt module gt  AttributeError   C  object has no attribute  a method   gt  gt  gt  cls b method   B  gt  gt  gt  type cls    bases     lt class    main   A  gt     gt  gt  gt  type cls    mro     lt class    main   C  gt    lt class    main   B  gt    lt class  object  gt     It s probably hidden for a good reason

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Negative round  The round   function rounds a float number to given precision in decimal digits  but precision can be negative    gt  gt  gt  str round 1234 5678  -2    1200 0   gt  gt  gt  str round 1234 5678  2    1234 57    Note  round   always returns a float  str   used in the above example because floating point math is inexact  and under 2 x the second example can print as 1234 5700000000001  Also see the decimal module

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When using the interactive shell      contains the value of the last printed item    gt  gt  gt  range 10   0  1  2  3  4  5  6  7  8  9   gt  gt  gt     0  1  2  3  4  5  6  7  8  9   gt  gt  gt

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insert vs append  not a feature  but may be interesting   suppose you want to insert some data in a list  and then reverse it  the easiest thing is  count   10    5 nums      for x in range count       nums append x  nums reverse     then you think  what about inserting the numbers from the beginning  instead  so   count   10    5  nums       for x in range count       nums insert 0  x    but it turns to be 100 times slower  if we set count   10    6  it will be 1 000 times slower  this is because insert is O n 2   while append is O n    the reason for that difference is that insert has to move each element in a list each time it s called  append just add at the end of the list that elements  sometimes it has to re-allocate everything  but it s still much more fast

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Multiply a string to get it repeated   print  SO  5    gives  SOSOSOSOSO

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Implicit concatenation    gt  gt  gt  print  Hello    World  Hello World   Useful when you want to make a long text fit on several lines in a script   hello    Greaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa Hello              Word    or  hello     Greaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa Hello              Word

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enumerate  Wrap an iterable with enumerate and it will yield the item along with its index   For example     gt  gt  gt  a     a    b    c    d    e    gt  gt  gt  for index  item in enumerate a   print index  item     0 a 1 b 2 c 3 d 4 e  gt  gt  gt    References    Python tutorial   looping techniques Python docs   built-in functions   enumerate PEP 279

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In-place value swapping   gt  gt  gt  a   10  gt  gt  gt  b   5  gt  gt  gt  a  b  10  5    gt  gt  gt  a  b   b  a  gt  gt  gt  a  b  5  10    The right-hand side of the assignment is an expression that creates a new tuple  The left-hand side of the assignment immediately unpacks that  unreferenced  tuple to the names a and b   After the assignment  the new tuple is unreferenced and marked for garbage collection  and the values bound to a and b have been swapped   As noted in the Python tutorial section on data structures      Note that multiple assignment is really just a combination of tuple packing and sequence unpacking

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Negative round  The round   function rounds a float number to given precision in decimal digits  but precision can be negative    gt  gt  gt  str round 1234 5678  -2    1200 0   gt  gt  gt  str round 1234 5678  2    1234 57    Note  round   always returns a float  str   used in the above example because floating point math is inexact  and under 2 x the second example can print as 1234 5700000000001  Also see the decimal module

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The pythonic idiom x       if     else     is far superior to x       and     or     and here is why   Although the statement   x   3 if  y    1  else 2   Is equivalent to  x   y    1 and 3 or 2   if you use the x       and     or     idiom  some day you may get bitten by this tricky situation   x   0 if True else 1      sets x equal to 0   and therefore is not equivalent to   x   True and 0 or 1     sets x equal to 1   For more on the proper way to do this  see Hidden features of Python

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Function argument unpacking  You can unpack a list or a dictionary as function arguments using   and      For example   def draw point x  y         do some magic  point foo    3  4  point bar     y   3   x   2   draw point  point foo  draw point   point bar    Very useful shortcut since lists  tuples and dicts are widely used as containers

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You can ask any object which module it came from by looking at its    module   property  This is useful  for example  if you re experimenting at the command line and have imported a lot of things   Along the same lines  you can ask a module where it came from by looking at its    file   property   This is useful when debugging path issues

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Python s positional and keyword expansions can be used on the fly  not just from a stored list   l lambda x y z x y z a 1 2 3 print l  a  print l   a 0  2 3     It is usually more useful with things like this   a  2 3  l   a  3

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Taking advantage of python s dynamic nature to have an apps config files in python syntax  For example if you had the following in a config file        name1    value1      name2    value2      Then you could trivially read it like   config   eval open  filename   read

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First-class functions  It s not really a hidden feature  but the fact that functions are first class objects is simply great  You can pass them around like any other variable    gt  gt  gt  def jim phrase         return  Jim says    s      phrase  gt  gt  gt  def say something person  phrase         print person phrase    gt  gt  gt  say something jim   hey guys    Jim says   hey guys

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Passing tuple to builtin functions  Much Python functions accept tuples  also it doesn t seem like  For example you want to test if your variable is a number  you could do     if isinstance  number  float  or isinstance  number  int        print  yaay    But if you pass us tuple this looks much cleaner     if isinstance  number   float  int         print  yaay

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Here is a helpful function I use when debugging type errors  def typePrint object       print str object      -      str type object            It simply prints the input followed by the type  for example   gt  gt  gt  a   101  gt  gt  gt  typePrint a      101 -   lt type  int  gt

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re can call functions   The fact that you can call a function every time something matches a regular expression is very handy  Here I have a sample of replacing every  Hello  with  Hi   and  there  with  Fred   etc   import re  def Main haystack       List of from replacements  can be a regex   finds     Hello    there    Bob     replaces     Hi     Fred     how are you       def ReplaceFunction matchobj       for found  rep in zip matchobj groups    replaces         if found    None          return rep        log error     return matchobj group 0     named groups        s     find for find in finds     ret   re sub     join named groups   ReplaceFunction  haystack    print ret  if   name         main       str    Hello there Bob    Main str      Prints  Hi  Fred  how are you

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Manipulating Recursion Limit  Getting or setting the maximum depth of recursion with sys getrecursionlimit    amp  sys setrecursionlimit     We can limit it to prevent a stack overflow caused by infinite recursion

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Sequence multiplication and reflected operands   gt  gt  gt   xyz    3  xyzxyzxyz    gt  gt  gt   1  2    3  1  2  1  2  1  2    gt  gt  gt   1  2    3  1  2  1  2  1  2    We get the same result with reflected  swapped  operands   gt  gt  gt  3    xyz   xyzxyzxyz    It works like this    gt  gt  gt  s    xyz   gt  gt  gt  num   3   To evaluate an expression s   num interpreter calls s    mul    num    gt  gt  gt  s   num  xyzxyzxyz    gt  gt  gt  s   mul   num   xyzxyzxyz    To evaluate an expression num   s interpreter calls num    mul    s    gt  gt  gt  num   s  xyzxyzxyz    gt  gt  gt  num   mul   s  NotImplemented   If the call returns NotImplemented then interpreter calls a reflected operation s    rmul    num  if operands have different types   gt  gt  gt  s   rmul   num   xyzxyzxyz    See http   docs python org reference datamodel html object rmul

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Be careful with mutable default arguments   gt  gt  gt  def foo x              x append 1          print x       gt  gt  gt  foo    1   gt  gt  gt  foo    1  1   gt  gt  gt  foo    1  1  1    Instead  you should use a sentinel value denoting  not given  and replace with the mutable you d like as default    gt  gt  gt  def foo x None           if x is None              x              x append 1          print x  gt  gt  gt  foo    1   gt  gt  gt  foo    1

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Everything is dynamic   There is no compile-time   Everything in Python is runtime  A module is  defined  by executing the module s source top-to-bottom  just like a script  and the resulting namespace is the module s attribute-space  Likewise  a class is  defined  by executing the class body top-to-bottom  and the resulting namespace is the class s attribute-space  A class body can contain completely arbitrary code -- including import statements  loops and other class statements  Creating a class  function or even module  dynamically   as is sometimes asked for  isn t hard  in fact  it s impossible to avoid  since everything is  dynamic

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The Object Data Model  You can override any operator in the language for your own classes   See this page for a complete list   Some examples    You can override any operator      -              lt   gt   lt    gt     etc    All this is done by overriding   mul      add    etc  in your objects   You can even override things like   rmul   to handle separately your object something else and something else your object     is attribute access  a b   and can be overridden to handle any arbitrary b by using   getattr     Also included here is a      using   call    You can create your own slice syntax  a stuff    which can be very complicated and quite different from the standard syntax used in lists  numpy has a good example of the power of this in their arrays  using any combination of       and     that you like  using Slice objects    Handle specially what happens with many keywords in the language   Included are del  in  import  and not  Handle what happens when many built in functions are called with your object   The standard   int      str    etc  go here  but so do   len      reversed      abs    and the three argument   pow    for modular exponentiation

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gt  gt  gt  from functools import partial  gt  gt  gt  bound func   partial range  0  10   gt  gt  gt  bound func    0  1  2  3  4  5  6  7  8  9   gt  gt  gt  bound func 2   0  2  4  6  8    not really a hidden feature but partial is extremely useful for having late evaluation of functions   you can bind as many or as few parameters in the initial call to partial as you want  and call it with any remaining parameters later  in this example i ve bound the begin end args to range  but call it the second time with a step arg   See the documentation

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If you use exec in a function the variable lookup rules change drastically   Closures are no longer possible but Python allows arbitrary identifiers in the function   This gives you a  modifiable locals    and can be used to star-import identifiers   On the downside it makes every lookup slower because the variables end up in a dict rather than slots in the frame    gt  gt  gt  def f         exec  a   42       return a       gt  gt  gt  def g         a   42      return a       gt  gt  gt  import dis  gt  gt  gt  dis dis f    2           0 LOAD CONST               1   a   42                 3 LOAD CONST               0  None                6 DUP TOP                            7 EXEC STMT               3           8 LOAD NAME                0  a               11 RETURN VALUE          gt  gt  gt  dis dis g    2           0 LOAD CONST               1  42                3 STORE FAST               0  a     3           6 LOAD FAST                0  a                9 RETURN VALUE

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Exposing Mutable Buffers  Using the Python Buffer Protocol to expose mutable byte-oriented buffers in Python  2 5 2 6     Sorry  no code here  Requires use of low-level C API or existing adapter module

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dict s constructor accepts keyword arguments    gt  gt  gt  dict foo 1  bar 2    foo   1   bar   2

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Python has GOTO     and it s implemented by external pure-Python module     from goto import goto  label for i in range 1  10       for j in range 1  20           for k in range 1  30               print i  j  k             if k    3                  goto  end   breaking out from a deeply nested loop label  end print  Finished

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Creating new types in a fully dynamic manner   gt  gt  gt  NewType   type  NewType    object      x    hello     gt  gt  gt  n   NewType    gt  gt  gt  n x  hello    which is exactly the same as   gt  gt  gt  class NewType object    gt  gt  gt      x    hello   gt  gt  gt  n   NewType    gt  gt  gt  n x  hello    Probably not the most useful thing  but nice to know   Edit  Fixed name of new type  should be NewType to be the exact same thing as with class statement   Edit  Adjusted the title to more accurately describe the feature

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Too lazy to initialize every field in a dictionary  No problem   In Python   2 3   from collections import defaultdict   In Python  lt   2 3   def defaultdict type        class Dict dict           def   getitem   self  key               return self setdefault key  type         return Dict     In any version   d   defaultdict list  for stuff in lots of stuff       d stuff name  append stuff    UPDATE   Thanks Ken Arnold  I reimplemented a more sophisticated version of defaultdict  It should behave exactly as the one in the standard library   def defaultdict default factory   args    kw                                      class defaultdict dict            def   missing   self  key               if default factory is None                  raise KeyError key              return self setdefault key  default factory             def   getitem   self  key               try                  return dict   getitem   self  key              except KeyError                  return self   missing   key       return defaultdict  args    kw

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A slight misfeature of python   The normal fast way to join a list of strings together is      join list of strings

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Method replacement for object instance  You can replace methods of already created object instances  It allows you to create object instance with different  exceptional  functionality    gt  gt  gt  class C object           def fun self               print  C a   self      gt  gt  gt  inst   C    gt  gt  gt  inst fun      C a method is executed C a  lt   main   C object at 0x00AE74D0 gt   gt  gt  gt  instancemethod   type C fun   gt  gt  gt   gt  gt  gt  def fun2 self           print  fun2   self      gt  gt  gt  inst fun   instancemethod fun2  inst  C     Now we are replace C a by fun2  gt  gt  gt  inst fun          and fun2 is executed fun2  lt   main   C object at 0x00AE74D0 gt    As we can C a was replaced by fun2   in inst instance  self didn t change    Alternatively we may use new module  but it s depreciated since Python 2 6    gt  gt  gt  def fun3 self           print  fun3   self      gt  gt  gt  import new  gt  gt  gt  inst fun   new instancemethod fun3  inst  C   gt  gt  gt  inst fun   fun3  lt   main   C object at 0x00AE74D0 gt    Node  This solution shouldn t be used as general replacement of inheritance mechanism  But it may be very handy in some specific situations  debugging  mocking    Warning  This solution will not work for built-in types and for new style classes using slots

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Nested Function Parameter Re-binding  def create printers n       for i in xrange n           def printer i i     Doesn t work without the i i             print i         yield printer

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Unpacking  to function parameters  def foo a  b  c           print a  b  c  bar    3  14  15  foo  bar    When executed prints   3 14 15

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The Python Interpreter   gt  gt  gt     Maybe not lesser known  but certainly one of my favorite features of Python

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ROT13 is a valid encoding for source code  when you use the right coding declaration at the top of the code file      usr bin env python   - - coding  rot13 - -  cevag  Uryyb fgnpxbiresybj   rapbqr  rot13

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I personally love the 3 different quotes  str    I m a string  but still I can use quotes  inside myself   str       For some messy multi line strings  Such as  lt html gt   lt head gt       lt  head gt       Also cool  not having to escape regular expressions  avoiding horrible backslash salad by using raw strings   str2   r  n   print str2  gt  gt   n

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Simulating the tertiary operator using and and or   and and or operators in python return the objects themselves rather than Booleans  Thus   In  18   a   True  In  19   a and 3 or 4 Out 19   3  In  20   a   False  In  21   a and 3 or 4 Out 21   4   However  Py 2 5 seems to have added an explicit tertiary operator      In  22   a   5 if True else  6       In  23   a     Out 23   5   Well  this works if you are sure that your true clause does not evaluate to False   example    gt  gt  gt  def foo             print  foo          return 0      gt  gt  gt  def bar             print  bar          return 1      gt  gt  gt  1 and foo   or bar   foo bar 1   To get it right  you ve got to just a little bit more     gt  gt  gt   1 and  foo    or  bar     0  foo 0   However  this isn t as pretty   if your version of python supports it  use the conditional operator    gt  gt  gt  foo   if True or bar   foo 0

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Taking advantage of python s dynamic nature to have an apps config files in python syntax  For example if you had the following in a config file        name1    value1      name2    value2      Then you could trivially read it like   config   eval open  filename   read

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Rounding Integers  Python has the function round  which returns numbers of type double     gt  gt  gt  print round 1123 456789  4  1123 4568   gt  gt  gt  print round 1123 456789  2  1123 46   gt  gt  gt  print round 1123 456789  0  1123 0   This function has a wonderful magic property     gt  gt  gt  print round 1123 456789  -1  1120 0   gt  gt  gt  print round 1123 456789  -2  1100 0   If you need an integer as a result use int to convert type     gt  gt  gt  print int round 1123 456789  -2   1100   gt  gt  gt  print int round 8359980  -2   8360000   Thank you Gregor

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Ability to substitute even things like file deletion  file opening etc  - direct manipulation of language library  This is a huge advantage when testing  You don t have to wrap everything in complicated containers  Just substitute a function method and go  This is also called monkey-patching

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Builtin methods or functions don t implement the descriptor protocol which makes it impossible to do stuff like this    gt  gt  gt  class C object        id   id       gt  gt  gt  C   id   Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  TypeError  id   takes exactly one argument  0 given    However you can create a small bind descriptor that makes this possible    gt  gt  gt  from types import MethodType  gt  gt  gt  class bind object        def   init   self  callable         self callable   callable      def   get   self  obj  type None         if obj is None         return self       return MethodType self callable  obj  type        gt  gt  gt  class C object        id   bind id        gt  gt  gt  C   id   7414064

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Set Comprehensions   gt  gt  gt   i  2 for i in range 5                                                          set  0  1  4  16  9     Python documentation  Wikipedia Entry

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Main messages     import this   btw look at this module s source        De-cyphered      The Zen of Python  by Tim Peters          Beautiful is better than ugly    Explicit is better than implicit    Simple is better than complex    Complex is better than complicated    Flat is better than nested    Sparse is better than dense    Readability counts    Special cases aren t special enough to break the rules    Although practicality beats purity    Errors should never pass silently    Unless explicitly silenced    In the face of ambiguity  refuse the temptation to guess    There should be one-- and preferably only one --obvious way to do it    Although that way may not be obvious at first unless you re Dutch    Now is better than never    Although never is often better than right now    If the implementation is hard to explain  it s a bad idea    If the implementation is easy to explain  it may be a good idea    Namespaces are one honking great idea -- let s do more of those

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Re-raising exceptions     Python 2 syntax try      some operation   except SomeError  e      if is fatal e           raise     handle nonfatal e     Python 3 syntax try      some operation   except SomeError as e      if is fatal e           raise     handle nonfatal e    The  raise  statement with no arguments inside an error handler tells Python to re-raise the exception with the original traceback intact  allowing you to say  oh  sorry  sorry  I didn t mean to catch that  sorry  sorry    If you wish to print  store or fiddle with the original traceback  you can get it with sys exc info    and printing it like Python would is done with the  traceback  module

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Some of the builtin favorites  map    reduce    and filter    All extremely fast and powerful

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If you are using descriptors on your classes Python completely bypasses   dict   for that key which makes it a nice place to store such values    gt  gt  gt  class User object        def  get username self         return self   dict    username        def  set username self  value         print  username set        self   dict    username     value      username   property  get username   set username       del  get username   set username       gt  gt  gt  u   User    gt  gt  gt  u username    foo  username set  gt  gt  gt  u   dict     username    foo     This helps to keep dir   clean

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Creating enums  In Python  you can do this to quickly create an enumeration    gt  gt  gt  FOO  BAR  BAZ   range 3   gt  gt  gt  FOO 0   But the  enums  don t have to have integer values  You can even do this   class Colors object       RED  GREEN  BLUE  YELLOW    255 0 0    0 255 0    0 0 255    0 255 255    now Colors RED is a 3-tuple that returns the 24-bit 8bpp RGB   value for saturated red

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Method replacement for object instance  You can replace methods of already created object instances  It allows you to create object instance with different  exceptional  functionality    gt  gt  gt  class C object           def fun self               print  C a   self      gt  gt  gt  inst   C    gt  gt  gt  inst fun      C a method is executed C a  lt   main   C object at 0x00AE74D0 gt   gt  gt  gt  instancemethod   type C fun   gt  gt  gt   gt  gt  gt  def fun2 self           print  fun2   self      gt  gt  gt  inst fun   instancemethod fun2  inst  C     Now we are replace C a by fun2  gt  gt  gt  inst fun          and fun2 is executed fun2  lt   main   C object at 0x00AE74D0 gt    As we can C a was replaced by fun2   in inst instance  self didn t change    Alternatively we may use new module  but it s depreciated since Python 2 6    gt  gt  gt  def fun3 self           print  fun3   self      gt  gt  gt  import new  gt  gt  gt  inst fun   new instancemethod fun3  inst  C   gt  gt  gt  inst fun   fun3  lt   main   C object at 0x00AE74D0 gt    Node  This solution shouldn t be used as general replacement of inheritance mechanism  But it may be very handy in some specific situations  debugging  mocking    Warning  This solution will not work for built-in types and for new style classes using slots

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Using sets to reference contents in sets of frozensets     As you probably know  sets are mutable and thus not hashable  so it s necessary to use frozensets if you want to make a set of sets  or use sets as dictionary keys     gt  gt  gt  fabc   frozenset  abc    gt  gt  gt  fxyz   frozenset  xyz    gt  gt  gt  mset   set  fabc  fxyz    gt  gt  gt  mset  frozenset   a    c    b     frozenset   y    x    z       However  it s possible to test for membership and remove discard members using just ordinary sets    gt  gt  gt  abc   set  abc    gt  gt  gt  abc in mset True  gt  gt  gt  mset remove abc   gt  gt  gt  mset  frozenset   y    x    z       To quote from the Python Standard Library docs      Note  the elem argument to the   contains      remove    and discard         methods may be a set  To support searching for an equivalent frozenset  the        elem set is temporarily mutated during the search and then restored  During        the search  the elem set should not be read or mutated since it does not        have a meaningful value     Unfortunately  and perhaps astonishingly  the same is not true of dictionaries    gt  gt  gt  mdict    fabc 1  fxyz 2   gt  gt  gt  fabc in mdict True  gt  gt  gt  abc in mdict Traceback  most recent call last   File   lt interactive input gt    line 1  in  lt module gt  TypeError  unhashable type   set

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A module exports EVERYTHING in its namespace  Including names imported from other modules     this is  answer42 py  from operator import   from inspect  import     Now test what s importable from the module    gt  gt  gt  import answer42  gt  gt  gt  answer42   dict   keys     gt    imul    ge    setslice    ArgInfo    getfile    isCallable    getsourcelines    CO OPTIMIZED    le    re    isgenerator    ArgSpec    imp    lt    delslice    BlockFinder    getargspec    currentframe    CO NOFREE    namedtuple    rshift    string    getframeinfo      file      strseq    iconcat    getmro    mod    getcallargs    isub    getouterframes    isdatadescriptor    modulesbyfile    setitem    truth    Attribute    div    CO NESTED    ixor    getargvalues    ismemberdescriptor    getsource    isMappingType    eq    index    xor    sub    getcomments    neg    getslice    isframe      builtins      abs    getmembers    mul    getclasstree    irepeat    is     getitem    indexOf    Traceback    findsource    ModuleInfo    ipow    TPFLAGS IS ABSTRACT    or     joinseq    is not    itruediv    getsourcefile    dis    os    iand    countOf    getinnerframes    pow    pos    and     lshift      name      sequenceIncludes    isabstract    isbuiltin    invert    contains    add    isSequenceType    irshift    types    tokenize    isfunction    not     istraceback    getmoduleinfo    isgeneratorfunction    getargs    CO GENERATOR    cleandoc    classify class attrs    EndOfBlock    walktree      doc      getmodule    isNumberType    ilshift    ismethod    ifloordiv    formatargvalues    indentsize    getmodulename    inv    Arguments    iscode    CO NEWLOCALS    formatargspec    iadd    getlineno    imod    CO VARKEYWORDS    ne    idiv      package      CO VARARGS    attrgetter    methodcaller    truediv    repeat    trace    isclass    ior    ismethoddescriptor    sys    isroutine    delitem    stack    concat    getdoc    getabsfile    ismodule    linecache    floordiv    isgetsetdescriptor    itemgetter    getblock    gt  gt  gt  from answer42 import getmembers  gt  gt  gt  getmembers  lt function getmembers at 0xb74b2924 gt   gt  gt  gt     That s a good reason not to from x import   and to define   all

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The reversed   builtin  It makes iterating much cleaner in many cases   quick example   for i in reversed  1  2  3        print i    produces   3 2 1   However  reversed   also works with arbitrary iterators  such as lines in a file  or generator expressions

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While debugging complex data structures pprint module comes handy   Quoting from the docs     gt  gt  gt  import pprint      gt  gt  gt  stuff   sys path     gt  gt  gt  stuff insert 0  stuff   gt  gt  gt  pprint pprint stuff    lt Recursion on list with id 869440 gt           usr local lib python1 5      usr local lib python1 5 test      usr local lib python1 5 sunos5      usr local lib python1 5 sharedmodules      usr local lib python1 5 tkinter

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is ok   and  Yes  or  No

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gt  gt  gt  x  1 1 2  a   a  3   gt  gt  gt  y      x for  x in x if not  x in locals      1       gt  gt  gt  y  1  2   a   3      locals      1     is the  secret name  of the list being created  Very useful when state of list being built affects subsequent build decisions

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Mod works correctly with negative numbers  -1   5 is 4  as it should be  not -1 as it is in other languages like JavaScript  This makes  wraparound windows  cleaner in Python  you just do this   index    index   increment    WINDOW SIZE

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I m not sure where  or whether  this is in the Python docs  but for python 2 x  at least 2 5 and 2 6  which I just tried   the print statement can be called with parenthenses  This can be useful if you want to be able to easily port some Python 2 x code to Python 3 x    Example  print  We want Moshiach Now   should print We want Moshiach Now work in python 2 5  2 6  and 3 x   Also  the not operator can be called with parenthenses in Python 2 and 3  not False and not False  should both return True   Parenthenses might also work with other statements and operators   EDIT  NOT a good idea to put parenthenses around not operators  and probably any other operators   since it can make for surprising situations  like so  this happens because the parenthenses are just really around the 1     gt  gt  gt   not 1     9 False   gt  gt  gt  not 1     9 True   This also can work  for some values  I think where it is not a valid identifier name   like this  not val  should return False  and print We want Moshiach Now  should return We want Moshiach Now   but not552 would raise a NameError since it is a valid identifier name

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You can override the mro of a class with a metaclass   gt  gt  gt  class A object           def a method self               print  A         gt  gt  gt  class B object           def b method self               print  B         gt  gt  gt  class MROMagicMeta type           def mro cls               return  cls  B  object        gt  gt  gt  class C A  metaclass MROMagicMeta           def c method self               print  C         gt  gt  gt  cls   C    gt  gt  gt  cls c method   C  gt  gt  gt  cls a method   Traceback  most recent call last    File   lt stdin gt    line 1  in  lt module gt  AttributeError   C  object has no attribute  a method   gt  gt  gt  cls b method   B  gt  gt  gt  type cls    bases     lt class    main   A  gt     gt  gt  gt  type cls    mro     lt class    main   C  gt    lt class    main   B  gt    lt class  object  gt     It s probably hidden for a good reason

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You can use property to make your class interfaces more strict   class C object       def   init   self  foo  bar           self foo   foo   read-write property         self bar   bar   simple attribute      def  set foo self  value           self  foo   value      def  get foo self           return self  foo      def  del foo self           del self  foo        any of fget  fset  fdel and doc are optional        so you can make a write-only and or delete-only property      foo   property fget    get foo  fset    set foo                     fdel    del foo  doc    Hello  I am foo     class D C       def  get foo self           return self  foo   2      def  set foo self  value           self  foo   value   2      foo   property fget    get foo  fset    set foo                     fdel   C foo fdel  doc   C foo   doc      In Python 2 6 and 3 0   class C object       def   init   self  foo  bar           self foo   foo   read-write property         self bar   bar   simple attribute       property     def foo self              Hello  I am foo              return self  foo       foo setter     def foo self  value           self  foo   value       foo deleter     def foo self           del self  foo  class D C        C foo getter     def foo self           return self  foo   2       foo setter     def foo self  value           self  foo   value   2   To learn more about how property works refer to descriptors

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You can easily transpose an array with zip   a     1 2    3 4    5 6   zip  a      1  3  5    2  4  6

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The first-classness of everything   everything is an object    and the mayhem this can cause    gt  gt  gt  x   5  gt  gt  gt  y   10  gt  gt  gt    gt  gt  gt  def sq x         return x   x       gt  gt  gt  def plus x         return x   x       gt  gt  gt   sq plus  y gt x  y  20   The last line creates a tuple containing the two functions  then evaluates y x  True  and uses that as an index to the tuple  by casting it to an int  1   and then calls that function with parameter y and shows the result   For further abuse  if you were returning an object with an index  e g  a list  you could add further square brackets on the end  if the contents were callable  more parentheses  and so on  For extra perversion  use the result of code like this as the expression in another example  i e  replace y x with this code     sq plus  y gt x  y  4  x    This showcases two facets of Python - the  everything is an object  philosophy taken to the extreme  and the methods by which improper or poorly-conceived use of the language s syntax can lead to completely unreadable  unmaintainable spaghetti code that fits in a single expression

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Private methods and data hiding  encapsulation   There s a common idiom in Python of denoting methods and other class members that are not intended to be part of the class s external API by giving them names that start with underscores   This is convenient and works very well in practice  but it gives the false impression that Python does not support true encapsulation of private code and or data   In fact  Python automatically gives you lexical closures  which make it very easy to encapsulate data in a much more bulletproof way when the situation really warrants it   Here s a contrived example of a class that makes use of this technique   class MyClass object     def   init   self        privateData           self publicData   123      def privateMethod k         print privateData k    self publicData      def privilegedMethod          privateData  foo      hello         privateMethod  foo        self privilegedMethod   privilegedMethod    def publicMethod self       print self publicData   And here s a contrived example of its use    gt  gt  gt  obj   MyClass    gt  gt  gt  obj publicMethod   123  gt  gt  gt  obj publicData    World   gt  gt  gt  obj publicMethod   World  gt  gt  gt  obj privilegedMethod   hello World  gt  gt  gt  obj privateMethod   Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  AttributeError   MyClass  object has no attribute  privateMethod   gt  gt  gt  obj privateData Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  AttributeError   MyClass  object has no attribute  privateData    The key is that privateMethod and privateData aren t really attributes of obj at all  so they can t be accessed from outside  nor do they show up in dir   or similar   They re local variables in the constructor  completely inaccessible outside of   init     However  because of the magic of closures  they really are per-instance variables with the same lifetime as the object with which they re associated  even though there s no way to access them from outside except  in this example  by invoking privilegedMethod   Often this sort of very strict encapsulation is overkill  but sometimes it really can be very handy for keeping an API or a namespace squeaky clean   In Python 2 x  the only way to have mutable private state is with a mutable object  such as the dict in this example    Many people have remarked on how annoying this can be   Python 3 x will remove this restriction by introducing the nonlocal keyword described in PEP 3104

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Tuple unpacking in for loops  list comprehensions and generator expressions    gt  gt  gt  l   1 2   3 4    gt  gt  gt   a b for a b in l     3 7    Useful in this idiom for iterating over  key data  pairs in dictionaries   d      x   y    f   e   for name  value in d items       one can also use iteritems      print  name  s  value  s     name value    prints   name x  value y name f  value e

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Python 2 x ignores commas if found after the last element of the sequence    gt  gt  gt  a tuple for instance    0 1 2 3    gt  gt  gt  another tuple    0 1 2 3   gt  gt  gt  a tuple for instance    another tuple True   A trailing comma causes a single parenthesized element to be treated as a sequence    gt  gt  gt  a tuple with one element    8

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Reloading modules enables a  live-coding  style  But class instances don t update  Here s why  and how to get around it  Remember  everything  yes  everything is an object    gt  gt  gt  from a package import a module  gt  gt  gt  cls   a module SomeClass  gt  gt  gt  obj   cls    gt  gt  gt  obj method    old method output    Now you change the method in a module py and want to update your object    gt  gt  gt  reload a module   gt  gt  gt  a module SomeClass is cls False   Because it just got freshly created by reload   gt  gt  gt  obj method    old method output    Here s one way to update it  but consider it running with scissors     gt  gt  gt  obj   class   is cls True   it s the old class object  gt  gt  gt  obj   class     a module SomeClass   pick up the new class  gt  gt  gt  obj method    new method output    This is  running with scissors  because the object s internal state may be different than what the new class expects  This works for really simple cases  but beyond that  pickle is your friend  It s still helpful to understand why this works  though

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Not  hidden  but quite useful and not commonly used   Creating string joining functions quickly like so   comma join       join  semi join        join   print comma join   foo   bar   baz      foo bar baz   and  Ability to create lists of strings more elegantly than the quote  comma mess    l     item1    item2    item3     replaced by  l    item1 item2 item3  split

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Monkeypatching objects  Every object in Python has a   dict   member  which stores the object s attributes  So  you can do something like this   class Foo object       def   init   self  arg1  arg2    kwargs            do stuff with arg1 and arg2         self   dict   update kwargs   f   Foo  arg1    arg2   bar 20  baz 10   now f is a Foo object with two extra attributes   This can be exploited to add both attributes and functions arbitrarily to objects  This can also be exploited to create a quick-and-dirty struct type   class struct object       def   init     kwargs          self   dict   update kwargs   s   struct foo 10  bar 11  baz  i m a string

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The step argument in slice operators  For example   a    1 2 3 4 5   gt  gt  gt  a   2     iterate over the whole list in 2-increments  1 3 5    The special case x   -1  is a useful idiom for  x reversed     gt  gt  gt  a   -1   5 4 3 2 1

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Tuple unpacking    gt  gt  gt   a   b  c   d      1  2    3  4    5  6    gt  gt  gt  a  1  2   gt  gt  gt  b 3  gt  gt  gt  c  d  4   5  6     More obscurely  you can do this in function arguments  in Python 2 x  Python 3 x will not allow this anymore     gt  gt  gt  def addpoints  x1  y1    x2  y2            return  x1 x2  y1 y2   gt  gt  gt  addpoints  5  0    3  5    8  5

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An interpreter within the interpreter  The standard library s code module let s you include your own read-eval-print loop inside a program  or run a whole nested interpreter  E g   copied my example from here     python Python 2 5 1  r251 54863  Jan 17 2008  19 35 17    GCC 4 0 1  Apple Inc  build 5465   on darwin Type  help    copyright    credits  or  license  for more information   gt  gt  gt  shared var    Set in main console   gt  gt  gt  import code  gt  gt  gt  ic   code InteractiveConsole    shared var   shared var     gt  gt  gt  try          ic interact  My custom console banner        except SystemExit  e          print  Got SystemExit        My custom console banner   gt  gt  gt  shared var  Set in main console   gt  gt  gt  shared var    Set in sub-console   gt  gt  gt  import sys  gt  gt  gt  sys exit   Got SystemExit   gt  gt  gt  shared var  Set in main console    This is extremely useful for situations where you want to accept scripted input from the user  or query the state of the VM in real-time   TurboGears uses this to great effect by having a WebConsole from which you can query the state of you live web app

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Assigning and deleting slices    gt  gt  gt  a   range 10   gt  gt  gt  a  0  1  2  3  4  5  6  7  8  9   gt  gt  gt  a  5     42   gt  gt  gt  a  42  5  6  7  8  9   gt  gt  gt  a  1    range 5   gt  gt  gt  a  0  1  2  3  4  5  6  7  8  9   gt  gt  gt  del a   2   gt  gt  gt  a  1  3  5  7  9   gt  gt  gt  a   2    a   -2   gt  gt  gt  a  9  3  5  7  1    Note  when assigning to extended slices  s start stop step    the assigned iterable must have the same length as the slice

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Slices  amp  Mutability   Copying lists   gt  gt  gt  x    1 2 3   gt  gt  gt  y   x     gt  gt  gt  y pop   3  gt  gt  gt  y  1  2   gt  gt  gt  x  1  2  3    Replacing lists   gt  gt  gt  x    1 2 3   gt  gt  gt  y   x  gt  gt  gt  y       4 5 6   gt  gt  gt  x  4  5  6

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Extending properties  defined as descriptor  in subclasses  Sometimes it s useful to extent  modify  value  returned  by descriptor in subclass  It can be easily done with super     class A object        property     def prop self           return   a   1   class B A        property     def prop self           return dict super B  self  prop  b 2    Store this in test py and run python -i test py  another hidden feature  -i option executed the script and allow you to continue in interactive mode     gt  gt  gt  B   prop   a   1   b   2

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Referencing a list comprehension as it is being built    You can reference a list comprehension as it is being built by the symbol    1     For example  the following function unique-ifies a list of elements without changing their order by referencing its list comprehension  def unique my list       return  x for x in my list if x not in locals      1

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A slight misfeature of python   The normal fast way to join a list of strings together is      join list of strings

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The underscore  it contains the most recent output value displayed by the interpreter  in an interactive session           a for a in xrange 10000    lt generator object at 0x81a8fcc      b    blah         lt generator object at 0x81a8fcc     A convenient Web-browser controller        import webbrowser     webbrowser open new tab  http   www stackoverflow com     A built-in http server  To serve the files in the current directory    python -m SimpleHTTPServer 8000   AtExit       import atexit

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The Zen of Python   gt  gt  gt  import this The Zen of Python  by Tim Peters  Beautiful is better than ugly  Explicit is better than implicit  Simple is better than complex  Complex is better than complicated  Flat is better than nested  Sparse is better than dense  Readability counts  Special cases aren t special enough to break the rules  Although practicality beats purity  Errors should never pass silently  Unless explicitly silenced  In the face of ambiguity  refuse the temptation to guess  There should be one-- and preferably only one --obvious way to do it  Although that way may not be obvious at first unless you re Dutch  Now is better than never  Although never is often better than  right  now  If the implementation is hard to explain  it s a bad idea  If the implementation is easy to explain  it may be a good idea  Namespaces are one honking great idea -- let s do more of those

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The re Scanner class  http   code activestate com recipes 457664-hidden-scanner-functionality-in-re-module

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To add more python modules  espcially 3rd party ones   most people seem to use PYTHONPATH environment variables or they add symlinks or directories in their site-packages directories  Another way  is to use   pth files  Here s the official python doc s explanation       The most convenient way  to modify   python s search path  is to add a path   configuration file to a directory   that s already on Python s path    usually to the     site-packages    directory  Path configuration files   have an extension of  pth  and each   line must contain a single path that   will be appended to sys path   Because   the new paths are appended to   sys path  modules in the added   directories will not override standard   modules  This means you can t use this   mechanism for installing fixed   versions of standard modules

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If you don t like using whitespace to denote scopes  you can use the C-style    by issuing   from   future   import braces

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Readable regular expressions  In Python you can split a regular expression over multiple lines  name your matches and insert comments   Example verbose syntax  from Dive into Python     gt  gt  gt  pattern                                 beginning of string     M 0 4                 thousands - 0 to 4 M s      CM CD D C 0 3        hundreds - 900  CM   400  CD   0-300  0 to 3 C s                                        or 500-800  D  followed by 0 to 3 C s       XC XL L X 0 3        tens - 90  XC   40  XL   0-30  0 to 3 X s                                    or 50-80  L  followed by 0 to 3 X s       IX IV V I 0 3        ones - 9  IX   4  IV   0-3  0 to 3 I s                                    or 5-8  V  followed by 0 to 3 I s                            end of string          gt  gt  gt  re search pattern   M   re VERBOSE    Example naming matches  from Regular Expression HOWTO    gt  gt  gt  p   re compile r   P lt word gt  b w  b     gt  gt  gt  m   p search        Lots of punctuation         gt  gt  gt  m group  word    Lots    You can also verbosely write a regex without using re VERBOSE thanks to string literal concatenation    gt  gt  gt  pattern                                   beginning of string          M 0 4                thousands - 0 to 4 M s           CM CD D C 0 3       hundreds - 900  CM   400  CD   0-300  0 to 3 C s                                            or 500-800  D  followed by 0 to 3 C s            XC XL L X 0 3       tens - 90  XC   40  XL   0-30  0 to 3 X s                                        or 50-80  L  followed by 0 to 3 X s            IX IV V I 0 3       ones - 9  IX   4  IV   0-3  0 to 3 I s                                        or 5-8  V  followed by 0 to 3 I s                                end of string        gt  gt  gt  print pattern   M 0 4  CM CD D C 0 3   XC XL L X 0 3   IX IV V I 0 3

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Interleaving if and for in list comprehensions   gt  gt  gt    x  y  for x in range 4  if x   2    1 for y in range 4     1  0    1  1    1  2    1  3    3  0    3  1    3  2    3  3     I never realized this until I learned Haskell

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Exception else clause   try    put 4000000000 volts through it parrot  except Voom    print   E s pining   else    print  This parrot is no more   finally    end sketch     The use of the else clause is better than adding additional code to the try clause because it avoids accidentally catching an exception that wasn   t raised by the code being protected by the try     except statement   See http   docs python org tut node10 html

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From python 3 1   2 7   dictionary and set comprehensions are supported      a a for a in range 10      a for a in range 10

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One word  IPython  Tab introspection  pretty-printing   debug  history management  pylab      well worth the time to learn well

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to activate the autocompletion in IDE that accepts it  like IDLE  Editra  IEP  instead of making   hi    and then you hit TAB   you can cheat in the IDE  just make hi    and you heat TAB   as you can see  there is no single quote in the beginning  because it will only follows the latest punctuation  it s like when you add   and hit enter  it adds directly an indentation  dont know if it will make change  but it s a tip no more

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Creating generators objects  If you write   x  n for n in foo if bar n     you can get out the generator and assign it to x  Now it means you can do  for n in x    The advantage of this is that you don t need intermediate storage  which you would need if you did  x    n for n in foo if bar n     In some cases this can lead to significant speed up   You can append many if statements to the end of the generator  basically replicating nested for loops    gt  gt  gt  n     a b  for a in range 0 2  for b in range 4 6    gt  gt  gt  for i in n        print i    0  4   0  5   1  4   1  5

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The Borg Pattern This is a killer from Alex Martelli  All instances of Borg share state  This removes the need to employ the singleton pattern  instances don t matter when state is shared  and is rather elegant  but is more complicated with new classes   The value of foo can be reassigned in any instance and all will be updated  you can even reassign the entire dict  Borg is the perfect name  read more here  class Borg        shared state     foo    bar       def   init   self           self   dict     self   shared state       rest of your class here  This is perfect for sharing an eventlet GreenPool to control concurrency

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reversing an iterable using negative step   gt  gt  gt  s    Hello World   gt  gt  gt  s   -1   dlroW olleH   gt  gt  gt  a    1 2 3 4 5 6   gt  gt  gt  a   -1   6  5  4  3  2  1   gt  gt  gt  a    5 4 3 2 1   gt  gt  gt  a   -1   1  2  3  4  5

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threading enumerate   gives access to all Thread objects in the system and sys  current frames   returns  the current stack frames of all threads in the system  so combine these two and you get Java style stack dumps   def dumpstacks signal  frame       id2name   dict   th ident  th name  for th in threading enumerate         code          for threadId  stack in sys  current frames   items            code append   n  Thread   s  d      id2name threadId   threadId           for filename  lineno  name  line in traceback extract stack stack               code append  File    s   line  d  in  s     filename  lineno  name               if line                  code append     s     line strip         print   n  join code   import signal signal signal signal SIGQUIT  dumpstacks    Do this at the beginning of a multi-threaded python program and you get access to current state of threads at any time by sending a SIGQUIT  You may also choose signal SIGUSR1 or signal SIGUSR2   See

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In addition to this mentioned earlier by haridsv    gt  gt  gt  foo   bar   baz   1  gt  gt  gt  foo  bar  baz  1  1  1    it s also possible to do this    gt  gt  gt  foo  bar  baz   1  2  3  gt  gt  gt  foo  bar  baz  1  2  3

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Dict Comprehensions   gt  gt  gt   i  i  2 for i in range 5    0  0  1  1  2  4  3  9  4  16    Python documentation  Wikipedia Entry

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Changing function label at run time    gt  gt  gt  class foo        def normal call self   print  normal call        def call self            print  first call          self call   self normal call   gt  gt  gt  y   foo    gt  gt  gt  y call   first call  gt  gt  gt  y call   normal call  gt  gt  gt  y call   normal call

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Not very hidden  but functions have attributes   def doNothing        pass  doNothing monkeys   4 print doNothing monkeys 4

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Python has  private  variables  Variables that start  but not end  with a double underscore become private  and not just by convention  Actually   var turns into  Classname  var  where Classname is the class in which the variable was created  They are not inherited and cannot be overriden        class A          def   init   self                   self   var   5         def getvar self                   return self   var          a   A       a   var Traceback  most recent call last     File     line 1  in  AttributeError  A instance has no attribute    var      a getvar   5     dir a     A  var      doc        init        module      getvar

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Flattening a list with sum     The sum   built-in function can be used to   add   lists together  providing a handy way to flatten a list of lists   Python 2 7 1  r271 86832  May 27 2011  21 41 45    GCC 4 2 1  Apple Inc  build 5664   on darwin Type  help    copyright    credits  or  license  for more information   gt  gt  gt  l     1  2  3    4  5    6    7  8  9    gt  gt  gt  sum l       1  2  3  4  5  6  7  8  9

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Ever used xrange INT  instead of range INT       It s got less memory usage and doesn t really depend on the size of the integer  Yey   Isn t that good

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Access Dictionary elements as   attributes  properties   so if an   a1 AttrDict   has key  name  -    instead of a1  name   we can easily   access name attribute of a1 using -    a1 name     class AttrDict dict        def   getattr   self  name           if name in self              return self name          raise AttributeError   s not found    name       def   setattr   self  name  value           self name    value      def   delattr   self  name           del self name   person   AttrDict   name    John Doe    age   66   print person  name   print person name  person name    Frodo G  print person name  del person age  print person

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Dictionaries have a get   method  Dictionaries have a  get    method  If you do d  key   and key isn t there  you get an exception  If you do d get  key    you get back None if  key  isn t there  You can add a second argument to get that item back instead of None  eg  d get  key   0    It s great for things like adding up numbers   sum value    sum get value  0    1

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In Python 2 you can generate a string representation of an expression by enclosing it with backticks         gt  gt  gt   sorted    lt built-in function sorted gt     This is gone in python 3 X

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Obviously  the antigravity module  xkcd  353

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Not really a hidden feature but something that might come in handy   for looping through items in a list pairwise  for x  y in zip s  s 1

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Braces  def g        print  hi    def f          g       gt  gt  gt  f   hi

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Not a programming feature but is useful when using Python with bash or shell scripts   python -c import os  print os getcwd        See the python documentation here  Additional things to note when writing longer Python scripts can be seen in this discussion

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Named formatting    -formatting takes a dictionary  also applies  i  s etc  validation     gt  gt  gt  print  The   foo s is   bar i       foo    answer    bar  42  The answer is 42    gt  gt  gt  foo  bar    question   123   gt  gt  gt  print  The   foo s is   bar i     locals   The question is 123    And since locals   is also a dictionary  you can simply pass that as a dict and have   -substitions from your local variables  I think this is frowned upon  but simplifies things    New Style Formatting   gt  gt  gt  print  The  foo  is  bar   format foo  answer   bar 42

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The textwrap dedent utility function in python can come quite in handy testing that a multiline string returned is equal to the expected output without breaking the indentation of your unittests   import unittest  textwrap  class XMLTests unittest TestCase       def test returned xml value self           returned xml   call to function that returns xml           expected value   textwrap dedent               lt  xml version  1 0  encoding  utf-8   gt           lt root node gt               lt my node gt my content lt  my node gt           lt  root node gt                        self assertEqual expected value  returned xml

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mapreduce using map and reduce functions  create a simple sumproduct this way   def sumprod x y       return reduce lambda a b a b  map lambda a  b  a b x y     example   In  2   sumprod  1 2 3   4 5 6   Out 2   32

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Python have exceptions for very unexpected things   Imports  This let you import an alternative if a lib is missing  try      import json except ImportError      import simplejson as json   Iteration  For loops do this internally  and catch StopIteration   iter     next   Traceback  most recent call last     File   lt pyshell 4 gt    line 1  in  lt module gt      iter a  next   StopIteration   Assertion   gt  gt  gt  try          assert        except AssertionError          print  This list should not be empty  This list should not be empty   While this is more verbose for one check  multiple checks mixing exceptions and boolean operators with the same error message can be shortened this way

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An interpreter within the interpreter  The standard library s code module let s you include your own read-eval-print loop inside a program  or run a whole nested interpreter  E g   copied my example from here     python Python 2 5 1  r251 54863  Jan 17 2008  19 35 17    GCC 4 0 1  Apple Inc  build 5465   on darwin Type  help    copyright    credits  or  license  for more information   gt  gt  gt  shared var    Set in main console   gt  gt  gt  import code  gt  gt  gt  ic   code InteractiveConsole    shared var   shared var     gt  gt  gt  try          ic interact  My custom console banner        except SystemExit  e          print  Got SystemExit        My custom console banner   gt  gt  gt  shared var  Set in main console   gt  gt  gt  shared var    Set in sub-console   gt  gt  gt  import sys  gt  gt  gt  sys exit   Got SystemExit   gt  gt  gt  shared var  Set in main console    This is extremely useful for situations where you want to accept scripted input from the user  or query the state of the VM in real-time   TurboGears uses this to great effect by having a WebConsole from which you can query the state of you live web app

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Get the python regex parse tree to debug your regex   Regular expressions are a great feature of python  but debugging them can be a pain  and it s all too easy to get a regex wrong   Fortunately  python can print the regex parse tree  by passing the undocumented  experimental  hidden flag re DEBUG  actually  128  to re compile    gt  gt  gt  re compile     font      P lt size gt  -   0-9  1 2             font        re DEBUG  at at beginning literal 91 literal 102 literal 111 literal 110 literal 116 max repeat 0 1   subpattern None     literal 61     subpattern 1       in         literal 45         literal 43       max repeat 1 2         in           range  48  57  literal 93 subpattern 2   min repeat 0 65535     any None in   literal 47   literal 102   literal 111   literal 110   literal 116   Once you understand the syntax  you can spot your errors   There we can see that I forgot to escape the    in   font    Of course you can combine it with whatever flags you want  like commented regexes    gt  gt  gt  re compile                       start of a line    font           the font tag        P lt size gt     optional  font  size    -   0-9  1 2    size specification                        end of tag                   text between the tags     font          end of the tag       re DEBUG re VERBOSE re DOTALL

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The unpacking syntax has been upgraded in the recent version as can be seen in the example    gt  gt  gt  a   b   range 5   gt  gt  gt  a  b  0   1  2  3  4    gt  gt  gt   a  b   range 5   gt  gt  gt  a  b   0  1  2  3   4   gt  gt  gt  a   b  c   range 5   gt  gt  gt  a  b  c  0   1  2  3   4

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Python sort function sorts tuples correctly  i e  using the familiar lexicographical order    a     2   b     1   a     2   a     3   c    print sorted a     1   a     2   a     2   b     3   c      Useful if you want to sort a list of persons after age and then name

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Chaining comparison operators    gt  gt  gt  x   5  gt  gt  gt  1  lt  x  lt  10 True  gt  gt  gt  10  lt  x  lt  20  False  gt  gt  gt  x  lt  10  lt  x 10  lt  100 True  gt  gt  gt  10  gt  x  lt   9 True  gt  gt  gt  5    x  gt  4 True   In case you re thinking it s doing 1  lt  x  which comes out as True  and then comparing True  lt  10  which is also True  then no  that s really not what happens  see the last example   It s really translating into 1  lt  x and x  lt  10  and x  lt  10 and 10  lt  x   10 and x 10  lt  100  but with less typing and each term is only evaluated once

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import antigravity

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Conditional Assignment  x   3 if  y    1  else 2   It does exactly what it sounds like   assign 3 to x if y is 1  otherwise assign 2 to x   Note that the parens are not necessary  but I like them for readability  You can also chain it if you have something more complicated   x   3 if  y    1  else 2 if  y    -1  else 1   Though at a certain point  it goes a little too far   Note that you can use if     else in any expression  For example    func1 if y    1 else func2  arg1  arg2     Here func1 will be called if y is 1 and func2  otherwise  In both cases the corresponding function will be called with arguments arg1 and arg2   Analogously  the following is also valid   x    class1 if y    1 else class2  arg1  arg2    where class1 and class2 are two classes

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while not very pythonic you can write to a file using print  print gt  gt outFile   I am Being Written   Explanation      This form is sometimes referred to as      print chevron     In this form  the   first expression after the    must   evaluate to a    file-like    object    specifically an object that has a   write   method as described above    With this extended form  the   subsequent expressions are printed to   this file object  If the first   expression evaluates to None  then   sys stdout is used as the file for   output

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Objects of small intgers  -5    256  never created twice         a1   -5  b1   256     a2   -5  b2   256     id a1     id a2   id b1     id b2   True  True          c1   -6  d1   257     c2   -6  d2   257     id c1     id c2   id d1     id d2   False  False         Edit  List objects never destroyed  only objects in lists   Python has array in which it keeps up to 80 empty lists  When you destroy list object - python puts it to that array and when you create new list - python gets last puted list from this array         a    1 2 3   a id   id a      b    1 2 3   b id   id b      del a  del b     c    1 2 3   id c     b id True     d    1 2 3   id d     a id True

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set frozenset  Probably an easily overlooked python builtin is  set frozenset    Useful when you have a list like this   1 2 1 1 2 3 4  and only want the uniques like this  1 2 3 4    Using set   that s exactly what you get    gt  gt  gt  x    1 2 1 1 2 3 4    gt  gt  gt    gt  gt  gt  set x   set  1  2  3  4     gt  gt  gt   gt  gt  gt  for i in set x           print i     1 2 3 4   And of course to get the number of uniques in a list    gt  gt  gt  len set  1 2 1 1 2 3 4    4   You can also find if a list is a subset of another list using set   issubset      gt  gt  gt  set  1 2 3 4   issubset  0 1 2 3 4 5   True   As of Python 2 7 and 3 0 you can use curly braces to create a set   myset    1 2 3 4    as well as set comprehensions    x for x in stuff    For more details  http   docs python org library stdtypes html set

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Private methods and data hiding  encapsulation   There s a common idiom in Python of denoting methods and other class members that are not intended to be part of the class s external API by giving them names that start with underscores   This is convenient and works very well in practice  but it gives the false impression that Python does not support true encapsulation of private code and or data   In fact  Python automatically gives you lexical closures  which make it very easy to encapsulate data in a much more bulletproof way when the situation really warrants it   Here s a contrived example of a class that makes use of this technique   class MyClass object     def   init   self        privateData           self publicData   123      def privateMethod k         print privateData k    self publicData      def privilegedMethod          privateData  foo      hello         privateMethod  foo        self privilegedMethod   privilegedMethod    def publicMethod self       print self publicData   And here s a contrived example of its use    gt  gt  gt  obj   MyClass    gt  gt  gt  obj publicMethod   123  gt  gt  gt  obj publicData    World   gt  gt  gt  obj publicMethod   World  gt  gt  gt  obj privilegedMethod   hello World  gt  gt  gt  obj privateMethod   Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  AttributeError   MyClass  object has no attribute  privateMethod   gt  gt  gt  obj privateData Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  AttributeError   MyClass  object has no attribute  privateData    The key is that privateMethod and privateData aren t really attributes of obj at all  so they can t be accessed from outside  nor do they show up in dir   or similar   They re local variables in the constructor  completely inaccessible outside of   init     However  because of the magic of closures  they really are per-instance variables with the same lifetime as the object with which they re associated  even though there s no way to access them from outside except  in this example  by invoking privilegedMethod   Often this sort of very strict encapsulation is overkill  but sometimes it really can be very handy for keeping an API or a namespace squeaky clean   In Python 2 x  the only way to have mutable private state is with a mutable object  such as the dict in this example    Many people have remarked on how annoying this can be   Python 3 x will remove this restriction by introducing the nonlocal keyword described in PEP 3104

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is ok   and  Yes  or  No

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Built-in base64  zlib  and rot13 codecs  Strings have encode and decode methods   Usually this is used for converting str to unicode and vice versa  e g  with u   s encode  utf8     But there are some other handy builtin codecs   Compression and decompression with zlib  and bz2  is available without an explicit import    gt  gt  gt  s    a    100  gt  gt  gt  s encode  zlib    x x9cKL xa4  x00 x00zG  xe5    Similarly you can encode and decode base64    gt  gt  gt   Hello world  encode  base64    SGVsbG8gd29ybGQ  n   gt  gt  gt   SGVsbG8gd29ybGQ  n  decode  base64    Hello world    And  of course  you can rot13    gt  gt  gt   Secret message  encode  rot13    Frperg zrffntr

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Top Secret Attributes   gt  gt  gt  class A object   pass  gt  gt  gt  a   A    gt  gt  gt  setattr a   can t touch this   123   gt  gt  gt  dir a      class        delattr        dict        doc        format        getattribute        hash        init        module        new        reduce        reduce ex        repr        setattr        sizeof        str        subclasshook        weakref      can t touch this    gt  gt  gt  a can t touch this   duh   File   lt stdin gt    line 1     a can t touch this                        SyntaxError  EOL while scanning string literal  gt  gt  gt  getattr a   can t touch this   123  gt  gt  gt  setattr a     class     name       O    gt  gt  gt  a   class     name    A   gt  gt  gt  getattr a     class     name       O

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Python can understand any kind of unicode digits  not just the ASCII kind    gt  gt  gt  s   u 10585   gt  gt  gt  s u  uff11 uff10 uff15 uff18 uff15   gt  gt  gt  print s 10585  gt  gt  gt  int s  10585  gt  gt  gt  float s  10585 0

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pow   can also calculate  x    y    z efficiently   There is a lesser known third argument of the built-in pow   function that allows you to calculate xy modulo z more efficiently than simply doing  x    y    z    gt  gt  gt  x  y  z   1234567890  2345678901  17  gt  gt  gt  pow x  y  z               almost instantaneous 6   In comparison   x    y    z didn t given a result in one minute on my machine for the same values

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set frozenset  Probably an easily overlooked python builtin is  set frozenset    Useful when you have a list like this   1 2 1 1 2 3 4  and only want the uniques like this  1 2 3 4    Using set   that s exactly what you get    gt  gt  gt  x    1 2 1 1 2 3 4    gt  gt  gt    gt  gt  gt  set x   set  1  2  3  4     gt  gt  gt   gt  gt  gt  for i in set x           print i     1 2 3 4   And of course to get the number of uniques in a list    gt  gt  gt  len set  1 2 1 1 2 3 4    4   You can also find if a list is a subset of another list using set   issubset      gt  gt  gt  set  1 2 3 4   issubset  0 1 2 3 4 5   True   As of Python 2 7 and 3 0 you can use curly braces to create a set   myset    1 2 3 4    as well as set comprehensions    x for x in stuff    For more details  http   docs python org library stdtypes html set

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slots   is a nice way to save memory  but it s very hard to get a dict of the values of the object   Imagine the following object   class Point object         slots       x    y     Now that object obviously has two attributes   Now we can create an instance of it and build a dict of it this way    gt  gt  gt  p   Point    gt  gt  gt  p x   3  gt  gt  gt  p y   5  gt  gt  gt  dict  k  getattr p  k   for k in p   slots      y   5   x   3    This however won t work if point was subclassed and new slots were added   However Python automatically implements   reduce ex   to help the copy module   This can be abused to get a dict of values    gt  gt  gt  p   reduce ex   2  2  1    y   5   x   3

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The spam module in standard Python  It is used for testing purposes    I ve picked it from ctypes tutorial  Try it yourself    gt  gt  gt  import   hello   Hello world     gt  gt  gt  type   hello     lt type  module  gt   gt  gt  gt  from   phello   import spam Hello world    Hello world     gt  gt  gt  type spam   lt type  module  gt   gt  gt  gt  help spam  Help on module   phello   spam in   phello     NAME       phello   spam  FILE     c  python26  lt frozen gt

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Multi line strings  One approach is to use backslashes    gt  gt  gt  sql    select   from some table   where id  gt  10   gt  gt  gt  print sql select   from some table where id  gt  10   Another is to use the triple-quote    gt  gt  gt  sql      select   from some table  where id  gt  10     gt  gt  gt  print sql select   from some table where id  gt  10   Problem with those is that they are not indented  look poor in your code   If you try to indent  it ll just print the white-spaces you put    A third solution  which I found about recently  is to divide your string into lines and surround with parentheses    gt  gt  gt  sql     select   from some table      lt -- no comma  whitespace at end             where id  gt  10               order by name     gt  gt  gt  print sql select   from some table where id  gt  10 order by name   note how there s no comma between lines  this is not a tuple   and you have to account for any trailing leading white spaces that your string needs to have  All of these work with placeholders  by the way  such as  my name is  s    name

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that dict get   has a default value of None  thereby avoiding KeyErrors   In  1   test     1    a     In  2   test 2  ---------------------------------------------------------------------------  lt type  exceptions KeyError  gt               Traceback  most recent call last    amp lt ipython console amp gt  in  lt module gt      lt type  exceptions KeyError  gt   2  In  3   test get  2    In  4   test get  1   Out 4    a   In  5   test get  2      None Out 5   True   and even to specify this  at the scene    In  6   test get  2   Some        Some  Out 6   True   And you can use setdefault   to have a value set and returned if it doesn t exist    gt  gt  gt  a       gt  gt  gt  b   a setdefault  foo    bar    gt  gt  gt  a   foo    bar    gt  gt  gt  b  bar

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Sending values into generator functions  For example having this function   def mygen           Yield 5 until something else is passed back via send          a   5     while True          f    yield a   yield a and possibly get f in return         if f is not None               a   f   store the new value   You can    gt  gt  gt  g   mygen    gt  gt  gt  g next   5  gt  gt  gt  g next   5  gt  gt  gt  g send 7    we send this back to the generator 7  gt  gt  gt  g next    now it will yield 7 until we send something else 7

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getattr      getattr is a really nice way to make generic classes  which is especially useful if you re writing an API   For example  in the FogBugz Python API  getattr is used to pass method calls on to the web service seamlessly   class FogBugz               def   getattr   self  name             Let s leave the private stuff to Python         if name startswith                    raise AttributeError  No such attribute   s     name           if not self   handlerCache has key name               def handler   kwargs                   return self   makerequest name    kwargs              self   handlerCache name    handler         return self   handlerCache name            When someone calls FogBugz search q  bug    they don t get actually call a search method   Instead  getattr handles the call by creating a new function that wraps the makerequest method  which crafts the appropriate HTTP request to the web API   Any errors will be dispatched by the web service and passed back to the user

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Special methods  Absolute power

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There are no secrets in Python

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namedtuple is a tuple   gt  gt  gt  node   namedtuple  node    a b    gt  gt  gt  node 1 2    node 5 6   1  2  5  6   gt  gt  gt   node 1 2   node 5 6    node a 1  b 2   node a 5  b 6    gt  gt  gt     Some more experiments to respond to comments    gt  gt  gt  from collections import namedtuple  gt  gt  gt  from operator import    gt  gt  gt  mytuple   namedtuple  A    a b    gt  gt  gt  yourtuple   namedtuple  Z    x y    gt  gt  gt  mytuple 1 2    yourtuple 5 6   1  2  5  6   gt  gt  gt  q    mytuple 1 2   yourtuple 5 6    gt  gt  gt  q  A a 1  b 2   Z x 5  y 6    gt  gt  gt  reduce operator   add    q   1  2  5  6    So  namedtuple is an interesting subtype of tuple

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You can assign several variables to the same value   gt  gt  gt  foo   bar   baz   1  gt  gt  gt  foo  bar  baz  1  1  1    Useful to initialize several variable to None  in a compact way

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Simple built-in benchmarking tool  The Python Standard Library comes with a very easy-to-use benchmarking module called  timeit   You can even use it from the command line to see which of several language constructs is the fastest   E g      python -m timeit  r   range 0  1000    for i in r  pass  10000 loops  best of 3  48 4 usec per loop    python -m timeit  r   xrange 0  1000    for i in r  pass  10000 loops  best of 3  37 4 usec per loop

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pdb     The Python Debugger  As a programmer  one of the first things that you need for serious program development is a debugger  Python has one built-in which is available as a module called pdb  for  Python DeBugger   naturally     http   docs python org library pdb html

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While debugging complex data structures pprint module comes handy   Quoting from the docs     gt  gt  gt  import pprint      gt  gt  gt  stuff   sys path     gt  gt  gt  stuff insert 0  stuff   gt  gt  gt  pprint pprint stuff    lt Recursion on list with id 869440 gt           usr local lib python1 5      usr local lib python1 5 test      usr local lib python1 5 sunos5      usr local lib python1 5 sharedmodules      usr local lib python1 5 tkinter

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Metaclasses  of course  -  What is a metaclass in Python

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enumerate with different starting index  enumerate has partly been covered in this answer  but recently I ve found an even more hidden feature of enumerate that I think deserves its own post instead of just a comment   Since Python 2 6  you can specify a starting index to enumerate in its second argument    gt  gt  gt  l     spam    ham    eggs    gt  gt  gt  list enumerate l    gt  gt  gt    0   spam     1   ham     2   eggs     gt  gt  gt  list enumerate l  1    gt  gt  gt    1   spam     2   ham     3   eggs      One place where I ve found it utterly useful is when I am enumerating over entries of a symmetric matrix  Since the matrix is symmetric  I can save time by iterating over the upper triangle only  but in that case  I have to use enumerate with a different starting index in the inner for loop to keep track of the row and column indices properly   for ri  row in enumerate matrix       for ci  column in enumerate matrix ri    ri             ci now refers to the proper column index   Strangely enough  this behaviour of enumerate is not documented in help enumerate   only in the online documentation

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Here are 2 easter eggs     One in python itself    gt  gt  gt  import   hello   Hello world        And another one in the Werkzeug module  which is a bit complicated to reveal  here it is   By looking at Werkzeug s source code  in werkzeug   init   py  there is a line that should draw your attention    werkzeug  internal        easteregg     If you re a bit curious  this should lead you to have a look at the werkzeug  internal py  there  you ll find an  easteregg   function which takes a wsgi application in argument  it also contains some base64 encoded data and 2 nested functions  that seem to do something special if an argument named macgybarchakku is found in the query string   So  to reveal this easter egg  it seems you need to wrap an application in the  easteregg   function  let s go   from werkzeug import Request  Response  run simple from werkzeug import  easteregg   Request application def application request       return Response  Hello World     run simple  localhost   8080   easteregg application     Now  if you run the app and visit http   localhost 8080  macgybarchakku  you should see the easter egg

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for line in open  foo        print line    which is equivalent  but better  to   f   open  foo    r   for line in f readlines       print line  f close

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Not at all a hidden feature but still nice   import os path as op  root dir   op abspath op join op dirname   file              Saves lots of characters when manipulating paths

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Operator overloading for the set builtin    gt  gt  gt  a   set  1 2 3 4    gt  gt  gt  b   set  3 4 5 6    gt  gt  gt  a   b   Union  1  2  3  4  5  6   gt  gt  gt  a  amp  b   Intersection  3  4   gt  gt  gt  a  lt  b   Subset False  gt  gt  gt  a - b   Difference  1  2   gt  gt  gt  a   b   Symmetric Difference  1  2  5  6    More detail from the standard library reference  Set Types

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Operators can be called as functions   from operator import add print reduce add   1 2 3 4 5 6

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Nested list comprehensions and generator expressions     i j  for i in range 3  for j in range i          i j  for i in range 4  for j in range i      These can replace huge chunks of nested-loop code

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From 2 5 onwards dicts have a special method   missing   that is invoked for missing items    gt  gt  gt  class MyDict dict        def   missing   self  key         self key    rv            return rv       gt  gt  gt  m   MyDict    gt  gt  gt  m  foo   append 1   gt  gt  gt  m  foo   append 2   gt  gt  gt  dict m    foo    1  2     There is also a dict subclass in collections called defaultdict that does pretty much the same but calls a function without arguments for not existing items    gt  gt  gt  from collections import defaultdict  gt  gt  gt  m   defaultdict list   gt  gt  gt  m  foo   append 1   gt  gt  gt  m  foo   append 2   gt  gt  gt  dict m    foo    1  2     I recommend converting such dicts to regular dicts before passing them to functions that don t expect such subclasses   A lot of code uses d a key  and catches KeyErrors to check if an item exists which would add a new item to the dict

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Many people don t know about the  dir  function  It s a great way to figure out what an object can do from the interpreter  For example  if you want to see a list of all the string methods    gt  gt  gt  dir  foo       add        class        contains      snipped a bunch    title     translate    upper    zfill     And then if you want more information about a particular method you can call  help  on it    gt  gt  gt  help  foo  upper      Help on built-in function upper   upper          S upper   - gt  string      Return a copy of the string S converted to uppercase

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Multiplying by a boolean  One thing I m constantly doing in web development is optionally printing HTML parameters  We ve all seen code like this in other languages   class   lt   isSelected    selected         gt     In Python  you can multiply by a boolean and it does exactly what you d expect   class   lt    selected    isSelected   gt     This is because multiplication coerces the boolean to an integer  0 for False  1 for True   and in python multiplying a string by an int repeats the string N times

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Memory Management  Python dynamically allocates memory and uses garbage collection to recover unused space  Once an object is out of scope  and no other variables reference it  it will be recovered  I do not have to worry about buffer overruns and slowly growing server processes  Memory management is also a feature of other dynamic languages but Python just does it so well   Of course  we must watch out for circular references and keeping references to objects which are no longer needed  but weak references help a lot here

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You can build up a dictionary from a set of length-2 sequences   Extremely handy when you have a list of values and a list of arrays    gt  gt  gt  dict     foo   bar     a  1    b  2       a   1   b   2   foo    bar     gt  gt  gt  names     Bob    Marie    Alice    gt  gt  gt  ages    23  27  36   gt  gt  gt  dict zip names  ages     Alice   36   Bob   23   Marie   27

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pdb     The Python Debugger  As a programmer  one of the first things that you need for serious program development is a debugger  Python has one built-in which is available as a module called pdb  for  Python DeBugger   naturally     http   docs python org library pdb html

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Multiple references to an iterator  You can create multiple references to the same iterator using list multiplication    gt  gt  gt  i    1 2 3 4 5 6 7 8 9 10    or any iterable object  gt  gt  gt  iterators    iter i     2  gt  gt  gt  iterators 0  next   1  gt  gt  gt  iterators 1  next   2  gt  gt  gt  iterators 0  next   3   This can be used to group an iterable into chunks  for example  as in this example from the itertools documentation  def grouper n  iterable  fillvalue None        grouper 3   ABCDEFG    x   -- gt  ABC DEF Gxx      args    iter iterable     n     return izip longest fillvalue fillvalue   args

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Unicode identifier in Python3    gt  gt  gt   Unicode         Variable  isidentifier   True  gt  gt  gt  Unicode         Variable  Python3 rules    gt  gt  gt  Unicode         Variable  Python3 rules

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Getter functions in module operator  The functions attrgetter   and itemgetter   in module operator can be used to generate fast access functions for use in sorting and search objects and dictionaries  Chapter 6 7 in the Python Library Docs

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Arguably  this is not a programming feature per se  but so useful that I ll post it nevertheless     python -m http server      followed by   wget http    lt ipnumber gt  8000 filename somewhere else   If you are still running an older  2 x  version of Python     python -m SimpleHTTPServer   You can also specify a port e g  python -m http server 80  so you can omit the port in the url if you have the root on the server side

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Python has GOTO     and it s implemented by external pure-Python module     from goto import goto  label for i in range 1  10       for j in range 1  20           for k in range 1  30               print i  j  k             if k    3                  goto  end   breaking out from a deeply nested loop label  end print  Finished

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Interactive Debugging of Scripts  and doctest strings   I don t think this is as widely known as it could be  but add this line to any python script   import pdb  pdb set trace    will cause the PDB debugger to pop up with the run cursor at that point in the code   What s even less known  I think  is that you can use that same line in a doctest        gt  gt  gt  1 in  1 2 3     Becomes  gt  gt  gt  import pdb  pdb set trace    1 in  1 2 3        You can then use the debugger to checkout the doctest environment   You can t really step through a doctest because the lines are each run autonomously  but it s a great tool for debugging the doctest globs and environment

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itertools  This module is often overlooked   The following example uses itertools chain   to flatten a list    gt  gt  gt  from itertools import    gt  gt  gt  l     1  2    3  4    gt  gt  gt  list chain  l    1  2  3  4    See http   docs python org library itertools html recipes for more applications

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Guessing integer base   gt  gt  gt  int  10   0  10  gt  gt  gt  int  0x10   0  16  gt  gt  gt  int  010   0     does not work on Python 3 x 8  gt  gt  gt  int  0o10   0     Python  gt  2 6 and Python 3 x 8  gt  gt  gt  int  0b10   0     Python  gt  2 6 and Python 3 x 2

[python] Hidden features of Python

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