Why are Python lambdas useful

Question

I m trying to figure out Python lambdas   Is lambda one of those  interesting  language items that in real life should be forgotten    I m sure there are some edge cases where it might be needed  but given the obscurity of it  the potential of it being redefined in future releases  my assumption based on the various definitions of it  and the reduced coding clarity - should it be avoided     This reminds me of overflowing  buffer overflow  of C types - pointing to the top variable and overloading to set the other field values   It feels like sort of a techie showmanship but maintenance coder nightmare

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As stated above  the lambda operator in Python defines an anonymous function  and in Python functions are closures  It is important not to confuse the concept of closures with the operator lambda  which is merely syntactic methadone for them   When I started in Python a few years ago  I used lambdas a lot  thinking they were cool  along with list comprehensions  However  I wrote and have to maintain a big website written in Python  with on the order of several thousand function points  I ve learnt from experience that lambdas might be OK to prototype things with  but offer nothing over inline functions  named closures  except for saving a few key-stokes  or sometimes not   Basically this boils down to several points    it is easier to read software that is explicitly written using meaningful names  Anonymous closures by definition cannot have a meaningful name  as they have no name  This brevity seems  for some reason  to also infect lambda parameters  hence we often see examples like lambda x  x 1  it is easier to reuse named closures  as they can be referred to by name more than once  when there is a name to refer to them by  it is easier to debug code that is using named closures instead of lambdas  because the name will appear in tracebacks  and around the error    That s enough reason to round them up and convert them to named closures  However  I hold two other grudges against anonymous closures   The first grudge is simply that they are just another unnecessary keyword cluttering up the language   The second grudge is deeper and on the paradigm level  i e  I do not like that they promote a functional-programming style  because that style is less flexible than the message passing  object oriented or procedural styles  because the lambda calculus is not Turing-complete  luckily in Python  we can still break out of that restriction even inside a lambda   The reasons I feel lambdas promote this style are    There is an implicit return  i e  they seem like they  should  be functions  They are an alternative state-hiding mechanism to another  more explicit  more readable  more reusable and more general mechanism  methods    I try hard to write lambda-free Python  and remove lambdas on sight  I think Python would be a slightly better language without lambdas  but that s just my opinion

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I use lambda to create callbacks that include parameters  It s cleaner writing a lambda in one line than to write a method to perform the same functionality   For example   import imported module  def func        return lambda  imported module method  foo    bar     as opposed to   import imported module  def func        def cb            return imported module method  foo    bar       return cb

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lambda is just a fancy way of saying function  Other than its name  there is nothing obscure  intimidating or cryptic about it  When you read the following line  replace lambda by function in your mind    gt  gt  gt  f   lambda x  x   1  gt  gt  gt  f 3  4   It just defines a function of x  Some other languages  like R  say it explicitly    gt  f   function x    x   1    gt  f 3  4   You see  It s one of the most natural things to do in programming

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I use lambdas to avoid code duplication  It would make the function easily comprehensible Eg   def a func           if some conditon                call some big func arg1  arg2  arg3  arg4       else               call some big func arg1  arg2  arg3  arg4       I replace that with a temp lambda  def a func           call big f   lambda args that change  call some big func arg1  arg2  arg3  args that change    if some conditon                call big f argX    else               call big f argY

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Pretty much anything you can do with lambda you can do better with either named functions or list and generator expressions   Consequently  for the most part you should just one of those in basically any situation  except maybe for scratch code written in the interactive interpreter

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I use it quite often  mainly as a null object or to partially bind parameters to a function   Here are examples   to implement null object pattern         DATA PACKET  self handle data packets     NET PACKET  self handle hardware packets   get packet type  lambda x   None  payload    for parameter binding   let say that I have the following API  def dump hex file  var        some code     pass  class X object                def packet received data             some kind of preprocessing         self callback data             Then  when I wan t to quickly dump the recieved data to a file I do that      dump file   file  hex dump txt   w   X callback   lambda  x   dump hex dump file  x      dump file close

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Lambdas are deeply linked to functional programming style in general  The idea that you can solve problems by applying a function to some data  and merging the results  is what google uses to implement most of its algorithms     Programs written in functional programming style  are easily parallelized  and hence are becoming more and more important with modern multi-core machines  So in short  NO you should not forget them

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Lambda function it s a non-bureaucratic way to create a function   That s it  For example  let s supose you have your main function and need to square values  Let s see the traditional way and the lambda way to do this   Traditional way   def main            y   square some number      return something  def square x       return x  2   The lambda way   def main        square   lambda x  x  2 y   square some number  return something   See the difference   Lambda functions go very well with lists  like lists comprehensions or map  In fact  list comprehension it s a  pythonic  way to express yourself using lambda  Ex    gt  gt  gt a    1 2 3 4   gt  gt  gt  x  2 for x in a   1 4 9 16    Let s see what each elements of the syntax means            Give me a list       x  2    using this new-born function       for x in a   into each element in a    That s convenient uh  Creating functions like this  Let s rewrite it using lambda    gt  gt  gt  square   lambda x  x  2  gt  gt  gt   square s  for x in a   1 4 9 16    Now let s use map  which is the same thing  but more language-neutral  Maps takes 2 arguments     i  one function   ii  an iterable  And gives you a list where each element it s the function applied to each element of the iterable   So  using map we would have    gt  gt  gt  a    1 2 3 4   gt  gt  gt  squared list   map lambda x  x  2  a    If you master lambdas and mapping  you will have a great power to manipulate data and in a concise way  Lambda functions are neither obscure nor take away code clarity  Don t confuse something hard with something new  Once you start using them  you will find it very clear

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One of the nice things about lambda that s in my opinion understated is that it s way of deferring an evaluation for simple forms till the value is needed  Let me explain    Many library routines are implemented so that they allow certain parameters to be callables  of whom lambda is one   The idea is that the actual value will be computed only at the time when it s going to be used  rather that when it s called   An  contrived  example might help to illustrate the point  Suppose you have a routine which which was going to do log a given timestamp  You want the routine to use the current time minus 30 minutes  You d call it like so  log timestamp datetime datetime now   - datetime timedelta minutes   30     Now suppose the actual function is going to be called only when a certain event occurs and you want the timestamp to be computed only at that time  You can do this like so  log timestamp lambda   datetime datetime now   - datetime timedelta minutes   30     Assuming the log timestamp can handle callables like this  it will evaluate this when it needs it and you ll get the timestamp at that time    There are of course alternate ways to do this  using the operator module for example  but I hope I ve conveyed the point    Update  Here is a slightly more concrete real world example    Update 2  I think this is an example of what is called a thunk

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I doubt lambda will go away  See Guido s post about finally giving up trying to remove it  Also see an outline of the conflict   You might check out this post for more of a history about the deal behind Python s functional features  http   python-history blogspot com 2009 04 origins-of-pythons-functional-features html     Curiously  the map  filter  and reduce functions that originally motivated the introduction of lambda and other functional features have to a large extent been superseded by list comprehensions and generator expressions  In fact  the reduce function was removed from list of builtin functions in Python 3 0   However  it s not necessary to send in complaints about the removal of lambda  map or filter  they are staying   -    My own two cents  Rarely is lambda worth it as far as clarity goes  Generally there is a more clear solution that doesn t include lambda

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First congrats that managed to figure out lambda  In my opinion this is really powerful construct to act with  The trend these days towards functional programming languages is surely an indicator that it neither should be avoided nor it will be redefined in the near future   You just have to think a little bit different  I m sure soon you will love it  But be careful if you deal only with python  Because the lambda is not a real closure  it is  broken  somehow  pythons lambda is broken

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Lambda is a procedure constructor  You can synthesize programs at run-time  although Python s lambda is not very powerful  Note that few people understand that kind of programming

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Are you talking about lambda functions  Like  lambda x  x  2   2 x - 5   Those things are actually quite useful   Python supports a style of programming called functional programming where you can pass functions to other functions to do stuff  Example   mult3   filter lambda x  x   3    0   1  2  3  4  5  6  7  8  9     sets mult3 to  3  6  9   those elements of the original list that are multiples of 3  This is shorter  and  one could argue  clearer  than  def filterfunc x       return x   3    0 mult3   filter filterfunc   1  2  3  4  5  6  7  8  9     Of course  in this particular case  you could do the same thing as a list comprehension   mult3    x for x in  1  2  3  4  5  6  7  8  9  if x   3    0     or even as range 3 10 3    but there are many other  more sophisticated use cases where you can t use a list comprehension and a lambda function may be the shortest way to write something out    Returning a function from another function   gt  gt  gt  def transform n           return lambda x  x   n      gt  gt  gt  f   transform 3   gt  gt  gt  f 4  7   This is often used to create function wrappers  such as Python s decorators  Combining elements of an iterable sequence with reduce     gt  gt  gt  reduce lambda a  b           format a  b    1  2  3  4  5  6  7  8  9    1  2  3  4  5  6  7  8  9   Sorting by an alternate key   gt  gt  gt  sorted  1  2  3  4  5  6  7  8  9   key lambda x  abs 5-x    5  4  6  3  7  2  8  1  9     I use lambda functions on a regular basis  It took me a while to get used to them  but eventually I came to understand that they re a very valuable part of the language

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I can give you an example where I actually needed lambda serious   I m making a graphical program  where the use right clicks on a file and assigns it one of three options   It turns out that in Tkinter  the GUI interfacing program I m writing this in   when someone presses a button  it can t be assigned to a command that takes in arguments   So if I chose one of the options and wanted the result of my choice to be   print  hi there    Then no big deal   But what if I need my choice to have a particular detail   For example  if I choose choice A  it calls a function that takes in some argument that is dependent on the choice A  B or C  TKinter could not support this   Lamda was the only option to get around this actually

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A useful case for using lambdas is to improve the readability of long list comprehensions  In this example loop dic is short for clarity but imagine loop dic being very long  If you would just use a plain value that includes i instead of the lambda version of that value you would get a NameError    gt  gt  gt  lis      name    Peter      name    Josef      gt  gt  gt  loop dic   lambda i    name   i  name       Wallace     gt  gt  gt  new lis    loop dic i  for i in lis    gt  gt  gt  new lis    name    Peter Wallace      name    Josef Wallace      Instead of    gt  gt  gt  lis      name    Peter      name    Josef      gt  gt  gt  new lis      name   i  name       Wallace   for i in lis    gt  gt  gt  new lis    name    Peter Wallace      name    Josef Wallace

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I find lambda useful for a list of functions that do the same  but for different circumstances    Like the Mozilla plural rules   plural rules         lambda n   all       lambda n   singular  if n    1 else  plural       lambda n   singular  if 0  lt   n  lt   1 else  plural               Call plural rule  1 with argument 4 to find out which sentence form to use  plural rule 1  4    returns  plural    If you d have to define a function for all of those you d go mad by the end of it  Also  it wouldn t be nice with function names like plural rule 1  plural rule 2  etc  And you d need to eval   it when you re depending on a variable function id

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I m a python beginner  so to getter a clear idea of lambda I compared it with a  for  loop  in terms of efficiency  Here s the code  python 2 7  -  import time start   time time     Measure the time taken for execution  def first        squares   map lambda x  x  2  range 10           Lambda     end   time time       elapsed   end - start     print elapsed     seconds      return elapsed   gives 0 0 seconds  def second        lst          for i in range 10           lst append i  2          a  for  loop     end   time time       elapsed   end - start     print elapsed     seconds      return elapsed   gives 0 0019998550415 seconds   print abs second   - first      Gives 0 0019998550415 seconds  duh

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I started reading David Mertz s book today  Text Processing in Python   While he has a fairly terse description of Lambda s the examples in the first chapter combined with the explanation in Appendix A made them jump off the page for me  finally  and all of a sudden I understood their value   That is not to say his explanation will work for you and I am still at the discovery stage so I will not attempt to add to these responses other than the following  I am new to Python I am new to OOP Lambdas were a struggle for me Now that I read Mertz  I think I get them and I see them as very useful as I think they allow a cleaner approach to programming     He reproduces the Zen of Python  one line of which is Simple is better than complex  As a non-OOP programmer reading code with lambdas  and until last week list comprehensions  I have thought-This is simple    I finally realized today that actually these features make the code much more readable  and understandable than the alternative-which is invariably a loop of some sort   I also realized that like financial statements-Python was not designed for the novice user  rather it is designed for the user that wants to get educated   I can t believe how powerful this language is   When it dawned on me  finally  the purpose and value of lambdas I wanted to rip up about 30 programs and start over putting in lambdas where appropriate

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I m just beginning Python and ran head first into Lambda- which took me a while to figure out    Note that this isn t a condemnation of anything   Everybody has a different set of things that don t come easily      Is lambda one of those  interesting  language items that in real life should be forgotten    No      I m sure there are some edge cases where it might be needed  but given the obscurity of it    It s not obscure   The past 2 teams I ve worked on  everybody used this feature all the time      the potential of it being redefined in future releases  my assumption based on the various definitions of it    I ve seen no serious proposals to redefine it in Python  beyond fixing the closure semantics a few years ago      and the reduced coding clarity - should it be avoided    It s not less clear  if you re using it right   On the contrary  having more language constructs available increases clarity      This reminds me of overflowing  buffer overflow  of C types - pointing to the top variable and overloading to set the other field values   sort of a techie showmanship but maintenance coder nightmare     Lambda is like buffer overflow   Wow   I can t imagine how you re using lambda if you think it s a  maintenance nightmare

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lambdas are extremely useful in GUI programming  For example  lets say you re creating a group of buttons and you want to use a single paramaterized callback rather than a unique callback per button  Lambda lets you accomplish that with ease   for value in   one   two   three        b   tk Button label value  command lambda arg value  my callback arg       b pack      Note  although this question is specifically asking about lambda  you can also use functools partial to get the same type of result   The alternative is to create a separate callback for each button which can lead to duplicated code

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A lambda is part of a very important abstraction mechanism which deals with higher order functions  To get proper understanding of its value  please watch high quality lessons from Abelson and Sussman  and read the book SICP  These are relevant issues in modern software business  and becoming ever more popular

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In Python  lambda is just a way of defining functions inline   a   lambda x  x   1 print a 1    and    def a x   return x   1 print a 1      are the exact same   There is nothing you can do with lambda which you cannot do with a regular function mdash in Python functions are an object just like anything else  and lambdas simply define a function    gt  gt  gt  a   lambda x  x   1  gt  gt  gt  type a   lt type  function  gt    I honestly think the lambda keyword is redundant in Python mdash I have never had the need to use them  or seen one used where a regular function  a list-comprehension or one of the many builtin functions could have been better used instead   For a completely random example  from the article  Python   s lambda is broken        To see how lambda is broken  try generating a list of functions fs  f0     f9  where fi n  i n  First attempt    gt  gt  gt  fs     lambda n  i   n  for i in range 10    gt  gt  gt  fs 3  4  13    I would argue  even if that did work  it s horribly and  unpythonic   the same functionality could be written in countless other ways  for example    gt  gt  gt  n   4  gt  gt  gt   i   n for i in range 10    4  5  6  7  8  9  10  11  12  13    Yes  it s not the same  but I have never seen a cause where generating a group of lambda functions in a list has been required  It might make sense in other languages  but Python is not Haskell  or Lisp  or          Please note that we can use lambda and still achieve the desired   results in this way     gt  gt  gt  fs     lambda n i i  i   n  for i in range 10    gt  gt  gt  fs 3  4  7    Edit   There are a few cases where lambda is useful  for example it s often convenient when connecting up signals in PyQt applications  like this   w   PyQt4 QtGui QLineEdit   w textChanged connect lambda event  dothing      Just doing w textChanged connect dothing  would call the dothing method with an extra event argument and cause an error  Using the lambda means we can tidily drop the argument without having to define a wrapping function

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I can t speak to python s particular implementation of lambda  but in general lambda functions are really handy   They re a core technique  maybe even THE technique  of functional programming  and they re also very useuful in object-oriented programs   For certain types of problems  they re the best solution  so certainly shouldn t be forgotten   I suggest you read up on closures and the map function  that links to python docs  but it exists in nearly every language that supports functional constructs  to see why it s useful

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Lambdas are actually very powerful constructs that stem from ideas in functional programming  and it is something that by no means will be easily revised  redefined or removed in the near future of Python  They help you write code that is more powerful as it allows you to pass functions as parameters  thus the idea of functions as first-class citizens   Lambdas do tend to get confusing  but once a solid understanding is obtained  you can write clean elegant code like this   squared   map lambda x  x x   1  2  3  4  5     The above line of code returns a list of the squares of the numbers in the list  Ofcourse  you could also do it like   def square x       return x x  squared   map square   1  2  3  4  5     It is obvious the former code is shorter  and this is especially true if you intend to use the map function  or any similar function that takes a function as a parameter  in only one place  This also makes the code more intuitive and elegant    Also  as  David Zaslavsky mentioned in his answer  list comprehensions are not always the way to go especially if your list has to get values from some obscure mathematical way   From a more practical standpoint  one of the biggest advantages of lambdas for me recently has been in GUI and event-driven programming  If you take a look at callbacks in Tkinter  all they take as arguments are the event that triggered them  E g   def define bindings widget       widget bind   lt Button-1 gt    do-something-cool   def do-something-cool event        Your code to execute on the event trigger   Now what if you had some arguments to pass  Something as simple as passing 2 arguments to store the coordinates of a mouse-click  You can easily do it like this   def main          define widgets and other imp stuff     x  y   None  None     widget bind   lt Button-1 gt    lambda event  do-something-cool x  y    def do-something-cool event  x  y       x   event x     y   event y      Do other cool stuff   Now you can argue that this can be done using global variables  but do you really want to bang your head worrying about memory management and leakage especially if the global variable will just be used in one particular place  That would be just poor programming style   In short  lambdas are awesome and should never be underestimated  Python lambdas are not the same as LISP lambdas though  which are more powerful   but you can really do a lot of magical stuff with them

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The two-line summary    Closures  Very useful   Learn them  use them  love them  Python s lambda keyword  unnecessary  occasionally useful   If you find yourself doing anything remotely complex with it  put it away and define a real function

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I ve been using Python for a few years and I ve never run in to a case where I ve needed lambda  Really  as the tutorial states  it s just for syntactic sugar

[python] Why are Python lambdas useful?

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