What is the difference between str and repr

Question

What is the difference between   str   and   repr   in Python

User · Accepted Answer

Alex summarized well but  surprisingly  was too succinct   First  let me reiterate the main points in Alex   s post    The default implementation is useless  it   s hard to think of one which wouldn   t be  but yeah    repr   goal is to be unambiguous   str   goal is to be readable Container   s   str   uses contained objects      repr     Default implementation is useless  This is mostly a surprise because Python   s defaults tend to be fairly useful  However  in this case  having a default for   repr   which would act like   return   s  r      self   class    self   dict      would have been too dangerous  for example  too easy to get into infinite recursion if objects reference each other   So Python cops out  Note that there is one default which is true  if   repr   is defined  and   str   is not  the object will behave as though   str     repr     This means  in simple terms  almost every object you implement should have a functional   repr   that   s usable for understanding the object  Implementing   str   is optional  do that if you need a    pretty print    functionality  for example  used by a report generator    The goal of   repr   is to be unambiguous  Let me come right out and say it     I do not believe in debuggers  I don   t really know how to use any debugger  and have never used one seriously  Furthermore  I believe that the big fault in debuggers is their basic nature     most failures I debug happened a long long time ago  in a galaxy far far away  This means that I do believe  with religious fervor  in logging  Logging is the lifeblood of any decent fire-and-forget server system  Python makes it easy to log  with maybe some project specific wrappers  all you need is a  log INFO   I am in the weird function and a is   a   and b is   b   but I got a null C     using default   default c    But you have to do the last step     make sure every object you implement has a useful repr  so code like that can just work  This is why the    eval    thing comes up  if you have enough information so eval repr c    c  that means you know everything there is to know about c  If that   s easy enough  at least in a fuzzy way  do it  If not  make sure you have enough information about c anyway  I usually use an eval-like format   MyClass this  r that  r      self this self that   It does not mean that you can actually construct MyClass  or that those are the right constructor arguments     but it is a useful form to express    this is everything you need to know about this instance      Note  I used  r above  not  s  You always want to use repr    or  r formatting character  equivalently  inside   repr   implementation  or you   re defeating the goal of repr  You want to be able to differentiate MyClass 3  and MyClass  3     The goal of   str   is to be readable  Specifically  it is not intended to be unambiguous     notice that str 3   str  3    Likewise  if you implement an IP abstraction  having the str of it look like 192 168 1 1 is just fine  When implementing a date time abstraction  the str can be  2010 4 12 15 35 22   etc  The goal is to represent it in a way that a user  not a programmer  would want to read it  Chop off useless digits  pretend to be some other class     as long is it supports readability  it is an improvement   Container   s   str   uses contained objects      repr    This seems surprising  doesn   t it  It is a little  but how readable would it be if it used their   str      moshe is  3  hello world  this is a list  oh I don t know  containing just 4 elements    Not very  Specifically  the strings in a container would find it way too easy to disturb its string representation  In the face of ambiguity  remember  Python resists the temptation to guess  If you want the above behavior when you   re printing a list  just  print            join l           you can probably also figure out what to do about dictionaries   Summary  Implement   repr   for any class you implement  This should be second nature  Implement   str   if you think it would be useful to have a string version which errs on the side of readability

User · Answer

Excellent answers already cover the difference between   str   and   repr    which for me boils down to the former being readable even by an end user  and the latter being as useful as possible to developers  Given that  I find that the default implementation of   repr   often fails to achieve this goal because it omits information useful to developers   For this reason  if I have a simple enough   str    I generally just try to get the best of both worlds with something like   def   repr   self       return   0    1    format object   repr   self   str self

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repr    representation of python object usually eval will convert it back to that object    str    is whatever you think is that object in text form  e g    gt  gt  gt  s    w o w     gt  gt  gt  repr s     w    o w     gt  gt  gt  str s   w  o w   gt  gt  gt  eval str s    s Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt    File   lt string gt    line 1     w o w          SyntaxError  EOL while scanning single-quoted string  gt  gt  gt  eval repr s    s True

User · Answer

Understand   str   and   repr   intuitively and permanently distinguish them at all     str   return the string disguised body of a given object for readable of eyes   repr   return the real flesh body of a given object  return itself  for unambiguity to identify   See it in an example  In  30   str datetime datetime now    Out 30    2017-12-07 15 41 14 002752  Disguised in string form   As to   repr    In  32   datetime datetime now   Out 32   datetime datetime 2017  12  7  15  43  27  297769  Presence in real body which allows to be manipulated directly    We can do arithmetic operation on   repr   results conveniently   In  33   datetime datetime now   Out 33   datetime datetime 2017  12  7  15  47  9  741521  In  34   datetime datetime 2017  12  7  15  47  9  741521  - datetime datetime 2          017  12  7  15  43  27  297769  Out 34   datetime timedelta 0  222  443752    if apply the operation on   str    In  35    2017-12-07 15 43 14 002752  -  2017-12-07 15 41 14 002752  TypeError  unsupported operand type s  for -   str  and  str    Returns nothing but error   Another example   In  36   str  string body   Out 36    string body    in string form  In  37   repr  real body   Out 37     real body    its real body hide inside   Hope this help you build concrete grounds to explore more answers

User · Answer

In all honesty  eval repr obj   is never used  If you find yourself using it  you should stop  because eval is dangerous  and strings are a very inefficient way to serialize your objects  use pickle instead     Therefore  I would recommend setting   repr       str    The reason is that str list  calls repr on the elements  I consider this to be one of the biggest design flaws of Python that was not addressed by Python 3   An actual repr will probably not be very helpful as the output of print  your  objects     To qualify this  in my experience  the most useful use case of the repr function is to put a string inside another string  using string formatting   This way  you don t have to worry about escaping quotes or anything  But note that there is no eval happening here

User · Answer

My rule of thumb     repr   is for developers    str   is for customers

User · Answer

gt  gt  gt  print decimal Decimal 23    decimal Decimal  1 05    21 90476190476190476190476190  gt  gt  gt  decimal Decimal 23    decimal Decimal  1 05   Decimal  21 90476190476190476190476190     When print   is called on the result of decimal Decimal 23    decimal Decimal  1 05   the raw number is printed  this output is in string form which can be achieved with   str      If we simply enter the expression we get a decimal Decimal output     this output is in representational form which can be achieved with   repr      All Python objects have two output forms  String form is designed to be human-readable  The representational form is designed to produce output that if fed to a Python interpreter would  when possible  reproduce the represented object

User · Answer

One important thing to keep in mind is that container s   str   uses contained objects    repr       gt  gt  gt  from datetime import datetime  gt  gt  gt  from decimal import Decimal  gt  gt  gt  print  Decimal  52    datetime now     Decimal  52    datetime datetime 2015  11  16  10  51  26  185000    gt  gt  gt  str  Decimal  52    datetime now       Decimal  52    datetime datetime 2015  11  16  10  52  22  176000      Python favors unambiguity over readability  the   str   call of a tuple calls the contained objects    repr    the  formal  representation of an object  Although the formal representation is harder to read than an informal one  it is unambiguous and more robust against bugs

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One aspect that is missing in other answers  It s true that in general the pattern is    Goal of   str    human-readable Goal of   repr    unambiguous  possibly machine-readable via eval   Unfortunately  this differentiation is flawed  because the Python REPL and also IPython use   repr   for printing objects in a REPL console  see related questions for Python and IPython   Thus  projects which are targeted for interactive console work  e g   Numpy or Pandas  have started to ignore above rules and provide a human-readable   repr   implementation instead

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str   can be invoked on an object by calling str obj  and should return a human readable string      repr   can be invoked on an object by calling repr obj  and should return internal object  object fields attributes   This example may help   class C1 pass  class C2              def   str   self           return str f  self   class     name    class str     class C3              def   repr   self                    return str f  self   class     name    class repr    class C4              def   str   self           return str f  self   class     name    class str        def   repr   self                    return str f  self   class     name    class repr     ci1   C1       ci2   C2     ci3   C3     ci4   C4    print ci1          lt   main   C1 object at 0x0000024C44A80C18 gt  print str ci1      lt   main   C1 object at 0x0000024C44A80C18 gt  print repr ci1     lt   main   C1 object at 0x0000024C44A80C18 gt  print ci2         C2 class str print str ci2     C2 class str print repr ci2     lt   main   C2 object at 0x0000024C44AE12E8 gt  print ci3         C3 class repr print str ci3     C3 class repr print repr ci3    C3 class repr print ci4         C4 class str  print str ci4     C4 class str  print repr ci4    C4 class repr

User · Answer

repr   is used everywhere  except by print and str methods  when a   str  is defined

User · Answer

Apart from all the answers given  I would like to add few points  -  1    repr     is invoked when you simply write object s name on interactive python console and press enter   2    str     is invoked when you use object with print statement   3  In case  if   str   is missing  then print and any function using str   invokes   repr     of object   4    str     of containers  when invoked will execute   repr     method of its contained elements   5  str   called within   str     could potentially recurse without a base case  and error on maximum recursion depth   6    repr     can call repr   which will attempt to avoid infinite recursion automatically  replacing an already represented object with

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From an  An Unofficial  Python Reference Wiki  archive copy  by effbot     str    computes the  informal  string representation of an object  This differs from   repr   in that it does not have to be a valid Python expression  a more convenient or concise representation may be used instead

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Unless you specifically act to ensure otherwise  most classes don t have helpful results for either    gt  gt  gt  class Sic object   pass       gt  gt  gt  print str Sic     lt   main   Sic object at 0x8b7d0 gt   gt  gt  gt  print repr Sic     lt   main   Sic object at 0x8b7d0 gt   gt  gt  gt     As you see -- no difference  and no info beyond the class and object s id   If you only override one of the two       gt  gt  gt  class Sic object          def   repr   object   return  foo        gt  gt  gt  print str Sic    foo  gt  gt  gt  print repr Sic    foo  gt  gt  gt  class Sic object         def   str   object   return  foo        gt  gt  gt  print str Sic    foo  gt  gt  gt  print repr Sic     lt   main   Sic object at 0x2617f0 gt   gt  gt  gt     as you see  if you override   repr    that s ALSO used for   str    but not vice versa   Other crucial tidbits to know    str   on a built-on container uses the   repr    NOT the   str    for the items it contains  And  despite the words on the subject found in typical docs  hardly anybody bothers making the   repr   of objects be a string that eval may use to build an equal object  it s just too hard  AND not knowing how the relevant module was actually imported makes it actually flat out impossible    So  my advice  focus on making   str   reasonably human-readable  and   repr   as unambiguous as you possibly can  even if that interferes with the fuzzy unattainable goal of making   repr   s returned value acceptable as input to   eval

User · Answer

What is the difference between   str   and   repr   in Python      str    read as  dunder  double-underscore  string   and   repr    read as  dunder-repper   for  representation    are both special methods that return strings based on the state of the object      repr   provides backup behavior if   str   is missing    So one should first write a   repr   that allows you to reinstantiate an equivalent object from the string it returns e g  using eval or by typing it in character-for-character in a Python shell    At any time later  one can write a   str   for a user-readable string representation of the instance  when one believes it to be necessary     str    If you print an object  or pass it to format  str format  or str  then if a   str   method is defined  that method will be called  otherwise    repr   will be used      repr    The   repr   method is called by the builtin function repr and is what is echoed on your python shell when it evaluates an expression that returns an object    Since it provides a backup for   str    if you can only write one  start with   repr    Here s the builtin help on repr   repr          repr object  - gt  string      Return the canonical string representation of the object      For most object types  eval repr object      object    That is  for most objects  if you type in what is printed by repr  you should be able to create an equivalent object  But this is not the default implementation   Default Implementation of   repr    The default object   repr   is  C Python source  something like   def   repr   self       return   lt  0   1  object at  2  gt   format        self   module    type self    name    hex id self      That means by default you ll print the module the object is from  the class name  and the hexadecimal representation of its location in memory - for example    lt   main   Foo object at 0x7f80665abdd0 gt    This information isn t very useful  but there s no way to derive how one might accurately create a canonical representation of any given instance  and it s better than nothing  at least telling us how we might uniquely identify it in memory   How can   repr   be useful   Let s look at how useful it can be  using the Python shell and datetime objects  First we need to import the datetime module   import datetime   If we call datetime now in the shell  we ll see everything we need to recreate an equivalent datetime object  This is created by the datetime   repr      gt  gt  gt  datetime datetime now   datetime datetime 2015  1  24  20  5  36  491180    If we print a datetime object  we see a nice human readable  in fact  ISO  format  This is implemented by datetime s   str      gt  gt  gt  print datetime datetime now    2015-01-24 20 05 44 977951   It is a simple matter to recreate the object we lost because we didn t assign it to a variable by copying and pasting from the   repr   output  and then printing it  and we get it in the same human readable output as the other object    gt  gt  gt  the past   datetime datetime 2015  1  24  20  5  36  491180   gt  gt  gt  print the past  2015-01-24 20 05 36 491180   How do I implement them   As you re developing  you ll want to be able to reproduce objects in the same state  if possible  This  for example  is how the datetime object defines   repr    Python source   It is fairly complex  because of all of the attributes needed to reproduce such an object   def   repr   self          Convert to formal string  for repr           L    self  year  self  month  self  day     These are never zero          self  hour  self  minute  self  second  self  microsecond      if L -1     0          del L -1      if L -1     0          del L -1      s     s  s  s      self   class     module                           self   class     qualname                                join map str  L        if self  tzinfo is not None          assert s -1                  s   s  -1       tzinfo  r    self  tzinfo           if self  fold          assert s -1                  s   s  -1       fold 1       return s   If you want your object to have a more human readable representation  you can implement   str   next  Here s how the datetime object  Python source  implements   str    which it easily does because it already has a function to display it in ISO format   def   str   self        Convert to string  for str         return self isoformat sep        Set   repr       str     This is a critique of another answer here that suggests setting   repr       str     Setting   repr       str   is silly -   repr   is a fallback for   str   and a   repr    written for developers usage in debugging  should be written before you write a   str     You need a   str   only when you need a textual representation of the object   Conclusion  Define   repr   for objects you write so you and other developers have a reproducible example when using it as you develop  Define   str   when you need a human readable string representation of it

User · Answer

Every object inherits   repr    from the base class that all objects created  class Person       pass  p Person    if you call repr p  you will get this as default    lt   main   Person object at 0x7fb2604f03a0 gt   But if you call str p  you will get the same output  it is because when   str   does not exist  Python calls   repr   Let s implement our own   str   class Person      def   init   self name age           self name name         self age age     def   repr   self           print  quot   repr   called quot           return f quot Person name   self name   age  self age   quot   p Person  quot ali quot  20   print p  and str p will return    repr   called      Person name  ali  age 20   let s add   str     class Person      def   init   self  name  age           self name   name         self age   age              def   repr   self           print    repr   called           return f quot Person name   self name   age self age   quot           def   str   self           print    str   called           return self name  p Person  quot ali quot  20   if we call print p  and str p   it will call   str     so it will return   str   called ali  repr p  will return repr called  quot Person name  ali  age self age   quot  Let s omit   repr   and just implement   str    class Person  def   init   self  name  age       self name   name     self age   age  def   str   self       print    str   called       return self name  p Person  ali  20   print p  will look for the   str   and will return    str   called ali  NOTE  if we had   repr   and   str   defined  f name is  p   would call   str

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To put it simply     str   is used in to show a string representation of your object to be read easily by others     repr   is used to show a string representation of the object   Let s say I want to create a Fraction class where the string representation of a fraction is   1 2   and the object  Fraction class  is to be represented as  Fraction  1 2    So we can create a simple Fraction class   class Fraction      def   init   self  num  den           self   num   num         self   den   den      def   str   self           return       str self   num          str self   den             def   repr   self           return  Fraction      str self   num          str self   den           f   Fraction 1 2  print  I want to represent the Fraction STRING as     str f      1 2  print  I want to represent the Fraction OBJECT as    repr f     Fraction  1 2

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str - Creates a new string object from the given object   repr - Returns the canonical string representation of the object   The differences   str      makes object readable generates output for end-user   repr      needs code that reproduces object generates output for developer

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In short  the goal of   repr   is to be unambiguous and   str   is to be   readable    Here is a good example    gt  gt  gt  import datetime  gt  gt  gt  today   datetime datetime now    gt  gt  gt  str today   2012-03-14 09 21 58 130922   gt  gt  gt  repr today   datetime datetime 2012  3  14  9  21  58  130922     Read this documentation for repr      repr object       Return a string containing a printable representation of an object  This is the same value yielded by conversions  reverse   quotes   It is sometimes useful to be able to access this operation as   an ordinary function  For many types  this function makes an attempt   to return a string that would yield an object with the same value when   passed to eval    otherwise the representation is a string enclosed in   angle brackets that contains the name of the type of the object   together with additional information often including the name and   address of the object  A class can control what this function returns   for its instances by defining a   repr     method    Here is the documentation for str      str object          Return a string containing a nicely printable   representation of an object  For strings  this returns the string   itself  The difference with repr object  is that str object  does not   always attempt to return a string that is acceptable to eval    its   goal is to return a printable string  If no argument is given  returns   the empty string

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In a nutshell   class Demo    def   repr   self       return  repr    def   str   self       return  str   demo   Demo   print demo    use   str    output  str  to stdout  s   str demo      str   is used  return  str  r   repr demo      repr   is used  return  repr   import logging logger   logging getLogger logging INFO  logger info demo    use   str    output  str  to stdout  from pprint import pprint  pformat pprint demo    use   repr    output  repr  to stdout result   pformat demo    use   repr    result is string which value is  str

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str   must return string object whereas   repr   can return any python expression  If   str   implementation is missing then   repr   function is used as fallback  There is no fallback if   repr   function implementation is missing  If   repr   function is returning String representation of the object  we can skip implementation of   str   function    Source  https   www journaldev com 22460 python-str-repr-functions

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On page 358 of the book Python scripting for computational science by Hans Petter Langtangen  it clearly states that    The   repr   aims at a complete string representation of the object  The   str   is to return a nice string for printing    So  I prefer to understand them as   repr   reproduce str   string  representation    from the user s point of view although this is a misunderstanding I made when learning python   A small but good example is also given on the same page as follows   Example  In  38   str  s   Out 38    s   In  39   repr  s   Out 39     s    In  40   eval str  s    Traceback  most recent call last      File   lt ipython-input-40-abd46c0c43e7 gt    line 1  in  lt module gt      eval str  s       File   lt string gt    line 1  in  lt module gt   NameError  name  s  is not defined   In  41   eval repr  s    Out 41    s

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From the book Fluent Python      A basic requirement for a Python object is to provide usable         string   representations of itself  one used for debugging and        logging  another for presentation to end users  That is why the        special methods   repr   and   str   exist in the data model

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Q  What s the difference between   str     and   repr      TL DR   LONG This question has been around a long time  and there are a variety of answers of which most are correct  not to mention from several Python community legends      However when it comes down to the nitty-gritty  this question is analogous to asking for the difference between the str   and repr   built-in functions  I m going to describe the differences in my own words  which means I may be  quot borrowing quot  liberally from Core Python Programming so pls forgive me   Both str   and repr   have the same basic job  their goal is to return a string representation of a Python object  What kind of string representation is what differentiates them   str    amp    str     return a printable string representation of an object    something human-readable for human consumption repr    amp    repr     return a string representation of an object that is a valid Python object  something you can pass to eval   or type into the Python shell without getting an error   For example  let s assign a string to x and an int to y   gt  gt  gt  x  y    foo   123  gt  gt  gt  str x   str y    foo    123    Can we take what s inside the quotes and enter them verbatim into the Python interpreter   gt  gt  gt  123 123  gt  gt  gt  foo Traceback  most recent call last     File  quot  lt stdin gt  quot   line 1  in  lt module gt  NameError  name  foo  is not defined  Clearly you can for an int but not necessarily for a str  While I can pass  123  to eval    that doesn t work for  foo    gt  gt  gt  eval  123   123  gt  gt  gt  eval  foo   Traceback  most recent call last     File  quot  lt stdin gt  quot   line 1  in  lt module gt    File  quot  lt string gt  quot   line 1  in  lt module gt  NameError  name  foo  is not defined  Now let s try repr    again  dump what s in the pair of quotes for each string   gt  gt  gt  repr x   repr y    quot  foo  quot    123    gt  gt  gt  123 123  gt  gt  gt   foo   foo   Wow  they both work  That s because  foo   while a printable string representation of that string  it s not evaluatable  but  quot  foo  quot  is  123 is a valid Python int called by either str   or repr    What happens when we call eval   with these   gt  gt  gt  eval  123   123  gt  gt  gt  eval  quot  foo  quot    foo   It works because 123 and  foo  are valid Python objects  Another key takeaway is that while sometimes both return the same thing  the same string representation   that s not always the case   And yes  yes  I can go create a variable foo where the eval   works  but that s not the point   More factoids about both pairs  Sometimes  str   and repr   are called implicitly  meaning they re called on behalf of users  when users execute print  Py1 Py2  or call print    Py3    even if users don t call str   explicitly  such a call is made on their behalf before the object is displayed  In the Python shell  interactive interpreter   if you enter a variable at the  gt  gt  gt  prompt and press RETURN  the interpreter displays the results of repr   implicitly called on that object  To connect str   and repr   to   str     and   repr      realize that calls to the built-in functions  i e   str x  or repr y  result in calling their object s corresponding special methods  x   str     or y   repr     By implementing   str     and   repr     for your Python classes  you overload the built-in functions  str   and repr     allowing instances of your classes to be passed in to str   and repr    When such calls are made  they turn around and call the class    str     and   repr      per  3

[python] What is the difference between str and repr?

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[python] What is the difference between str and repr?