How to print instances of a class using print

Question

I am learning the ropes in Python  When I try to print an object of class Foobar using the print   function  I get an output like this    lt   main   Foobar instance at 0x7ff2a18c gt    Is there a way I can set the printing behaviour  or the string representation  of a class and its objects  For instance  when I call print   on a class object  I would like to print its data members in a certain format  How to achieve this in Python   If you are familiar with C   classes  the above can be achieved for the standard ostream by adding a friend ostream amp  operator  lt  lt   ostream amp   const Foobar amp   method for the class

User · Answer

If you re in a situation like  Keith you could try  print a   dict     It goes against what I would consider good style but if you re just trying to debug then it should do what you want

User · Answer

A generic way that can be applied to any class without specific formatting could be done as follows   class Element      def   init   self  name  symbol  number           self name   name         self symbol   symbol         self number   number      def   str   self           return str self   class             str self   dict      And then   elem   Element  my name    some symbol   3  print elem    produces    main   Element    symbol    some symbol    name    my name    number   3

User · Answer

gt  gt  gt  class Test          def   repr   self               return  Test            def   str   self               return  member of Test        gt  gt  gt  t   Test    gt  gt  gt  t Test    gt  gt  gt  print t  member of Test   The   str   method is what happens when you print it  and the   repr   method is what happens when you use the repr   function  or when you look at it with the interactive prompt   If this isn t the most Pythonic method  I apologize  because I m still learning too - but it works   If no   str   method is given  Python will print the result of   repr   instead  If you define   str   but not   repr    Python will use what you see above as the   repr    but still use   str   for printing

User · Answer

A prettier version of response by  user394430  class Element      def   init   self  name  symbol  number           self name   name         self symbol   symbol         self number   number      def   str   self           return  str self   class        n     n  join            format item  self   dict   item   for item in self   dict      elem   Element  my name    some symbol   3  print elem    Produces visually nice list of the names and values    lt class    main   Element  gt  name   my name symbol   some symbol number   3   An even fancier version  thanks Ruud  sorts the items   def   str   self       return  str self   class        n      n  join  str item            str self   dict   item   for item in sorted self   dict

User · Answer

For Python 3   If the specific format isn t important  e g  for debugging  just inherit from the Printable class below   No need to write code for every object     Inspired by this answer  class Printable      def   repr   self           from pprint import pformat         return   lt     type self    name       gt      pformat vars self   indent 4  width 1     Example Usage class MyClass Printable       pass  my obj   MyClass   my obj msg    Hello  my obj number    46  print my obj

User · Answer

Just to add my two cents to  dbr s answer  following is an example of how to implement this sentence from the official documentation he s cited             to return a string that would yield an object with the same value when passed to eval             Given this class definition   class Test object       def   init   self  a  b           self  a   a         self  b   b      def   str   self           return  An instance of class Test with state  a  s b  s     self  a  self  b       def   repr   self           return  Test   s    s       self  a  self  b    Now  is easy to serialize instance of Test class   x   Test  hello    world   print  Human readable     str x  print  Object representation     repr x  print  y   eval repr x   print  Human readable     str y  print  Object representation     repr y  print   So  running last piece of code  we ll get   Human readable   An instance of class Test with state  a hello b world Object representation   Test  hello   world    Human readable   An instance of class Test with state  a hello b world Object representation   Test  hello   world     But  as I said in my last comment  more info is just here

User · Answer

As Chris Lutz mentioned  this is defined by the   repr   method in your class   From the documentation of repr        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    Given the following class Test   class Test      def   init   self  a  b           self a   a         self b   b      def   repr   self           return   lt Test a  s b  s gt      self a  self b       def   str   self           return  From str method of Test  a is  s  b is  s     self a  self b      it will act the following way in the Python shell    gt  gt  gt  t   Test 123  456   gt  gt  gt  t  lt Test a 123 b 456 gt   gt  gt  gt  print repr t   lt Test a 123 b 456 gt   gt  gt  gt  print t  From str method of Test  a is 123  b is 456  gt  gt  gt  print str t   From str method of Test  a is 123  b is 456   If no   str   method is defined  print t   or print str t    will use the result of   repr   instead  If no   repr   method is defined then the default is used  which is pretty much equivalent to    def   repr   self       return   lt  s instance at  s gt      self   class     name    id self

User · Answer

You need to use   repr    This is a standard function like   init    For example   class Foobar           This will create Foobar type object          def   init   self           print  Foobar object is created        def   repr   self           return  Type what do you want to see here    a   Foobar    print a

User · Answer

Simple  In the print  do  print foobar   dict     as long as the constructor is   init

User · Answer

There are already a lot of answers in this thread but none of them particularly helped me  I had to work it out myself  so I hope this one is a little more informative   You just have to make sure you have parentheses at the end of your class  e g   print class      Here s an example of code from a project I was working on   class Element      def   init   self  name  symbol  number           self name   name         self symbol   symbol         self number   number     def   str   self           return         nAtomic Number     n  format self name  self symbol  self number  class Hydrogen Element       def   init   self           super     init   name    Hydrogen   symbol    H   number    1     To print my Hydrogen class  I used the following   print Hydrogen      Please note  this will not work without the parentheses at the end of Hydrogen  They are necessary   Hope this helps  let me know if you have anymore questions

[python] How to print instances of a class using print()?

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