Elegant ways to support equivalence equality in Python classes

Question

When writing custom classes it is often important to allow equivalence by means of the    and    operators  In Python  this is made possible by implementing the   eq   and   ne   special methods  respectively  The easiest way I ve found to do this is the following method   class Foo      def   init   self  item           self item   item      def   eq   self  other           if isinstance other  self   class                 return self   dict      other   dict           else              return False      def   ne   self  other           return not self   eq   other    Do you know of more elegant means of doing this  Do you know of any particular disadvantages to using the above method of comparing   dict  s   Note  A bit of clarification--when   eq   and   ne   are undefined  you ll find this behavior    gt  gt  gt  a   Foo 1   gt  gt  gt  b   Foo 1   gt  gt  gt  a is b False  gt  gt  gt  a    b False   That is  a    b evaluates to False because it really runs a is b  a test of identity  i e    Is a the same object as b      When   eq   and   ne   are defined  you ll find this behavior  which is the one we re after     gt  gt  gt  a   Foo 1   gt  gt  gt  b   Foo 1   gt  gt  gt  a is b False  gt  gt  gt  a    b True

User · Answer

You need to be careful with inheritance    gt  gt  gt  class Foo      def   eq   self  other           if isinstance other  self   class                 return self   dict      other   dict           else              return False   gt  gt  gt  class Bar Foo  pass   gt  gt  gt  b   Bar    gt  gt  gt  f   Foo    gt  gt  gt  f    b True  gt  gt  gt  b    f False   Check types more strictly  like this   def   eq   self  other       if type other  is type self           return self   dict      other   dict       return False   Besides that  your approach will work fine  that s what special methods are there for

User · Answer

You don t have to override both   eq   and   ne   you can override only   cmp   but this will make an implication on the result of           lt      and so on   is tests for object identity  This means a is b will be True in the case when a and b both hold the reference to the same object  In python you always hold a reference to an object in a variable not the actual object  so essentially for a is b to be true the objects in them should be located in the same memory location  How and most importantly why would you go about overriding this behaviour   Edit  I didn t know   cmp   was removed from python 3 so avoid it

User · Answer

You don t have to override both   eq   and   ne   you can override only   cmp   but this will make an implication on the result of           lt      and so on   is tests for object identity  This means a is b will be True in the case when a and b both hold the reference to the same object  In python you always hold a reference to an object in a variable not the actual object  so essentially for a is b to be true the objects in them should be located in the same memory location  How and most importantly why would you go about overriding this behaviour   Edit  I didn t know   cmp   was removed from python 3 so avoid it

User · Answer

Instead of using subclassing mixins  I like to use a generic class decorator  def comparable cls           Class decorator providing generic comparison functionality          def   eq   self  other           return isinstance other  self   class    and self   dict      other   dict        def   ne   self  other           return not self   eq   other       cls   eq       eq       cls   ne       ne       return cls   Usage    comparable class Number object       def   init   self  x           self x   x  a   Number 1  b   Number 1  assert a    b

User · Answer

You need to be careful with inheritance    gt  gt  gt  class Foo      def   eq   self  other           if isinstance other  self   class                 return self   dict      other   dict           else              return False   gt  gt  gt  class Bar Foo  pass   gt  gt  gt  b   Bar    gt  gt  gt  f   Foo    gt  gt  gt  f    b True  gt  gt  gt  b    f False   Check types more strictly  like this   def   eq   self  other       if type other  is type self           return self   dict      other   dict       return False   Besides that  your approach will work fine  that s what special methods are there for

User · Answer

I think that the two terms you re looking for are equality      and identity  is   For example    gt  gt  gt  a    1 2 3   gt  gt  gt  b    1 2 3   gt  gt  gt  a    b True        lt -- a and b have values which are equal  gt  gt  gt  a is b False       lt -- a and b are not the same list object

User · Answer

This incorporates the comments on Algorias  answer  and compares objects by a single attribute because I don t care about the whole dict  hasattr other   id   must be true  but I know it is because I set it in the constructor   def   eq   self  other       if other is self          return True      if type other  is not type self             delegate to superclass         return NotImplemented      return other id    self id

User · Answer

Instead of using subclassing mixins  I like to use a generic class decorator  def comparable cls           Class decorator providing generic comparison functionality          def   eq   self  other           return isinstance other  self   class    and self   dict      other   dict        def   ne   self  other           return not self   eq   other       cls   eq       eq       cls   ne       ne       return cls   Usage    comparable class Number object       def   init   self  x           self x   x  a   Number 1  b   Number 1  assert a    b

User · Answer

You need to be careful with inheritance    gt  gt  gt  class Foo      def   eq   self  other           if isinstance other  self   class                 return self   dict      other   dict           else              return False   gt  gt  gt  class Bar Foo  pass   gt  gt  gt  b   Bar    gt  gt  gt  f   Foo    gt  gt  gt  f    b True  gt  gt  gt  b    f False   Check types more strictly  like this   def   eq   self  other       if type other  is type self           return self   dict      other   dict       return False   Besides that  your approach will work fine  that s what special methods are there for

User · Answer

Not a direct answer but seemed relevant enough to be tacked on as it saves a bit of verbose tedium on occasion  Cut straight from the docs       functools total ordering cls   Given a class defining one or more rich comparison ordering methods  this class decorator supplies the rest  This simplifies the effort involved in specifying all of the possible rich comparison operations   The class must define one of   lt        le        gt      or   ge      In addition  the class should supply an   eq     method   New in version 2 7   total ordering class Student      def   eq   self  other           return   self lastname lower    self firstname lower                        other lastname lower    other firstname lower         def   lt   self  other           return   self lastname lower    self firstname lower     lt                   other lastname lower    other firstname lower

User · Answer

I think that the two terms you re looking for are equality      and identity  is   For example    gt  gt  gt  a    1 2 3   gt  gt  gt  b    1 2 3   gt  gt  gt  a    b True        lt -- a and b have values which are equal  gt  gt  gt  a is b False       lt -- a and b are not the same list object

User · Answer

Consider this simple problem   class Number       def   init   self  number           self number   number   n1   Number 1  n2   Number 1   n1    n2   False -- oops   So  Python by default uses the object identifiers for comparison operations   id n1    140400634555856 id n2    140400634555920   Overriding the   eq   function seems to solve the problem   def   eq   self  other          Overrides the default implementation        if isinstance other  Number           return self number    other number     return False   n1    n2   True n1    n2   True in Python 2 -- oops  False in Python 3   In Python 2  always remember to override the   ne   function as well  as the documentation states      There are no implied relationships among the comparison operators  The   truth of x  y does not imply that x  y is false  Accordingly  when   defining   eq      one should also define   ne     so that the   operators will behave as expected    def   ne   self  other          Overrides the default implementation  unnecessary in Python 3         return not self   eq   other    n1    n2   True n1    n2   False   In Python 3  this is no longer necessary  as the documentation states      By default    ne     delegates to   eq     and inverts the result   unless it is NotImplemented  There are no other implied   relationships among the comparison operators  for example  the truth   of  x lt y or x  y  does not imply x lt  y    But that does not solve all our problems  Let   s add a subclass   class SubNumber Number       pass   n3   SubNumber 1   n1    n3   False for classic-style classes -- oops  True for new-style classes n3    n1   True n1    n3   True for classic-style classes -- oops  False for new-style classes n3    n1   False   Note  Python 2 has two kinds of classes    classic-style  or old-style  classes  that do not inherit from object and that are declared as class A   class A    or class A B   where B is a classic-style class  new-style classes  that do inherit from object and that are declared as class A object  or class A B   where B is a new-style class  Python 3 has only new-style classes that are declared as class A   class A object   or class A B       For classic-style classes  a comparison operation always calls the method of the first operand  while for new-style classes  it always calls the method of the subclass operand  regardless of the order of the operands    So here  if Number is a classic-style class    n1    n3 calls n1   eq    n3    n1 calls n3   eq    n1    n3 calls n1   ne    n3    n1 calls n3   ne      And if Number is a new-style class    both n1    n3 and n3    n1 call n3   eq    both n1    n3 and n3    n1 call n3   ne      To fix the non-commutativity issue of the    and    operators for Python 2 classic-style classes  the   eq   and   ne   methods should return the NotImplemented value when an operand type is not supported  The documentation defines the NotImplemented value as      Numeric methods and rich comparison methods may return this value if   they do not implement the operation for the operands provided   The   interpreter will then try the reflected operation  or some other   fallback  depending on the operator   Its truth value is true    In this case the operator delegates the comparison operation to the reflected method of the other operand  The documentation defines reflected methods as      There are no swapped-argument versions of these methods  to be used   when the left argument does not support the operation but the right   argument does   rather    lt     and   gt     are each other   s   reflection    le     and   ge     are each other   s reflection  and     eq     and   ne     are their own reflection    The result looks like this   def   eq   self  other          Overrides the default implementation        if isinstance other  Number           return self number    other number     return NotImplemented  def   ne   self  other          Overrides the default implementation  unnecessary in Python 3         x   self   eq   other      if x is NotImplemented          return NotImplemented     return not x   Returning the NotImplemented value instead of False is the right thing to do even for new-style classes if commutativity of the    and    operators is desired when the operands are of unrelated types  no inheritance    Are we there yet  Not quite  How many unique numbers do we have   len set  n1  n2  n3      3 -- oops   Sets use the hashes of objects  and by default Python returns the hash of the identifier of the object  Let   s try to override it   def   hash   self          Overrides the default implementation        return hash tuple sorted self   dict   items       len set  n1  n2  n3      1   The end result looks like this  I added some assertions at the end for validation    class Number       def   init   self  number           self number   number      def   eq   self  other              Overrides the default implementation            if isinstance other  Number               return self number    other number         return NotImplemented      def   ne   self  other              Overrides the default implementation  unnecessary in Python 3             x   self   eq   other          if x is not NotImplemented              return not x         return NotImplemented      def   hash   self              Overrides the default implementation            return hash tuple sorted self   dict   items        class SubNumber Number       pass   n1   Number 1  n2   Number 1  n3   SubNumber 1  n4   SubNumber 4   assert n1    n2 assert n2    n1 assert not n1    n2 assert not n2    n1  assert n1    n3 assert n3    n1 assert not n1    n3 assert not n3    n1  assert not n1    n4 assert not n4    n1 assert n1    n4 assert n4    n1  assert len set  n1  n2  n3         1 assert len set  n1  n2  n3  n4       2

User · Answer

The way you describe is the way I ve always done it  Since it s totally generic  you can always break that functionality out into a mixin class and inherit it in classes where you want that functionality   class CommonEqualityMixin object        def   eq   self  other           return  isinstance other  self   class                and self   dict      other   dict         def   ne   self  other           return not self   eq   other   class Foo CommonEqualityMixin        def   init   self  item           self item   item

User · Answer

The  is  test will test for identity using the builtin  id    function which essentially returns the memory address of the object and therefore isn t overloadable   However in the case of testing the equality of a class you probably want to be a little bit more strict about your tests and only compare the data attributes in your class   import types  class ComparesNicely object        def   eq   self  other           for key  value in self   dict   iteritems                if  isinstance value  types FunctionType  or                      key startswith                         continue              if key not in other   dict                    return False              if other   dict   key     value                  return False           return True   This code will only compare non function data members of your class as well as skipping anything private which is generally what you want  In the case of Plain Old Python Objects I have a base class which implements   init      str      repr   and   eq   so my POPO objects don t carry the burden of all that extra  and in most cases identical  logic

User · Answer

Consider this simple problem   class Number       def   init   self  number           self number   number   n1   Number 1  n2   Number 1   n1    n2   False -- oops   So  Python by default uses the object identifiers for comparison operations   id n1    140400634555856 id n2    140400634555920   Overriding the   eq   function seems to solve the problem   def   eq   self  other          Overrides the default implementation        if isinstance other  Number           return self number    other number     return False   n1    n2   True n1    n2   True in Python 2 -- oops  False in Python 3   In Python 2  always remember to override the   ne   function as well  as the documentation states      There are no implied relationships among the comparison operators  The   truth of x  y does not imply that x  y is false  Accordingly  when   defining   eq      one should also define   ne     so that the   operators will behave as expected    def   ne   self  other          Overrides the default implementation  unnecessary in Python 3         return not self   eq   other    n1    n2   True n1    n2   False   In Python 3  this is no longer necessary  as the documentation states      By default    ne     delegates to   eq     and inverts the result   unless it is NotImplemented  There are no other implied   relationships among the comparison operators  for example  the truth   of  x lt y or x  y  does not imply x lt  y    But that does not solve all our problems  Let   s add a subclass   class SubNumber Number       pass   n3   SubNumber 1   n1    n3   False for classic-style classes -- oops  True for new-style classes n3    n1   True n1    n3   True for classic-style classes -- oops  False for new-style classes n3    n1   False   Note  Python 2 has two kinds of classes    classic-style  or old-style  classes  that do not inherit from object and that are declared as class A   class A    or class A B   where B is a classic-style class  new-style classes  that do inherit from object and that are declared as class A object  or class A B   where B is a new-style class  Python 3 has only new-style classes that are declared as class A   class A object   or class A B       For classic-style classes  a comparison operation always calls the method of the first operand  while for new-style classes  it always calls the method of the subclass operand  regardless of the order of the operands    So here  if Number is a classic-style class    n1    n3 calls n1   eq    n3    n1 calls n3   eq    n1    n3 calls n1   ne    n3    n1 calls n3   ne      And if Number is a new-style class    both n1    n3 and n3    n1 call n3   eq    both n1    n3 and n3    n1 call n3   ne      To fix the non-commutativity issue of the    and    operators for Python 2 classic-style classes  the   eq   and   ne   methods should return the NotImplemented value when an operand type is not supported  The documentation defines the NotImplemented value as      Numeric methods and rich comparison methods may return this value if   they do not implement the operation for the operands provided   The   interpreter will then try the reflected operation  or some other   fallback  depending on the operator   Its truth value is true    In this case the operator delegates the comparison operation to the reflected method of the other operand  The documentation defines reflected methods as      There are no swapped-argument versions of these methods  to be used   when the left argument does not support the operation but the right   argument does   rather    lt     and   gt     are each other   s   reflection    le     and   ge     are each other   s reflection  and     eq     and   ne     are their own reflection    The result looks like this   def   eq   self  other          Overrides the default implementation        if isinstance other  Number           return self number    other number     return NotImplemented  def   ne   self  other          Overrides the default implementation  unnecessary in Python 3         x   self   eq   other      if x is NotImplemented          return NotImplemented     return not x   Returning the NotImplemented value instead of False is the right thing to do even for new-style classes if commutativity of the    and    operators is desired when the operands are of unrelated types  no inheritance    Are we there yet  Not quite  How many unique numbers do we have   len set  n1  n2  n3      3 -- oops   Sets use the hashes of objects  and by default Python returns the hash of the identifier of the object  Let   s try to override it   def   hash   self          Overrides the default implementation        return hash tuple sorted self   dict   items       len set  n1  n2  n3      1   The end result looks like this  I added some assertions at the end for validation    class Number       def   init   self  number           self number   number      def   eq   self  other              Overrides the default implementation            if isinstance other  Number               return self number    other number         return NotImplemented      def   ne   self  other              Overrides the default implementation  unnecessary in Python 3             x   self   eq   other          if x is not NotImplemented              return not x         return NotImplemented      def   hash   self              Overrides the default implementation            return hash tuple sorted self   dict   items        class SubNumber Number       pass   n1   Number 1  n2   Number 1  n3   SubNumber 1  n4   SubNumber 4   assert n1    n2 assert n2    n1 assert not n1    n2 assert not n2    n1  assert n1    n3 assert n3    n1 assert not n1    n3 assert not n3    n1  assert not n1    n4 assert not n4    n1 assert n1    n4 assert n4    n1  assert len set  n1  n2  n3         1 assert len set  n1  n2  n3  n4       2

User · Answer

The way you describe is the way I ve always done it  Since it s totally generic  you can always break that functionality out into a mixin class and inherit it in classes where you want that functionality   class CommonEqualityMixin object        def   eq   self  other           return  isinstance other  self   class                and self   dict      other   dict         def   ne   self  other           return not self   eq   other   class Foo CommonEqualityMixin        def   init   self  item           self item   item

User · Answer

The way you describe is the way I ve always done it  Since it s totally generic  you can always break that functionality out into a mixin class and inherit it in classes where you want that functionality   class CommonEqualityMixin object        def   eq   self  other           return  isinstance other  self   class                and self   dict      other   dict         def   ne   self  other           return not self   eq   other   class Foo CommonEqualityMixin        def   init   self  item           self item   item

User · Answer

The  is  test will test for identity using the builtin  id    function which essentially returns the memory address of the object and therefore isn t overloadable   However in the case of testing the equality of a class you probably want to be a little bit more strict about your tests and only compare the data attributes in your class   import types  class ComparesNicely object        def   eq   self  other           for key  value in self   dict   iteritems                if  isinstance value  types FunctionType  or                      key startswith                         continue              if key not in other   dict                    return False              if other   dict   key     value                  return False           return True   This code will only compare non function data members of your class as well as skipping anything private which is generally what you want  In the case of Plain Old Python Objects I have a base class which implements   init      str      repr   and   eq   so my POPO objects don t carry the burden of all that extra  and in most cases identical  logic

User · Answer

This incorporates the comments on Algorias  answer  and compares objects by a single attribute because I don t care about the whole dict  hasattr other   id   must be true  but I know it is because I set it in the constructor   def   eq   self  other       if other is self          return True      if type other  is not type self             delegate to superclass         return NotImplemented      return other id    self id

User · Answer

The  is  test will test for identity using the builtin  id    function which essentially returns the memory address of the object and therefore isn t overloadable   However in the case of testing the equality of a class you probably want to be a little bit more strict about your tests and only compare the data attributes in your class   import types  class ComparesNicely object        def   eq   self  other           for key  value in self   dict   iteritems                if  isinstance value  types FunctionType  or                      key startswith                         continue              if key not in other   dict                    return False              if other   dict   key     value                  return False           return True   This code will only compare non function data members of your class as well as skipping anything private which is generally what you want  In the case of Plain Old Python Objects I have a base class which implements   init      str      repr   and   eq   so my POPO objects don t carry the burden of all that extra  and in most cases identical  logic

User · Answer

I think that the two terms you re looking for are equality      and identity  is   For example    gt  gt  gt  a    1 2 3   gt  gt  gt  b    1 2 3   gt  gt  gt  a    b True        lt -- a and b have values which are equal  gt  gt  gt  a is b False       lt -- a and b are not the same list object

User · Answer

I think that the two terms you re looking for are equality      and identity  is   For example    gt  gt  gt  a    1 2 3   gt  gt  gt  b    1 2 3   gt  gt  gt  a    b True        lt -- a and b have values which are equal  gt  gt  gt  a is b False       lt -- a and b are not the same list object

User · Answer

You need to be careful with inheritance    gt  gt  gt  class Foo      def   eq   self  other           if isinstance other  self   class                 return self   dict      other   dict           else              return False   gt  gt  gt  class Bar Foo  pass   gt  gt  gt  b   Bar    gt  gt  gt  f   Foo    gt  gt  gt  f    b True  gt  gt  gt  b    f False   Check types more strictly  like this   def   eq   self  other       if type other  is type self           return self   dict      other   dict       return False   Besides that  your approach will work fine  that s what special methods are there for

User · Answer

The  is  test will test for identity using the builtin  id    function which essentially returns the memory address of the object and therefore isn t overloadable   However in the case of testing the equality of a class you probably want to be a little bit more strict about your tests and only compare the data attributes in your class   import types  class ComparesNicely object        def   eq   self  other           for key  value in self   dict   iteritems                if  isinstance value  types FunctionType  or                      key startswith                         continue              if key not in other   dict                    return False              if other   dict   key     value                  return False           return True   This code will only compare non function data members of your class as well as skipping anything private which is generally what you want  In the case of Plain Old Python Objects I have a base class which implements   init      str      repr   and   eq   so my POPO objects don t carry the burden of all that extra  and in most cases identical  logic

User · Answer

From this answer  https   stackoverflow com a 30676267 541136 I have demonstrated that  while it s correct to define   ne   in terms   eq   - instead of   def   ne   self  other       return not self   eq   other    you should use   def   ne   self  other       return not self    other

User · Answer

You don t have to override both   eq   and   ne   you can override only   cmp   but this will make an implication on the result of           lt      and so on   is tests for object identity  This means a is b will be True in the case when a and b both hold the reference to the same object  In python you always hold a reference to an object in a variable not the actual object  so essentially for a is b to be true the objects in them should be located in the same memory location  How and most importantly why would you go about overriding this behaviour   Edit  I didn t know   cmp   was removed from python 3 so avoid it

User · Answer

You don t have to override both   eq   and   ne   you can override only   cmp   but this will make an implication on the result of           lt      and so on   is tests for object identity  This means a is b will be True in the case when a and b both hold the reference to the same object  In python you always hold a reference to an object in a variable not the actual object  so essentially for a is b to be true the objects in them should be located in the same memory location  How and most importantly why would you go about overriding this behaviour   Edit  I didn t know   cmp   was removed from python 3 so avoid it

User · Answer

Not a direct answer but seemed relevant enough to be tacked on as it saves a bit of verbose tedium on occasion  Cut straight from the docs       functools total ordering cls   Given a class defining one or more rich comparison ordering methods  this class decorator supplies the rest  This simplifies the effort involved in specifying all of the possible rich comparison operations   The class must define one of   lt        le        gt      or   ge      In addition  the class should supply an   eq     method   New in version 2 7   total ordering class Student      def   eq   self  other           return   self lastname lower    self firstname lower                        other lastname lower    other firstname lower         def   lt   self  other           return   self lastname lower    self firstname lower     lt                   other lastname lower    other firstname lower

User · Answer

From this answer  https   stackoverflow com a 30676267 541136 I have demonstrated that  while it s correct to define   ne   in terms   eq   - instead of   def   ne   self  other       return not self   eq   other    you should use   def   ne   self  other       return not self    other

User · Answer

The way you describe is the way I ve always done it  Since it s totally generic  you can always break that functionality out into a mixin class and inherit it in classes where you want that functionality   class CommonEqualityMixin object        def   eq   self  other           return  isinstance other  self   class                and self   dict      other   dict         def   ne   self  other           return not self   eq   other   class Foo CommonEqualityMixin        def   init   self  item           self item   item

[python] Elegant ways to support equivalence ("equality") in Python classes

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