How is eq handled in Python and in what order

Question

Since Python does not provide left right versions of its comparison operators  how does it decide which function to call  class A object       def   eq   self  other           print  quot A   eq   called quot          return self value    other class B object       def   eq   self  other           print  quot B   eq   called quot          return self value    other   gt  gt  gt  a   A    gt  gt  gt  a value   3  gt  gt  gt  b   B    gt  gt  gt  b value   4  gt  gt  gt  a    b  quot A   eq   called quot   quot B   eq   called quot  False  This seems to call both   eq   functions  I am looking for the official decision tree

User · Answer

When Python2 x sees a    b  it tries the following    If type b  is a new-style class  and type b  is a subclass of type a   and type b  has overridden   eq    then the result is b   eq   a   If type a  has overridden   eq    that is  type a    eq   isn t object   eq     then the result is a   eq   b   If type b  has overridden   eq    then the result is b   eq   a   If none of the above are the case  Python repeats the process looking for   cmp    If it exists  the objects are equal iff it returns zero  As a final fallback  Python calls object   eq   a  b   which is True iff a and b are the same object    If any of the special methods return NotImplemented  Python acts as though the method didn t exist   Note that last step carefully  if neither a nor b overloads     then a    b is the same as a is b     From https   eev ee blog 2012 03 24 python-faq-equality

User · Answer

The a    b expression invokes A   eq    since it exists   Its code includes self value    other   Since int s don t know how to compare themselves to B s  Python tries invoking B   eq   to see if it knows how to compare itself to an int   If you amend your code to show what values are being compared   class A object       def   eq   self  other           print  A   eq   called   r     r       self  other           return self value    other class B object       def   eq   self  other           print  B   eq   called   r     r       self  other           return self value    other  a   A   a value   3 b   B   b value   4 a    b   it will print   A   eq   called   lt   main   A object at 0x013BA070 gt      lt   main   B object at 0x013BA090 gt    B   eq   called   lt   main   B object at 0x013BA090 gt     3

User · Answer

I m writing an updated answer for Python 3 to this question   How is   eq   handled in Python and in what order  a    b   It is generally understood  but not always the case  that a    b invokes a   eq   b   or type a    eq   a  b   Explicitly  the order of evaluation is   if b s type is a strict subclass  not the same type  of a s type and has an   eq    call it and return the value if the comparison is implemented  else  if a has   eq    call it and return it if the comparison is implemented  else  see if we didn t call b s   eq   and it has it  then call and return it if the comparison is implemented  else  finally  do the comparison for identity  the same comparison as is   We know if a comparison isn t implemented if the method returns NotImplemented   In Python 2  there was a   cmp   method that was looked for  but it was deprecated and removed in Python 3   Let s test the first check s behavior for ourselves by letting B subclass A  which shows that the accepted answer is wrong on this count  class A      value   3     def   eq   self  other           print  A   eq   called           return self value    other value  class B A       value   4     def   eq   self  other           print  B   eq   called           return self value    other value  a  b   A    B   a    b  which only prints B   eq   called before returning False  How do we know this full algorithm  The other answers here seem incomplete and out of date  so I m going to update the information and show you how how you could look this up for yourself  This is handled at the C level  We need to look at two different bits of code here - the default   eq   for objects of class object  and the code that looks up and calls the   eq   method regardless of whether it uses the default   eq   or a custom one  Default   eq   Looking   eq   up in the relevant C api docs shows us that   eq   is handled by tp richcompare - which in the  quot object quot  type definition in cpython Objects typeobject c is defined in object richcompare for case Py EQ       case Py EQ             Return NotImplemented instead of False  so if two            objects are compared  both get a chance at the            comparison   See issue  1393             res    self    other    Py True   Py NotImplemented          Py INCREF res           break   So here  if self    other we return True  else we return the NotImplemented object  This is the default behavior for any subclass of object that does not implement its own   eq   method  How   eq   gets called Then we find the C API docs  the PyObject RichCompare function  which calls do richcompare  Then we see that the tp richcompare function  created for the  quot object quot  C definition is called by do richcompare  so let s look at that a little more closely  The first check in this function is for the conditions the objects being compared   are not the same type  but the second s type is a subclass of the first s type  and the second s type has an   eq   method   then call the other s method with the arguments swapped  returning the value if implemented  If that method isn t implemented  we continue        if   Py IS TYPE v  Py TYPE w    amp  amp          PyType IsSubtype Py TYPE w   Py TYPE v    amp  amp           f   Py TYPE w - gt tp richcompare     NULL            checked reverse op   1          res     f  w  v   Py SwappedOp op            if  res    Py NotImplemented              return res          Py DECREF res    Next we see if we can lookup the   eq   method from the first type and call it  As long as the result is not NotImplemented  that is  it is implemented  we return it      if   f   Py TYPE v - gt tp richcompare     NULL            res     f  v  w  op           if  res    Py NotImplemented              return res          Py DECREF res    Else if we didn t try the other type s method and it s there  we then try it  and if the comparison is implemented  we return it      if   checked reverse op  amp  amp   f   Py TYPE w - gt tp richcompare     NULL            res     f  w  v   Py SwappedOp op            if  res    Py NotImplemented              return res          Py DECREF res          Finally  we get a fallback in case it isn t implemented for either one s type  The fallback checks for the identity of the object  that is  whether it is the same object at the same place in memory - this is the same check as for self is other         If neither object implements it  provide a sensible default        for    and     but raise an exception for ordering         switch  op        case Py EQ          res    v    w    Py True   Py False          break   Conclusion In a comparison  we respect the subclass implementation of comparison first  Then we attempt the comparison with the first object s implementation  then with the second s if it wasn t called  Finally we use a test for identity for comparison for equality

[python] How is eq handled in Python and in what order?

Examples related to python

Examples related to comparison

Examples related to user-defined

[python] How is __eq__ handled in Python and in what order?

Examples related to python

Examples related to comparison

Examples related to user-defined

[python] How is eq handled in Python and in what order?