a == b is false, but id(a) == id(b) is true?

Ran into the following:

>>> class A:
...     def __str__(self):
...             return "some A()"
... 
>>> class B(A):
...     def __str__(self):
...             return "some B()"
... 
>>> print A()
some A()
>开发者_如何转开发>> print B()
some B()
>>> A.__str__ == B.__str__
False # seems reasonable, since each method is an object
>>> id(A.__str__)==id(B.__str__)
True # what?!

What's going on here?

As the string id(A.__str__) == id(B.__str__) is evaluated, A.__str__ is created, its id taken, and then garbage collected. Then B.__str__ is created, and happens to end up at the exact same address that A.__str__ was at earlier, so it gets (in CPython) the same id.

Try assigning A.__str__ and B.__str__ to temporary variables and you'll see something different:

>>> f = A.__str__
>>> g = B.__str__
>>> id(f) == id(g)
False

For a simpler example of this phenomenon, try:

>>> id(float('3.0')) == id(float('4.0'))
True

The following works:

>>> id(A.__str__.im_func) == id(A.__str__.im_func)
True
>>> id(B.__str__.im_func) == id(A.__str__.im_func)
False

For those of us here attracted by your title, to determine whether a method was overridden:

class A:
    def __str__(self):
        return "some A()"

    def strWasOverridden(self):
        return A.__str__ != self.__str__