Understanding Python's “is” operator

Question

The is operator does not match the values of the variables, but the instances themselves.

What does it really mean?

Answer 1

x is y is same as id(x) == id(y), comparing identity of objects.

As @tomasz-kurgan pointed out in the comment below is operator behaves unusually with certain objects.

E.g.

>>> class A(object):
...   def foo(self):
...     pass
... 
>>> a = A()
>>> a.foo is a.foo
False
>>> id(a.foo) == id(a.foo)
True

Ref;
https://docs.python.org/2/reference/expressions.html#is-not
https://docs.python.org/2/reference/expressions.html#id24