Is there any difference between “foo is None” and “foo == None”?

Is there any difference between:

if foo is None: pass

and

if foo == None: pass

The convention that I’ve seen in most Python code (and the code I myself write) is the former, but I recently came across code which uses the latter. None is an instance (and the only instance, IIRC) of NoneType, so it shouldn’t matter, right? Are there any circumstances in which it might?

Answer

is always returns True if it compares the same object instance

Whereas == is ultimately determined by the __eq__() method

i.e.

>>> class Foo(object):
       def __eq__(self, other):
           return True

>>> f = Foo()
>>> f == None
True
>>> f is None
False