Python List Class __contains__ Method Functionality

It checks the value

>>> x = 8888
>>> y = 8888
>>> list1 = [x]
>>> print(id(x))
140452322647920
>>> print(id(y))
140452322648016
>>> y in list1
True

>>> a = [[]]
>>> b = []
>>> b in a
True
>>> b is a[0]
False

This proves that it is a value check (by default at least), not an identity check. Keep in mind though that a class can if desired override __contains__() to make it an identity check. But again, by default, no.

Python lists (and tuples) first check whether an object itself is an element of a list (using the is operator) and only if that is False then does it check whether the object is equal to an item in the list (using the == operator). You can see this by creating an object that is not equal to itself:

>>> class X:
...     def __eq__(self, other):
...             return False
... 
>>> x = X()
>>> x == x
False

However since x is x, __contains__ still recognises that this object is in a list

>>> x in [1, 'a', x, 7]
True

That is, a lists __contains__ method is roughly equivalent to:

def __contains__(self, other):
    return any(other is item or other == item for item in self)