Python List Class __contains__ Method Functionality

Question

Does the __contains__ method of a list class check whether an object itself is an element of a list, or does it check whether the list contains an element equivalent to the given parameter?

Could you give me an example to demonstrate?

Jakie

Answer 1

>>> 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.

Newsstand

No, Python performs the identity check first and then only checks the value if the identity check fails. See the example below. – Dulcle

Answer 2

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 equivalent to:

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

Dulcle

And a quote from the official documentation to back this up: "For container types such as list, tuple, set, frozenset, dict, or collections.deque, the expression x in y is equivalent to any(x is e or x == e for e in y)." – Longerich

Answer 3

It depends on the class how it does the check. For the builtin list it uses the == operator; otherwise you couldn't e.g. use 'something' in somelist safely.

To be more specific, it check if the item is equal to an item in the list - so internally it's most likely a hash(a) == hash(b) comparison; if the hashes are equal the objects itself are probably compared, too.

Lycurgus

It won't be just a check of hashes - hashes can be equal without the values being equal. – Kenon

This is wrong. I just defined a __eq__ method which has been (involuntarily) picked up by __contains__ or in. Therefore, it's most likely list implements a a.__eq__(b) comparison. – Winshell

Answer 4

It checks the value

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

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