Check if object is in list (not "by value", but by id)

Question

Consider the following code:

>>> class A(object):
...   def __init__(self, a):
...     self.a = a
...   def __eq__(self, other):
...     return self.a==other.a
... 
>>> a=A(1)
>>> b=A(1)
>>> c=A(2)

>>> a==b
True                   # because __eq__ says so
>>> a==c
False                  # because __eq__ says so
>>> a is b
False                  # because they're different objects

>>> l = [b,c]
>>> a in l
True                   # seems to use __eq__ under the hood

So, in seems to use __eq__ to determine whether or not something is in a container.

1. Where can one find documentation on this behavior?
2. Is it possible to make in use object identity, a.k.a. a in somelist if the object a is in somelist, and not some other object that compares equal to a?

Answer 1

Use the any() function and a generator expression:

any(o is a for o in l)

The behaviour of in is documented in the Common Sequence Operators section:

x in s
True if an item of s is equal to x, else False

Bold emphasis mine.

If you must use in, use a wrapper object with a custom __eq__ method that uses is, or build your own container where a custom __contains__ method uses is to test against each contained element.

The wrapper could look like this:

class IdentityWrapper(object):
    def __init__(self, ob):
        self.ob = ob
    def __eq__(self, other):
        return other is self.ob

Demo:

>>> IdentityWrapper(a) in l
False
>>> IdentityWrapper(a) in (l + [a])
True

The container could just use the same any() function outlined above:

class IdentityList(list):
    def __contains__(self, other):
        return any(o is other for o in self)

Demo:

>>> il = IdentityList(l)
>>> a in il
False
>>> a in IdentityList(l + [a])
True