Is it possible for `__contains__` to return non-boolean value?

Question

The documentation says that __contains__ should return true if item is in self, false otherwise. However, if the method returns a non-boolean value x, it seems that python automatically converts it to bool(x).

Is there any way to avoid that, and return the actual value x? Or is this feature behavior implemented directly in the interpreter and there's no way to change this?

Answer 1

Note that it's not __contains__ that converts the value to a Boolean, but the in operator that calls __contains__. With

class Foo(list):
    def __contains__(self, v):
        if super().__contains__(v):
            return v
        else:
            return False

>>> f = Foo([1,2,3])
>>> f.__contains__(2)
2
>>> 2 in f
True

Answer 2

A foo in bar will be compiled to COMPARE_OP (in) for CPython3. The implementation uses PySequence_Contain() and then coerces to result to a bool. So while you could return something else, you always end up with a bool after the call.