Can't this code:

    class Sequence(Sized, Iterable, Container):
    # ...
        def __contains__(self, value):
            for v in self:
                if v == value:
                    return True
            return False

Be shortened into this: 

    class Sequence(Sized, Iterable, Container):
    # ...
        def __contains__(self, value):
            return any(item == value for value in self)

Which can even fit on one line with a lambda: 

    class Sequence(Sized, Iterable, Container):
    # ...
        __contains__ = lambda self: any(item == value for value in self)

This is slower.

>>> import timeit
>>> class A(list):
...         def __contains__(self, value):
...             for v in self:
...                 if v == value:
...                     return True
...             return False
... 
>>> timeit.timeit('500 in x', setup='from __main__ import A; x = A(range(1000))', number=10000)
1.1222619999971357
>>> class B(list):
...         def __contains__(self, value):
...             return any(v == value for v in self)
... 
>>> timeit.timeit('500 in x', setup='from __main__ import B; x = B(range(1000))', number=10000)
2.05952100000286

Oh. I wonder why `any` is slow like that, you'd figure it's be optimized.

Because in first case there is one iterator, iter(self), and in second case 
there are two iterators: iter(self) and iter((v == value for v in self)).

Thanks for the clarification. Oh well, sad to see the more verbose code win, but I guess that's life.

I tried on PyPy but the difference was even more pronounced, 0.008922450399566156 for the long version and 0.042124665810088044 for the short version.