http://docs.python.org/3.3/glossary.html#term-sequence

__getitem__ and __len__ are required for sequence type.
(__iter__ is not required because types having __getitem__ are already iterator.)

.__contains__(), .index() and .count() is not required for sequence.

For example, following class should be sequence.

class Foo:
    def __getitem__(self, index):
        if not isinstance(index, int):
            raise TypeError
        if index >= 3:
            raise IndexError
        return index

    def __len__(self):
        return 3