issubclass of abstract base class Sequence

Question

This list shows what methods you need to implement for your class to be \"regarded\" as Sequence: __getitem__, __len__, __contains__,

Answer 1

Use the source, Luke!

Sequence does not implement its own __subclasshook__, and all the implementations of __subclasshook__ from the parents of Sequence have checks like this:

class Iterable:
    ...

    @classmethod
    def __subclasshook__(cls, C):
        if cls is Iterable:  # <<<<
            if _hasattr(C, "__iter__"):
                return True
        return NotImplemented

You can however explicitly register() your class as a Sequence:

Sequence.register(S)

As for the reason why Sequence does not implement __subclasshook__, see issue 16728 (which title was initially "collections.abc.Sequence shoud provide __subclasshook__"). The issue can be summarized by saying that a sequence can be many things, depending on the needs of who uses it:

Many algorithms that require a sequence only need __len__ and __getitem__. [...] collections.abc.Sequence is a much richer interface.