Add Sequence to documentation

[alexchamberlain]

Apologies in advance if I've failed to search the docs. As far as I can tell, Sequence isn't formally documented - it would fit quite nicely in Predefined Protocols I think.

I will look to make this change myself over the next couple of days, but I wanted to raise the issue first in case I had a complete fail with the docs.

[alexchamberlain]

After some searching, it is documented at https://docs.python.org/3/library/typing.html#typing.Sequence and implemented at https://github.com/python/cpython/blob/3.7/Lib/typing.py#L1220

[JelleZijlstra]

It's not listed as a Protocol because it's not actually a Protocol. There might be another place in the mypy docs where it could usefully be mentioned though.

[ilevkivskyi]

Yes, it is not a protocol. The remainder is essentially a duplicate of #31

[vectro]

Apologies for necro-posting, but why isn't it a protocol? The fact that it's not means you can't do something like:

@typing.runtime_checkable
class HashableProtocol(
    typing.Protocol[T_co], typing.Hashable, typing.Sequence[T_co]
):
    pass

[TeamSpen210]

@vectro, the reason the more complicated ABCs aren't protocols is that they don't just imply that specific method signatures exist, they imply behaviours that can't just be inferred. For instance if len(seq) > i, seq[i] won't raise an exception. In particular, distinguishing between Sequence and Mapping isn't possible just from methods. See this paragraph in the collections.abc docs:

... Interfaces specify semantics and relationships between methods that cannot be inferred solely from the presence of specific method names. For example, knowing that a class supplies __getitem__, __len__, and __iter__ is insufficient for distinguishing a Sequence from a Mapping.