Is this bug valid? 

I have checked that only bytearray, bytes, list, range, str and tuple are valid sequence types [using issubclass(type, collections.Sequence)] and all of them has index and count methods... 

I'm working on a script to discovery what types that are not collections.Sequence ABC yet, but must be.

Merwork said the docs are the authoritative reference to define what classes are sequences and one way to discovery it is by comparing the set of methods of each concrete class with the methods of collections.Sequence