Switched SortedList base class to Sequence

[pganssle]

Since SortedList does not implement the MutableSequence interface (many of MutableSequence's mixin methods return NotImplemented), it should not derive from MutableSequence.

This change will allow type checkers to properly reject SortedList as an argument to a function that takes MutableSequence.

This is a breaking change (albeit a minor one).

Fixes #138

[grantjenks]

Those methods are a remnant of blist's implementation which include all the MutableSequence methods. It's been helpful, as a drop-in replacement, to have those methods implemented and raising NotImplementedError with useful error messages and doc strings. Raising AttributeError will be much more cryptic for folks.

Otherwise, I don't see much benefit with this change except appeasing type checkers which seems kind of dubious to me. Did you see a specific case where a type checker would have caught your concern?

[pganssle]

Those methods are a remnant of blist's implementation which include all the MutableSequence methods. It's been helpful, as a drop-in replacement, to have those methods implemented and raising NotImplementedError with useful error messages and doc strings. Raising AttributeError will be much more cryptic for folks.

I have implemented a custom __getattr__ so that the same message is raised, just using AttributeError rather than NotImplementedError. The primary change here is the error type raised - if it raises AttributeError, hasattr and getattr will properly understand that these methods are not implemented, and any isinstance checks that work based on the interface will work correctly.

Otherwise, I don't see much benefit with this change except appeasing type checkers which seems kind of dubious to me. Did you see a specific case where a type checker would have caught your concern?

I have recently been trying to upgrade SortedList on a large code base with a lot of type checking. This would make it much easier to find situations where someone is programming as if something is a list and expecting .extend or .append to work, but what was actually passed was a SortedList.

Same thing with regards to dispatch - people dispatching on MutableSequence expect the thing to actually be a MutableSequence. This is just a mutable Sequence, which is a different thing.