MetaList inherits from both Element and MutableSequence, but their `index` attributes conflict

[castedo]

This is an example of what I would call a headache caused by adding type hints to an existing untyped library. 😅

It's probably fine to ignore this issue because probably nobody is ever going to try and call the method MutableSequence.index(...) on a panflue.MetaList. That said, it's clearly a bug.

I am working around the issue in panflute-stubs by not adding a type hint at all for index and making a specific exception for index to tell stubtest to ignore the absence of a type hint for this attribute.

In the current codebase the declaration of MetaList is

class MetaList(MetaValue, MutableSequence):

which means in inherits from Element the following index attribute:

class Element(object):
    ...
    def __new__(cls, *args, **kwargs):
        ...
        element.index = None

The documentation says it's a int | None and other code in Element confirms.

On the other hand, collections.abc.Sequence (from which collections.abc.MutableSequence derives) has the method:

index(value[, start[, stop])

which is a standard method of sequences in Python, see https://docs.python.org/3/library/stdtypes.html#sequence.index

Mypy if run on the panflute codebase or any attempt by panflute-stubs to type hint index will report the following error:

error: Definition of "index" in base class "Element" is incompatible with definition in base class "Sequence"  [misc]