bpo-47178: Improve the implementations of Sequence.index and MutableSequence.extend in collections.abc

[geryogam]

This P.R. will make the following changes to the collections.abc module:

• simplify the implementation with slicing in function Sequence.index.
• remove an unnecessary copy of self when a sequence extends itself in function MutableSequence.extend.

https://bugs.python.org/issue47178

[bedevere-bot]

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[rhettinger]

Am marking this as closed because the changes are incorrect.

[rhettinger]

Example that succeeds with the current code but fails with the proposed revision to index()

class LL(Sequence):
    def __len__(self):
        return 3
    def __getitem__(self, i):
        if i >= 10:
            raise IndexError
        return i * 10

assert LL().index(70) == 7

Here the issue is that when stop is None, the index() code should iterate until it hits an IndexError rather than relying on _len_ which can be inaccurate or can reflect an actual size change during iteration.

[geryogam]

Thanks for the example. Indeed, I overlooked that Sequence.index did not rely on Sequence.__len__ but on IndexError to stop, except for shifting negative start and stop indices. My PR was incorrect, sorry.

I have opened a separate PR that moves the unrelated if statement out of the try clause.