Fix mypy annotations in fermion_partitioning (#1282)

[rosspeili]

First slice for #1282: correct generator return types and local variable annotations in measurements/fermion_partitioning.py. No runtime or API behavior changes.

[gemini-code-assist]

Code Review

This pull request adds type annotations to several functions in src/openfermion/measurements/fermion_partitioning.py to improve type safety and code readability. Since there are no review comments, I have no feedback to provide.

[mhucka]

Thank you for doing this!

There is one change I would recommend. Although Any is valid, it is better to define so-called generic parameter types, along the following lines:

from typing import TypeVar

T = TypeVar('T')

def pair_within(labels: list[T]) -> Generator[tuple[T, ...], None, None]:
    """Generates pairings of labels that contain each pair at least once."""
    ...

Applying this across other matching functions (like pair_between and pair_within_simultaneously) allows static analyzers to check that the output tuples contain elements of the exact same type as the input.

Would you be able to try to make the appropriate changes to this PR?

[rosspeili]

@mhucka reworked based on suggestion, mypy is clean on this file and all tests pass. Let me know how it looks, and thanks for the tip. <3

[mhucka]

I'm not done with the review on the update, but here's a start for a minor detail.

[rosspeili]

@mhucka applied all the Pairing[T] suggestions from above, but that made strict mypy complain about the None sentinel in pair_within, so I fixed that in a follow-up push, same behavior, all tests pass. Let me know how this looks or if I am missing something. Thanks again for the tips and direction <3

[mhucka]

Looks good. Thank you for working on this so diligently!

[rosspeili]

@mhucka thanks for merging #1349

Happy to keep going with the same small-PR approach. Any preference for the next batch? I was thinking another pure-annotation file with a small error count, unless you'd rather prioritize a specific error family first.

[mhucka]

@rosspeili Thank you for your work on this. I can't think of a reason to prefer any particular order, so it's up to you.