feat: a sympy backend

[Saransh-cpp]

Description

Kindly take a look at CONTRIBUTING.md.

Please describe the purpose of this pull request. Reference and link to any relevant issues or pull requests.

Checklist

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't any other open Pull Requests for the required change?
• Does your submission pass pre-commit? ($ pre-commit run --all-files or $ nox -s lint)
• Does your submission pass tests? ($ pytest or $ nox -s tests)
• Does the documentation build with your changes? ($ cd docs; make clean; make html or $ nox -s docs)
• Does your submission pass the doctests? ($ pytest --doctest-plus src/vector/ or $ nox -s doctests)

Before Merging

• Summarize the commit messages into a brief review of the Pull request.

[codecov]

Codecov Report

Attention: Patch coverage is 84.81013% with 120 lines in your changes are missing coverage. Please review.

Project coverage is 86.77%. Comparing base (a16da6d) to head (2a29f81).

Files	Patch %	Lines
src/vector/backends/sympy.py	85.08%	108 Missing ⚠️
src/vector/_lib.py	86.44%	8 Missing ⚠️
src/vector/__init__.py	42.85%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #442      +/-   ##
==========================================
+ Coverage   86.38%   86.77%   +0.39%     
==========================================
  Files          96       98       +2     
  Lines       11139    11927     +788     
==========================================
+ Hits         9622    10350     +728     
- Misses       1517     1577      +60     

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[Saransh-cpp]

TODO:

• add docs
• create a new release once this is reviewed and merged

[Saransh-cpp]

Should finally be ready for a review!

[jpivarski]

All of this looks great! I made some high-level comments below, and I'm trusting the tests to ensure that the implementation is line-by-line correct.

I see that there are a lot more tests, but strangely, it's failing because Codecov says it's undershooting the target line coverage. I don't see how that's possible, considering that you added a few more classes and a lot more tests, including tests of non-SymPy functionality that wasn't tested before.

Is Codecov wrong? If not, is there a way to satisfy it with just a few more tests? If not, maybe we should just lower its threshold. Going a little under 85% is not terrible. Some lines of code will never be tested, like

    else:
        raise AssertionError(
            f"unexpected case; should never get here {data = }"
        )

[jpivarski]

It looks like SymPy is an optional dependency and that's good! ✔️

[jpivarski]

SymPy's preferred capitalization includes a capital "P", but we ignore NumPy's preferred capitalization, too. This is fine. ✔️

(MomentumSymPy4D would be too bumpy!)

[Saransh-cpp]

Thank you for the review!

Is Codecov wrong? If not, is there a way to satisfy it with just a few more tests? If not, maybe we should just lower its threshold. Going a little under 85% is not terrible.

I am not sure how Codecov is processing coverage report here, but I'll try satisfying it for the lines that can be tested. I think the absolute coverage went up but the relative coverage went down.

[jpivarski]

I'm just surprised because you added a lot more tests than code. I know that lines of test code don't correspond one-to-one with lines covered in the codebase, but it's still surprising to me.

[Saransh-cpp]

Added a few more tests, coverage went up! Merging!