fix: Handle single-file module pypi-deps in py_pex_binary

[ekfeet]

Addresses the bug in #391.

Single-file modules such as six and typing-extensions does not work with the current py_pex_binary rule, since it's assumed that there exist a sub-directory within site-packages that is not dist-info.

An example of what a single-file module pypi-dep looks like:

ls bazel-reppro_pex_err/external/rules_python~~pip~pypi_311_six/site-packages/
__init__.py  six-1.16.0.dist-info  six.py

Since Distribution.load(..) takes in the site-packages directory, we only emit this part of the path, and let uniquify=True handle deduplication after _map_srcs is applied.

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Changes are visible to end-users: no

Test plan

• Manual testing; please provide instructions so we can reproduce:
  Add six to requirements and "@pypi_six//:pkg" as a dep to the py_pex_binary example; then import six in py_pex_binary's say.py. Printing the module or cowsay.cow(f"{six}") shows that the previous example and single-files modules now also work.

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[mattem]

Thanks! Would you be able to turn the manual test case into an automated one?

@thesayyn Can you take a look?

[ekfeet]

Thanks! Would you be able to turn the manual test case into an automated one?

Looks to be a bit more work than just adding a test case, as I don't see any tests for the py-pex-binary rule. I'll leave test automation up to you guys if that's okay

[thesayyn]

As i remember, Distribution.load has always wanted to read a dist-info, how does this work now?

[ekfeet]

It should be functionally equivalent to the previous version I believe, but I might be missing something. _map_srcs now emits the path up to (and including) 'site-packages', instead of 'site-packages/some-package', so +/../ isn't needed anymore

EDIT: options.dependencies only contain the deps and not the dist-infos afaict. Is the options.distinfos in pex/main.py used?

[thesayyn]

Could you also add a test for this?

[ekfeet]

Could you also add a test for this?

Gave it a shot in a11b203