gh-139847: Add PyType_Lookup() function

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• Issue: [C API] Add PyType_Lookup() function #139847

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📚 Documentation preview 📚: https://cpython-previews--139848.org.readthedocs.build/

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Check labels / Unresolved review (pull_request): Failing after 6s

The CI fails with:

Error: Label error. Requires exactly 1 of: awaiting merge. Found: type-feature, awaiting core review, interpreter-core, topic-C-API

I added the awaiting merge label to repair this CI.

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I reverted changes to schedule _PyType_Lookup() and _PyType_LookupRef() removal, just to make this PR easier to review and more "atomic". I prefer to write a separated PR for the removal.

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@encukou: So what do you think of exposing this PyType function?

[encukou]

I think there's benefit to exposing this but we need to do it carefully. The operation is different from getattr in surprising ways. The docs & tests should mention details -- not invoking descriptors or metaclass getattr. It might also be good for the docs to add an example of where Python uses this operaton, and/or equivalent Python code.

“Lookup” is a new term in the public C API; are we sure this is the right operation for the name?

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The docs & tests should mention details -- not invoking descriptors or metaclass getattr.

Good idea. I added tests for that and completed the doc.

It might also be good for the docs to add an example of where Python uses this operaton, and/or equivalent Python code.

I added Python pseudo-code to the doc.

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The issue was closed, so I close the PR as well: #139847 (comment).