[C API] Add PyType_Lookup() function

[vstinner]

Feature or enhancement

There is a private _PyType_Lookup() function which is used by 27 projects of the PyPI top 15,000 projects:

• Nuitka (2.7.16)
• ansys_pythonnet-3.1.0rc6
• casadi (3.7.2)
• catboost (1.2.8)
• cvxpy (1.7.2)
• cvxpy_base (1.7.2)
• cython (3.1.4)
• extensionclass (6.0)
• gamsapi (51.1.0)
• gdal (3.11.4)
• gevent (25.9.1)
• htseq (2.0.9)
• ihm (2.7)
• m2crypto (0.45.1)
• mecab-ko (1.0.1)
• mecab_python3 (1.0.10)
• msgspec (0.19.0)
• mupdf (1.21.1.20230112.1504)
• mypy (1.18.2)
• pybind11 (3.0.1)
• pycuda (2025.1.2)
• pygdal (3.6.4.11)
• pygraphviz (1.14)
• python_crfsuite (0.9.11)
• pythonnet (3.0.5)
• sentencepiece (0.2.1)
• yappi (1.6.10)

I propose to make the function public.

Linked PRs

• gh-139847: Add PyType_Lookup() function #139848

[markshannon]

This does seem to be filling a gap in the C API.

[encukou]

To me, _PyType_Lookup sounds like a special case -- a useful special case, but it might be good to consider in together with more variations. Off the top of my head, things that you might want to enable/disable are:

• (not) looking in the instance, direct class, superclasses, metaclass
• (not) invoking descriptors
• (not) binding methods with Py_TPFLAGS_METHOD_DESCRIPTOR

Also: when we walk the MRO we should return the class the attribute is on (the defining class).

[encukou]

I had some notes lying around; so I collected them here: https://discuss.python.org/t/api-for-special-attribute-lookup/104288