Importing `bz2` will succeed under `lazy_imports=all` even if bz2 is not supported

Bug report

Edit: Originally this bug was expressed through shutil's support detection. shutil now directly imports from _bz2 which works around this issue.

Bug description:

Under lazy_imports=all, modules imported in a try/except block are still imported eagerly. However, imports that occur within that module are still imported lazily.

This may be intentional, but this bug is the result of that behaviour.

On CPython built without bz2 support:

$ PYTHON_LAZY_IMPORTS=normal ./python -c "import shutil; print(shutil._BZ2_SUPPORTED)"
False
$ PYTHON_LAZY_IMPORTS=all ./python -c "import shutil; print(shutil._BZ2_SUPPORTED)"
True

The try/except check is on bz2, which succeeds because the import to _bz2 which would fail is converted to a lazy import and is not used at top level so the import which would fail isn't triggered.

https://github.com/python/cpython/blob/87a879f4d0ec2e545e84c898c5ce452a6c87b09e/Lib/shutil.py#L21-L26

shutil could directly check _bz2 in this particular case, but I expect this pattern isn't uncommon. I looked into this after I ran into a similar issue with pip and rich with an OS-specific import that should have failed in a similar manner.

Related: https://github.com/python/cpython/issues/149640 as this also causes test_shutil to fail under lazy_imports=all if _bz2 isn't present.

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-150279
• gh-154771
• gh-157321

Beginne mit Lib/shutil.py Zeilen 21-26 und reproduziere die beiden PYTHON_LAZY_IMPORTS-Modi in einem CPython-Build ohne bz2-Unterstützung. Lies die zugehörige test_shutil-Abdeckung und die verknüpften PRs, um die aktuelle Richtung zu verstehen. Die Aufgabe ist abgeschlossen, wenn sich die Prüfung auf nicht unterstütztes bz2 unter lazy_imports=all korrekt verhält, ohne bestehende shutil-Tests zu beeinträchtigen.