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

从 Lib/shutil.py 第 21-26 行开始，在不支持 bz2 的 CPython 构建中复现两种 PYTHON_LAZY_IMPORTS 模式。阅读相关的 test_shutil 覆盖情况和链接的 PRs，以了解当前方向。当不支持的 bz2 检查在 lazy_imports=all 下行为正确，且不会导致现有 shutil 测试回归时，即表示完成。