`lazy import a.b as c` incorrectly binds to `a.b.b`

Bug report

(Edited by @johnslavik)

To reproduce:

import importlib.metadata
import importlib.metadata as x
lazy import importlib.metadata as lazy_x

print("type(x):     ", type(x).__name__)  # reifies to importlib.metadata
print("type(lazy_x):", type(lazy_x).__name__)  # reifies to importlib.metadata.metadata

x.distributions()  # OK
lazy_x.distributions()  # AttributeError: 'function' object has no attribute 'distributions'

x is the importlib.metadata module, as expected. lazy_x is instead reified to importlib.metadata.metadata, the function.

The bug is in cached name resolution.

For lazy import a.b as c, the issue reproduces when:

1. a.b is already in sys.modules.
2. a.b has an attribute named b.
3. c is reified.

Possible root cause

The problem may be in _PyEval_LazyImportFrom: [Python/ceval.c:3305-3361](https://github.com/python/cpython/blob/cf312cfa64dcaa1700531ec8bc76d56ddd65fb15/Python/ceval.c#L3305-L3361) (3.15 at cf312cfa64dcaa1700531ec8bc76d56ddd65fb15, v3.15.0rc2-22-gcf312cf).

The sys.modules fast path at lines 3313-3331 runs before the base-module computation at 3333-3360 and looks up name directly in d->lz_from.

In the IMPORT_FROM chain generated for import a.b.c as x, however, d->lz_from does not appear to be the module the attribute should be taken from: on the first step it is the full dotted name, and on later steps it is one component short of the base.

CPython versions tested on:

3.15

Operating systems tested on:

Linux

Linked PRs

• gh-157626

まずリンクされている PR gh-157626 を確認し、次に Python/ceval.c の 3305～3361 行付近にある _PyEval_LazyImportFrom を調べます。sys.modules の fast path とキャッシュされた名前の解決に重点を置いてください。importlib.metadata の例で動作を再現します。lazy import a.b as c によって c が a.b.b ではなく a.b に束縛され、回帰が関連するテストでカバーされれば完了です。