Recently, when debugging a weird problem (see https://bugzilla.redhat.com/show_bug.cgi?id=2018551 for details if interested, but not important for this issue), I've realized that the _NamespacePath class (from importlib/_bootstrap_external.py) has a cache that (in some cases) uses tuple(sys.path) as the key. I was expecting importlib.invalidate_caches() to invalidate this cache, but it doesn't. Consider the following directory structure: . ├── PATH1 │   └── namespace │   └── sub1 │   └── __init__.py └── PATH2 └── namespace └── sub2 └── __init__.py Here is a helper to create it (on Linux-ish): $ mkdir -p PATH1/namespace/sub1 $ mkdir -p PATH2/namespace/sub2 $ touch PATH1/namespace/sub1/__init__.py $ touch PATH2/namespace/sub2/__init__.py Run Python with PYTHONPATH=PATH1:PATH2 (output slightly formatted for readability): $ PYTHONPATH=PATH1:PATH2 python3.11 >>> import namespace >>> namespace.__path__ _NamespacePath(['.../namespace_path_cache/PATH1/namespace', '.../namespace_path_cache/PATH2/namespace']) >>> import namespace.sub1 # works >>> import namespace.sub2 # works >>> exit() The namespace packages seem to work as expected. Now move PATH2/namespace out of the way: $ mv PATH2/namespace PATH2/cant-import-this Run Python again: $ PYTHONPATH=PATH1:PATH2 python3.11 >>> import namespace >>> namespace.__path__ _NamespacePath(['.../namespace_path_cache/PATH1/namespace']) >>> ... While this interpreter still runs, move the PATH2/namespace module back in: $ mv PATH2/cant-import-this PATH2/namespace >>> ... >>> namespace.__path__ _NamespacePath(['.../namespace_path_cache/PATH1/namespace']) >>> import namespace.sub2 Traceback (most recent call last): File "<stdin>", line 1, in <module> ModuleNotFoundError: No module named 'namespace.sub2' >>> import importlib >>> importlib.invalidate_caches() # invalidate the cache, not helpful >>> namespace.__path__ _NamespacePath(['.../namespace_path_cache/PATH1/namespace']) >>> import namespace.sub2 Traceback (most recent call last): File "<stdin>", line 1, in <module> ModuleNotFoundError: No module named 'namespace.sub2' >>> import sys >>> sys.path.remove('') # changing sys.path solves this >>> namespace.__path__ _NamespacePath(['.../namespace_path_cache/PATH1/namespace']) >>> import namespace.sub2 >>> namespace.__path__ _NamespacePath(['.../namespace_path_cache/PATH1/namespace', '.../namespace_path_cache/PATH2/namespace']) importlib.invalidate_caches() documentation says: > This function should be called if any modules are created/installed while your program is running to guarantee all finders will notice the new module’s existence. That makes me think calling importlib.invalidate_caches() should also invalidate the cache of _NamespacePaths. (This also affects older Pythons, but since it is a behavior change, I've only marked 3.11).

I have a fix in mind, will submit a draft pull request without tests shortly, continue with the tests later.

The PR is now ready for review.

 New changeset ae1965ccb4b1fad63fab40fe8805d1b8247668d3 by Miro Hrončok in branch 'main': bpo-45703: Invalidate _NamespacePath cache on importlib.invalidate_ca… (GH-29384) https://github.com/python/cpython/commit/ae1965ccb4b1fad63fab40fe8805d1b8247668d3 

Now, is this a bugfix, or a new feature? I lean toward calling it a bugfix and backporting to earlier versions.

I consider it a bugfix. I don't expect users to rely on the previous behavior and be surprised by the new, but yet I know https://xkcd.com/1172/ very well.

Sadly, the backport is non-trivial. I'm putting on my TODO list, but I doubt I'll get to it soon :(

The automated backport to Python 3.9 failed. Miro asked me to close the PR: https://github.com/python/cpython/pull/29964 Can someone backport the fix to 3.9? (create a PR)

Embarassingly, it seems it bust needs regen-importlib, at least for 3.10

 New changeset 5c39e474dbd61397c2ff877fa17d742bf4646702 by Petr Viktorin in branch '3.10': [3.10] bpo-45703: Invalidate _NamespacePath cache on importlib.invalidate_cache (GH-29384) (GH-30922) https://github.com/python/cpython/commit/5c39e474dbd61397c2ff877fa17d742bf4646702 

 New changeset 8d239bfdcc98f4e4a592d9cda569380a9c15a5c0 by Petr Viktorin in branch '3.9': [3.9] bpo-45703: Invalidate _NamespacePath cache on importlib.invalidate_cache (GH-29384) (GH-30922) (GH-31076) https://github.com/python/cpython/commit/8d239bfdcc98f4e4a592d9cda569380a9c15a5c0