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).