The attached file demonstrates the problem:

If importlib.invalidate_caches() is called while the zipfile used by the zip importer is not available the import system breaks entirely. I found this in a testsuite that accedently did this (it should have updated sys.path).

I get the following exception:

$ python3.10 t.py
Traceback (most recent call last):
  File "/Users/ronald/Projects/modulegraph2/t.py", line 27, in <module>
    import uu
  File "<frozen importlib._bootstrap>", line 1027, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1002, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 945, in _find_spec
  File "<frozen importlib._bootstrap_external>", line 1430, in find_spec
  File "<frozen importlib._bootstrap_external>", line 1402, in _get_spec
  File "<frozen zipimport>", line 168, in find_spec
  File "<frozen zipimport>", line 375, in _get_module_info
TypeError: argument of type 'NoneType' is not iterable

This exception is not very friendly....

This particular exception is caused by setting self._files to None in the importer's invalidate_caches method, while not checking for None in _get_modules_info.

I'm not sure what the best fix would be, setting self._files to an empty list would likely be the easiest fix.

Note that the script runs without errors in Python 3.9.

I bisected this to:

3abf6f0 is the first bad commit
commit 3abf6f0
Author: Desmond Cheong <desmondcheongzx@gmail.com>
Date: Tue Mar 9 04:06:02 2021 +0800

bpo-14678: Update zipimport to support importlib.invalidate_caches() (GH-24159)



Added an invalidate_caches() method to the zipimport.zipimporter class based on the implementation of importlib.FileFinder.invalidate_caches(). This was done by adding a get_files() method and an _archive_mtime attribute to zipimport.zipimporter to check for updates or cache invalidation whenever the cache of files and toc entry information in the zipimporter is accessed.

Doc/library/zipimport.rst | 9 +
Lib/test/test_zipimport.py | 41 +
Lib/zipimport.py | 10 +
.../2021-01-07-21-25-49.bpo-14678.1zniCH.rst | 3 +
Python/importlib_zipimport.h | 1896 ++++++++++----------
5 files changed, 1020 insertions(+), 939 deletions(-)
create mode 100644 Misc/NEWS.d/next/Library/2021-01-07-21-25-49.bpo-14678.1zniCH.rst
bisect run success

Which is https://bugs.python.org/issue14678

Brett, can you take a look when you have some time?

I just noticed that I'm unnecessarily obtuse in my description of a possible fix, the diff (without test update):

% git diff Lib/zipimport.py                                                                                        (main)cpython
diff --git a/Lib/zipimport.py b/Lib/zipimport.py
index c55fec6aa1..43ac6cbe57 100644
--- a/Lib/zipimport.py
+++ b/Lib/zipimport.py
@@ -334,7 +334,7 @@ def invalidate_caches(self):
             _zip_directory_cache[self.archive] = self._files
         except ZipImportError:
             _zip_directory_cache.pop(self.archive, None)
-            self._files = None
+            self._files = {}
 
 
     def __repr__(self):

With that change the exception should not happen, and the now stale zipimporter would be ignored when flushing the cache while the zipfile referenced by the zip importer instance has been removed.

That said, I haven't tested this and won't create a PR because my local tree is (still) a mess.

The proposed fix seems to be the right one based on my reading of the code.

New changeset 209b703 by Brett Cannon in branch 'main':
bpo-45183: don't raise an exception when calling zipimport.zipimporter.find_spec() when the zip file is missing and the internal cache has been reset (GH-28435)
209b703

New changeset e1bdecb by Brett Cannon in branch '3.10':
[3.10] bpo-45183: don't raise an exception when calling zipimport.zipimporter.find_spec() when the zip file is missing and the internal cache has been reset (GH-28435) (bpo-28438)
e1bdecb

I decided that find_spec() saying something wasn't available in a finder made sense even though it was because a zip file no longer existed as the loader would still fail as appropriate.