[3.7] bpo-33169: Remove values of `None` from sys.path_importer_cache when invalidating caches (GH-6402) (GH-6403)

An entry of None in sys.path_importer_cache represents a negative/missing finder for a path, so clearing it out makes sense.
(cherry picked from commit 9e2be60634914f23db2ae5624e4acc9335bf5fea)

1 files changed, 4 insertions, 2 deletions

diff --git a/Lib/importlib/_bootstrap_external.py b/Lib/importlib/_bootstrap_external.py
index e295174..490f20d 100644
--- a/Lib/importlib/_bootstrap_external.py
+++ b/Lib/importlib/_bootstrap_external.py
@@ -1179,8 +1179,10 @@ class PathFinder:
     def invalidate_caches(cls):
         """Call the invalidate_caches() method on all path entry finders
         stored in sys.path_importer_caches (where implemented)."""
-        for finder in sys.path_importer_cache.values():
-            if hasattr(finder, 'invalidate_caches'):
+        for name, finder in list(sys.path_importer_cache.items()):
+            if finder is None:
+                del sys.path_importer_cache[name]
+            elif hasattr(finder, 'invalidate_caches'):
                 finder.invalidate_caches()
 
     @classmethod