From 8593a7568817bf36039460e34826822bec9c3904 Mon Sep 17 00:00:00 2001
From: Brett Cannon <bcannon@gmail.com>
Date: Mon, 30 Mar 2009 19:57:15 +0000
Subject: Fix importlib.machinery.PathFinder.find_module() to essentially skip
 over None entries in sys.path_importer_cache. While this differs from
 semantics in how __import__ works, it prevents any implicit semantics from
 taking hold with users.

---
 Lib/importlib/_bootstrap.py             |  7 ++++---
 Lib/importlib/test/import_/test_path.py | 19 +++++++++++++++++++
 2 files changed, 23 insertions(+), 3 deletions(-)

diff --git a/Lib/importlib/_bootstrap.py b/Lib/importlib/_bootstrap.py
index d5b909e..949a612 100644
--- a/Lib/importlib/_bootstrap.py
+++ b/Lib/importlib/_bootstrap.py
@@ -661,9 +661,10 @@ class PathFinder:
                 finder = cls._path_importer_cache(entry)
             except ImportError:
                 continue
-            loader = finder.find_module(fullname)
-            if loader:
-                return loader
+            if finder:
+                loader = finder.find_module(fullname)
+                if loader:
+                    return loader
         else:
             return None
 
diff --git a/Lib/importlib/test/import_/test_path.py b/Lib/importlib/test/import_/test_path.py
index 3d9b1a1..ede9b5f 100644
--- a/Lib/importlib/test/import_/test_path.py
+++ b/Lib/importlib/test/import_/test_path.py
@@ -55,6 +55,25 @@ class FinderTests(unittest.TestCase):
             self.assert_(path in sys.path_importer_cache)
             self.assert_(sys.path_importer_cache[path] is importer)
 
+    def test_path_importer_cache_has_None(self):
+        # Test that if sys.path_importer_cache has None that None is returned.
+        clear_cache = {path: None for path in sys.path}
+        with util.import_state(path_importer_cache=clear_cache):
+            for name in ('asynchat', 'sys', '<test module>'):
+                self.assert_(machinery.PathFinder.find_module(name) is None)
+
+    def test_path_importer_cache_has_None_continues(self):
+        # Test that having None in sys.path_importer_cache causes the search to
+        # continue.
+        path = '<test path>'
+        module = '<test module>'
+        importer = util.mock_modules(module)
+        with util.import_state(path=['1', '2'],
+                            path_importer_cache={'1': None, '2': importer}):
+            loader = machinery.PathFinder.find_module(module)
+            self.assert_(loader is importer)
+
+
 
 class DefaultPathFinderTests(unittest.TestCase):
 
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