Issue #13591: import_module potentially imports a module twice.

1 files changed, 17 insertions, 0 deletions

diff --git a/Lib/importlib/test/test_api.py b/Lib/importlib/test/test_api.py
index 0ffa3c4d807..a151626de7d 100644
--- a/Lib/importlib/test/test_api.py
+++ b/Lib/importlib/test/test_api.py
@@ -67,6 +67,23 @@ class ImportModuleTests(unittest.TestCase):
             importlib.import_module('.support')
 
 
+    def test_loaded_once(self):
+        # Issue #13591: Modules should only be loaded once when
+        # initializing the parent package attempts to import the
+        # module currently being imported.
+        b_load_count = 0
+        def load_a():
+            importlib.import_module('a.b')
+        def load_b():
+            nonlocal b_load_count
+            b_load_count += 1
+        code = {'a': load_a, 'a.b': load_b}
+        modules = ['a.__init__', 'a.b']
+        with util.mock_modules(*modules, module_code=code) as mock:
+            with util.import_state(meta_path=[mock]):
+                importlib.import_module('a.b')
+        self.assertEqual(b_load_count, 1)
+
 def test_main():
     from test.support import run_unittest
     run_unittest(ImportModuleTests)