only clear a module's __dict__ if the module is the only one with a reference to it #7140

Lib/test/test_module.py

@@ -55,6 +55,14 @@ class ModuleTests(unittest.TestCase):
               {"__name__": "foo", "__doc__": "foodoc", "bar": 42})
         self.assertTrue(foo.__dict__ is d)
 
+    def test_dont_clear_dict(self):
+        # See issue 7140.
+        def f():
+            foo = ModuleType("foo")
+            foo.bar = 4
+            return foo
+        self.assertEqual(f().__dict__["bar"], 4)
+
 def test_main():
     run_unittest(ModuleTests)
 

Misc/NEWS

@@ -12,6 +12,9 @@ What's New in Python 2.7 alpha 1
 Core and Builtins
 -----------------
 
+- Issue #7140: The __dict__ of a module should not be cleared unless the module
+  is the only object holding a reference to it.
+
 - Issue #1754094: Improve the stack depth calculation in the compiler.
   There should be no other effect than a small decrease in memory use.
   Patch by Christopher Tur Lesniewski-Laas.

Objects/moduleobject.c

@@ -175,6 +175,9 @@ module_dealloc(PyModuleObject *m)
 {
 	PyObject_GC_UnTrack(m);
 	if (m->md_dict != NULL) {
+		/* If we are the only ones holding a reference, we can clear
+		   the dictionary. */
+		if (Py_REFCNT(m->md_dict) == 1)
 			_PyModule_Clear((PyObject *)m);
 		Py_DECREF(m->md_dict);
 	}