bpo-30441: Fix bug when modifying os.environ while iterating over it (#2409)

Lib/os.py

@@ -697,7 +697,9 @@ class _Environ(MutableMapping):
             raise KeyError(key) from None
 
     def __iter__(self):
-        for key in self._data:
+        # list() from dict object is an atomic operation
+        keys = list(self._data)
+        for key in keys:
             yield self.decodekey(key)
 
     def __len__(self):

Lib/test/test_os.py

@@ -835,6 +835,30 @@ class EnvironTests(mapping_tests.BasicTestMappingProtocol):
         self.assertIs(cm.exception.args[0], missing)
         self.assertTrue(cm.exception.__suppress_context__)
 
+    def _test_environ_iteration(self, collection):
+        iterator = iter(collection)
+        new_key = "__new_key__"
+
+        next(iterator)  # start iteration over os.environ.items
+
+        # add a new key in os.environ mapping
+        os.environ[new_key] = "test_environ_iteration"
+
+        try:
+            next(iterator)  # force iteration over modified mapping
+            self.assertEqual(os.environ[new_key], "test_environ_iteration")
+        finally:
+            del os.environ[new_key]
+
+    def test_iter_error_when_changing_os_environ(self):
+        self._test_environ_iteration(os.environ)
+
+    def test_iter_error_when_changing_os_environ_items(self):
+        self._test_environ_iteration(os.environ.items())
+
+    def test_iter_error_when_changing_os_environ_values(self):
+        self._test_environ_iteration(os.environ.values())
+
 
 class WalkTests(unittest.TestCase):
     """Tests for os.walk()."""

Misc/ACKS

@@ -1088,6 +1088,7 @@ Fredrik Nehr
 Tony Nelson
 Trent Nelson
 Andrew Nester
+Osvaldo Santana Neto
 Chad Netzer
 Max Neunhöffer
 Anthon van der Neut

Misc/NEWS.d/next/Library/2017-06-29-14-25-14.bpo-30441.3Wh9kc.rst

@@ -0,0 +1 @@
+Fix bug when modifying os.environ while iterating over it