bpo-27141: Fix collections.UserList and UserDict shallow copy. (GH-4094)

Lib/collections/__init__.py

@@ -1038,6 +1038,13 @@ class UserDict(_collections_abc.MutableMapping):
 
     # Now, add the methods in dicts but not in MutableMapping
     def __repr__(self): return repr(self.data)
+    def __copy__(self):
+        inst = self.__class__.__new__(self.__class__)
+        inst.__dict__.update(self.__dict__)
+        # Create a copy and avoid triggering descriptors
+        inst.__dict__["data"] = self.__dict__["data"].copy()
+        return inst
+
     def copy(self):
         if self.__class__ is UserDict:
             return UserDict(self.data.copy())
@@ -1050,6 +1057,7 @@ class UserDict(_collections_abc.MutableMapping):
             self.data = data
         c.update(self)
         return c
+
     @classmethod
     def fromkeys(cls, iterable, value=None):
         d = cls()
@@ -1118,6 +1126,12 @@ class UserList(_collections_abc.MutableSequence):
     def __imul__(self, n):
         self.data *= n
         return self
+    def __copy__(self):
+        inst = self.__class__.__new__(self.__class__)
+        inst.__dict__.update(self.__dict__)
+        # Create a copy and avoid triggering descriptors
+        inst.__dict__["data"] = self.__dict__["data"][:]
+        return inst
     def append(self, item): self.data.append(item)
     def insert(self, i, item): self.data.insert(i, item)
     def pop(self, i=-1): return self.data.pop(i)

Lib/test/test_collections.py

@@ -37,6 +37,20 @@ class TestUserObjects(unittest.TestCase):
                 b=b.__name__,
             ),
         )
+
+    def _copy_test(self, obj):
+        # Test internal copy
+        obj_copy = obj.copy()
+        self.assertIsNot(obj.data, obj_copy.data)
+        self.assertEqual(obj.data, obj_copy.data)
+
+        # Test copy.copy
+        obj.test = [1234]  # Make sure instance vars are also copied.
+        obj_copy = copy.copy(obj)
+        self.assertIsNot(obj.data, obj_copy.data)
+        self.assertEqual(obj.data, obj_copy.data)
+        self.assertIs(obj.test, obj_copy.test)
+
     def test_str_protocol(self):
         self._superset_test(UserString, str)
 
@@ -46,6 +60,16 @@ class TestUserObjects(unittest.TestCase):
     def test_dict_protocol(self):
         self._superset_test(UserDict, dict)
 
+    def test_list_copy(self):
+        obj = UserList()
+        obj.append(123)
+        self._copy_test(obj)
+
+    def test_dict_copy(self):
+        obj = UserDict()
+        obj[123] = "abc"
+        self._copy_test(obj)
+
 
 ################################################################################
 ### ChainMap (helper class for configparser and the string module)

Misc/NEWS.d/next/Library/2017-10-24-00-42-14.bpo-27141.zbAgSs.rst

+Added a ``__copy__()`` to ``collections.UserList`` and
+``collections.UserDict`` in order to correctly implement shallow copying of
+the objects. Patch by Bar Harel.