gh-157217: Add mappingproxy dict() and update tests

Author: KingLizard1020

Lib/test/test_dict_mappingproxy.py

@@ -0,0 +1,34 @@
+"""Tests for dict() / update / unpacking of types.MappingProxyType.
+
+gh-157217: merging from a mappingproxy wrapping a dict should take the
+locked dict-to-dict path instead of iterating the proxy unlocked.
+"""
+
+import collections
+import types
+import unittest
+
+
+class MappingProxyDictMergeTests(unittest.TestCase):
+    def test_update_from_mappingproxy_dict(self):
+        d = {}
+        d.update(types.MappingProxyType({1: 1, 2: 2, 3: 3}))
+        self.assertEqual(d, {1: 1, 2: 2, 3: 3})
+
+    def test_dict_constructor_from_mappingproxy(self):
+        view = types.MappingProxyType({'a': 1, 'b': 2})
+        self.assertEqual(dict(view), {'a': 1, 'b': 2})
+        self.assertEqual({**view}, {'a': 1, 'b': 2})
+        dest = {'z': 0}
+        dest.update(view)
+        self.assertEqual(dest, {'z': 0, 'a': 1, 'b': 2})
+
+    def test_dict_constructor_from_mappingproxy_userdict(self):
+        self.assertEqual(
+            dict(types.MappingProxyType(collections.UserDict(x=1))),
+            {'x': 1},
+        )
+
+
+if __name__ == '__main__':
+    unittest.main()