Track optional TypdeDict keys (#687)

Backport of python/cpython#17214 (BPO-38834)

Author: Zac-HD

src_py3/test_typing_extensions.py

@@ -428,6 +428,9 @@ class Point2D(TypedDict):
     x: int
     y: int
 
+class Point2Dor3D(Point2D, total=False):
+    z: int
+
 class LabelPoint2D(Point2D, Label): ...
 
 class Options(TypedDict, total=False):
@@ -442,7 +445,7 @@ class Options(TypedDict, total=False):
     ann_module = ann_module2 = ann_module3 = None
     A = B = CSub = G = CoolEmployee = CoolEmployeeWithDefault = object
     XMeth = XRepr = HasCallProtocol = NoneAndForward = Loop = object
-    Point2D = LabelPoint2D = Options = object
+    Point2D = Point2Dor3D = LabelPoint2D = Options = object
 
 gth = get_type_hints
 
@@ -1481,7 +1484,7 @@ def test_typeddict_create_errors(self):
 
     def test_typeddict_errors(self):
         Emp = TypedDict('Emp', {'name': str, 'id': int})
-        if hasattr(typing, 'TypedDict'):
+        if sys.version_info[:2] >= (3, 9):
             self.assertEqual(TypedDict.__module__, 'typing')
         else:
             self.assertEqual(TypedDict.__module__, 'typing_extensions')
@@ -1543,6 +1546,11 @@ def test_total(self):
             self.assertEqual(Options(log_level=2), {'log_level': 2})
             self.assertEqual(Options.__total__, False)
 
+    @skipUnless(PY36, 'Python 3.6 required')
+    def test_optional_keys(self):
+        assert Point2Dor3D.__required_keys__ == frozenset(['x', 'y'])
+        assert Point2Dor3D.__optional_keys__ == frozenset(['z'])
+
 
 @skipUnless(TYPING_3_5_3, "Python >= 3.5.3 required")
 class AnnotatedTests(BaseTestCase):
@@ -1817,7 +1825,14 @@ def test_typing_extensions_includes_standard(self):
             self.assertIn('runtime', a)
 
     def test_typing_extensions_defers_when_possible(self):
-        exclude = {'overload', 'Text', 'TYPE_CHECKING', 'Final', 'get_type_hints'}
+        exclude = {
+            'overload',
+            'Text',
+            'TypedDict',
+            'TYPE_CHECKING',
+            'Final',
+            'get_type_hints'
+        }
         for item in typing_extensions.__all__:
             if item not in exclude and hasattr(typing, item):
                 self.assertIs(

src_py3/typing_extensions.py

@@ -1569,7 +1569,9 @@ def runtime_checkable(cls):
     runtime = runtime_checkable
 
 
-if hasattr(typing, 'TypedDict'):
+if sys.version_info[:2] >= (3, 9):
+    # The standard library TypedDict in Python 3.8 does not store runtime information
+    # about which (if any) keys are optional.  See https://bugs.python.org/issue38834
     TypedDict = typing.TypedDict
 else:
     def _check_fails(cls, other):
@@ -1652,9 +1654,20 @@ def __new__(cls, name, bases, ns, total=True):
             anns = ns.get('__annotations__', {})
             msg = "TypedDict('Name', {f0: t0, f1: t1, ...}); each t must be a type"
             anns = {n: typing._type_check(tp, msg) for n, tp in anns.items()}
+            required = set(anns if total else ())
+            optional = set(() if total else anns)
+
             for base in bases:
-                anns.update(base.__dict__.get('__annotations__', {}))
+                base_anns = base.__dict__.get('__annotations__', {})
+                anns.update(base_anns)
+                if getattr(base, '__total__', True):
+                    required.update(base_anns)
+                else:
+                    optional.update(base_anns)
+
             tp_dict.__annotations__ = anns
+            tp_dict.__required_keys__ = frozenset(required)
+            tp_dict.__optional_keys__ = frozenset(optional)
             if not hasattr(tp_dict, '__total__'):
                 tp_dict.__total__ = total
             return tp_dict
@@ -1682,8 +1695,9 @@ class Point2D(TypedDict):
 
             assert Point2D(x=1, y=2, label='first') == dict(x=1, y=2, label='first')
 
-        The type info could be accessed via Point2D.__annotations__. TypedDict
-        supports two additional equivalent forms::
+        The type info can be accessed via the Point2D.__annotations__ dict, and
+        the Point2D.__required_keys__ and Point2D.__optional_keys__ frozensets.
+        TypedDict supports two additional equivalent forms::
 
             Point2D = TypedDict('Point2D', x=int, y=int, label=str)
             Point2D = TypedDict('Point2D', {'x': int, 'y': int, 'label': str})