bpo-40389: Improve repr of typing.Optional (#19714)

Lib/test/test_dataclasses.py

@@ -2028,7 +2028,7 @@ def test_docstring_one_field_with_default_none(self):
         class C:
             x: Union[int, type(None)] = None
 
-        self.assertDocStrEqual(C.__doc__, "C(x:Union[int, NoneType]=None)")
+        self.assertDocStrEqual(C.__doc__, "C(x:Optional[int]=None)")
 
     def test_docstring_list_field(self):
         @dataclass

Lib/test/test_typing.py

@@ -1750,7 +1750,7 @@ def test_extended_generic_rules_repr(self):
         self.assertEqual(repr(Union[Tuple, Tuple[int]]).replace('typing.', ''),
                          'Union[Tuple, Tuple[int]]')
         self.assertEqual(repr(Callable[..., Optional[T]][int]).replace('typing.', ''),
-                         'Callable[..., Union[int, NoneType]]')
+                         'Callable[..., Optional[int]]')
         self.assertEqual(repr(Callable[[], List[T]][int]).replace('typing.', ''),
                          'Callable[[], List[int]]')
 

Lib/typing.py

@@ -691,6 +691,13 @@ def copy_with(self, params):
         return _GenericAlias(self.__origin__, params, name=self._name, inst=self._inst)
 
     def __repr__(self):
+        if (self.__origin__ == Union and len(self.__args__) == 2
+                and type(None) in self.__args__):
+            if self.__args__[0] is not type(None):
+                arg = self.__args__[0]
+            else:
+                arg = self.__args__[1]
+            return (f'typing.Optional[{_type_repr(arg)}]')
         if (self._name != 'Callable' or
                 len(self.__args__) == 2 and self.__args__[0] is Ellipsis):
             if self._name:

Misc/NEWS.d/next/Library/2020-04-25-20-00-58.bpo-40389.FPA6f0.rst

@@ -0,0 +1 @@
+``repr()`` now returns ``typing.Optional[T]`` when called for ``typing.Union`` of two types, one of which is ``NoneType``.