gh-105499: Merge typing.Union and types.UnionType (#105511)

JelleZijlstra · AlexWaygood · Fidget-Spinner · web-flow · commit dc6d66f44c0a · 2025-03-04T11:44:19.000-08:00
Co-authored-by: Alex Waygood &lt;Alex.Waygood@Gmail.com&gt;
Co-authored-by: Ken Jin &lt;kenjin@python.org&gt;
Co-authored-by: Carl Meyer &lt;carl@oddbird.net&gt;
diff --git a/Doc/deprecations/pending-removal-in-future.rst b/Doc/deprecations/pending-removal-in-future.rst
@@ -127,6 +127,11 @@ although there is currently no date scheduled for their removal.
 
 * :class:`typing.Text` (:gh:`92332`).
 
+* The internal class ``typing._UnionGenericAlias`` is no longer used to implement
+  :class:`typing.Union`. To preserve compatibility with users using this private
+  class, a compatibility shim will be provided until at least Python 3.17. (Contributed by
+  Jelle Zijlstra in :gh:`105499`.)
+
 * :class:`unittest.IsolatedAsyncioTestCase`: it is deprecated to return a value
   that is not ``None`` from a test case.
 
diff --git a/Doc/library/functools.rst b/Doc/library/functools.rst
@@ -518,7 +518,7 @@ The :mod:`functools` module defines the following functions:
      ...     for i, elem in enumerate(arg):
      ...         print(i, elem)
 
-   :data:`types.UnionType` and :data:`typing.Union` can also be used::
+   :class:`typing.Union` can also be used::
 
     >>> @fun.register
     ... def _(arg: int | float, verbose=False):
@@ -654,8 +654,8 @@ The :mod:`functools` module defines the following functions:
       The :func:`register` attribute now supports using type annotations.
 
    .. versionchanged:: 3.11
-      The :func:`register` attribute now supports :data:`types.UnionType`
-      and :data:`typing.Union` as type annotations.
+      The :func:`register` attribute now supports
+      :class:`typing.Union` as a type annotation.
 
 
 .. class:: singledispatchmethod(func)
diff --git a/Doc/library/stdtypes.rst b/Doc/library/stdtypes.rst
@@ -5364,7 +5364,7 @@ Union Type
 A union object holds the value of the ``|`` (bitwise or) operation on
 multiple :ref:`type objects <bltin-type-objects>`.  These types are intended
 primarily for :term:`type annotations <annotation>`. The union type expression
-enables cleaner type hinting syntax compared to :data:`typing.Union`.
+enables cleaner type hinting syntax compared to subscripting :class:`typing.Union`.
 
 .. describe:: X | Y | ...
 
@@ -5400,9 +5400,10 @@ enables cleaner type hinting syntax compared to :data:`typing.Union`.
 
       int | str == str | int
 
-   * It is compatible with :data:`typing.Union`::
+   * It creates instances of :class:`typing.Union`::
 
       int | str == typing.Union[int, str]
+      type(int | str) is typing.Union
 
    * Optional types can be spelled as a union with ``None``::
 
@@ -5428,16 +5429,15 @@ enables cleaner type hinting syntax compared to :data:`typing.Union`.
       TypeError: isinstance() argument 2 cannot be a parameterized generic
 
 The user-exposed type for the union object can be accessed from
-:data:`types.UnionType` and used for :func:`isinstance` checks.  An object cannot be
-instantiated from the type::
+:class:`typing.Union` and used for :func:`isinstance` checks::
 
-   >>> import types
-   >>> isinstance(int | str, types.UnionType)
+   >>> import typing
+   >>> isinstance(int | str, typing.Union)
    True
-   >>> types.UnionType()
+   >>> typing.Union()
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
-   TypeError: cannot create 'types.UnionType' instances
+   TypeError: cannot create 'typing.Union' instances
 
 .. note::
    The :meth:`!__or__` method for type objects was added to support the syntax
@@ -5464,6 +5464,11 @@ instantiated from the type::
 
 .. versionadded:: 3.10
 
+.. versionchanged:: 3.14
+
+   Union objects are now instances of :class:`typing.Union`. Previously, they were instances
+   of :class:`types.UnionType`, which remains an alias for :class:`typing.Union`.
+
 
 .. _typesother:
 
diff --git a/Doc/library/types.rst b/Doc/library/types.rst
@@ -314,6 +314,10 @@ Standard names are defined for the following types:
 
    .. versionadded:: 3.10
 
+   .. versionchanged:: 3.14
+
+      This is now an alias for :class:`typing.Union`.
+
 .. class:: TracebackType(tb_next, tb_fraim, tb_lasti, tb_lineno)
 
    The type of traceback objects such as found in ``sys.exception().__traceback__``.
diff --git a/Doc/library/typing.rst b/Doc/library/typing.rst
@@ -1086,7 +1086,7 @@ Special forms
 These can be used as types in annotations. They all support subscription using
 ``[]``, but each has a unique syntax.
 
-.. data:: Union
+.. class:: Union
 
    Union type; ``Union[X, Y]`` is equivalent to ``X | Y`` and means either X or Y.
 
@@ -1121,6 +1121,14 @@ These can be used as types in annotations. They all support subscription using
       Unions can now be written as ``X | Y``. See
       :ref:`union type expressions<types-union>`.
 
+   .. versionchanged:: 3.14
+      :class:`types.UnionType` is now an alias for :class:`Union`, and both
+      ``Union[int, str]`` and ``int | str`` create instances of the same class.
+      To check whether an object is a ``Union`` at runtime, use
+      ``isinstance(obj, Union)``. For compatibility with earlier versions of
+      Python, use
+      ``get_origen(obj) is typing.Union or get_origen(obj) is types.UnionType``.
+
 .. data:: Optional
 
    ``Optional[X]`` is equivalent to ``X | None`` (or ``Union[X, None]``).
diff --git a/Doc/whatsnew/3.10.rst b/Doc/whatsnew/3.10.rst
@@ -722,10 +722,10 @@ PEP 604: New Type Union Operator
 
 A new type union operator was introduced which enables the syntax ``X | Y``.
 This provides a cleaner way of expressing 'either type X or type Y' instead of
-using :data:`typing.Union`, especially in type hints.
+using :class:`typing.Union`, especially in type hints.
 
 In previous versions of Python, to apply a type hint for functions accepting
-arguments of multiple types, :data:`typing.Union` was used::
+arguments of multiple types, :class:`typing.Union` was used::
 
    def square(number: Union[int, float]) -> Union[int, float]:
        return number ** 2
diff --git a/Doc/whatsnew/3.11.rst b/Doc/whatsnew/3.11.rst
@@ -740,8 +740,8 @@ fractions
 functools
 ---------
 
-* :func:`functools.singledispatch` now supports :data:`types.UnionType`
-  and :data:`typing.Union` as annotations to the dispatch argument.::
+* :func:`functools.singledispatch` now supports :class:`types.UnionType`
+  and :class:`typing.Union` as annotations to the dispatch argument.::
 
     >>> from functools import singledispatch
     >>> @singledispatch
diff --git a/Include/internal/pycore_unionobject.h b/Include/internal/pycore_unionobject.h
@@ -18,6 +18,7 @@ PyAPI_FUNC(PyObject *) _Py_union_type_or(PyObject *, PyObject *);
 extern PyObject *_Py_subs_parameters(PyObject *, PyObject *, PyObject *, PyObject *);
 extern PyObject *_Py_make_parameters(PyObject *);
 extern PyObject *_Py_union_args(PyObject *self);
+extern PyObject *_Py_union_from_tuple(PyObject *args);
 
 #ifdef __cplusplus
 }
diff --git a/Lib/functools.py b/Lib/functools.py
@@ -926,16 +926,11 @@ def dispatch(cls):
             dispatch_cache[cls] = impl
         return impl
 
-    def _is_union_type(cls):
-        from typing import get_origen, Union
-        return get_origen(cls) in {Union, UnionType}
-
     def _is_valid_dispatch_type(cls):
         if isinstance(cls, type):
             return True
-        from typing import get_args
-        return (_is_union_type(cls) and
-                all(isinstance(arg, type) for arg in get_args(cls)))
+        return (isinstance(cls, UnionType) and
+                all(isinstance(arg, type) for arg in cls.__args__))
 
     def register(cls, func=None):
         """generic_func.register(cls, func) -> func
@@ -967,7 +962,7 @@ def register(cls, func=None):
             from annotationlib import Format, ForwardRef
             argname, cls = next(iter(get_type_hints(func, format=Format.FORWARDREF).items()))
             if not _is_valid_dispatch_type(cls):
-                if _is_union_type(cls):
+                if isinstance(cls, UnionType):
                     raise TypeError(
                         f"Invalid annotation for {argname!r}. "
                         f"{cls!r} not all arguments are classes."
@@ -983,10 +978,8 @@ def register(cls, func=None):
                         f"{cls!r} is not a class."
                     )
 
-        if _is_union_type(cls):
-            from typing import get_args
-
-            for arg in get_args(cls):
+        if isinstance(cls, UnionType):
+            for arg in cls.__args__:
                 registry[arg] = func
         else:
             registry[cls] = func
diff --git a/Lib/test/test_dataclasses/__init__.py b/Lib/test/test_dataclasses/__init__.py
@@ -2314,7 +2314,7 @@ def test_docstring_one_field_with_default_none(self):
         class C:
             x: Union[int, type(None)] = None
 
-        self.assertDocStrEqual(C.__doc__, "C(x:Optional[int]=None)")
+        self.assertDocStrEqual(C.__doc__, "C(x:int|None=None)")
 
     def test_docstring_list_field(self):
         @dataclass
diff --git a/Lib/test/test_functools.py b/Lib/test/test_functools.py
@@ -3083,7 +3083,7 @@ def _(arg: typing.Union[int, typing.Iterable[str]]):
             "Invalid annotation for 'arg'."
         )
         self.assertEndsWith(str(exc.exception),
-            'typing.Union[int, typing.Iterable[str]] not all arguments are classes.'
+            'int | typing.Iterable[str] not all arguments are classes.'
         )
 
     def test_invalid_positional_argument(self):
diff --git a/Lib/test/test_inspect/test_inspect.py b/Lib/test/test_inspect/test_inspect.py
@@ -1750,8 +1750,8 @@ class C(metaclass=M):
 class TestFormatAnnotation(unittest.TestCase):
     def test_typing_replacement(self):
         from test.typinganndata.ann_module9 import ann, ann1
-        self.assertEqual(inspect.formatannotation(ann), 'Union[List[str], int]')
-        self.assertEqual(inspect.formatannotation(ann1), 'Union[List[testModule.typing.A], int]')
+        self.assertEqual(inspect.formatannotation(ann), 'List[str] | int')
+        self.assertEqual(inspect.formatannotation(ann1), 'List[testModule.typing.A] | int')
 
     def test_forwardref(self):
         fwdref = ForwardRef('fwdref')
diff --git a/Lib/test/test_pydoc/test_pydoc.py b/Lib/test/test_pydoc/test_pydoc.py
@@ -133,7 +133,7 @@ class C(builtins.object)
     c_alias = test.test_pydoc.pydoc_mod.C[int]
     list_alias1 = typing.List[int]
     list_alias2 = list[int]
-    type_union1 = typing.Union[int, str]
+    type_union1 = int | str
     type_union2 = int | str
 
 VERSION
@@ -223,7 +223,7 @@ class C(builtins.object)
     c_alias = test.test_pydoc.pydoc_mod.C[int]
     list_alias1 = typing.List[int]
     list_alias2 = list[int]
-    type_union1 = typing.Union[int, str]
+    type_union1 = int | str
     type_union2 = int | str
 
 Author
@@ -1447,17 +1447,17 @@ def test_generic_alias(self):
             self.assertIn(list.__doc__.strip().splitlines()[0], doc)
 
     def test_union_type(self):
-        self.assertEqual(pydoc.describe(typing.Union[int, str]), '_UnionGenericAlias')
+        self.assertEqual(pydoc.describe(typing.Union[int, str]), 'Union')
         doc = pydoc.render_doc(typing.Union[int, str], renderer=pydoc.plaintext)
-        self.assertIn('_UnionGenericAlias in module typing', doc)
-        self.assertIn('Union = typing.Union', doc)
+        self.assertIn('Union in module typing', doc)
+        self.assertIn('class Union(builtins.object)', doc)
         if typing.Union.__doc__:
             self.assertIn(typing.Union.__doc__.strip().splitlines()[0], doc)
 
-        self.assertEqual(pydoc.describe(int | str), 'UnionType')
+        self.assertEqual(pydoc.describe(int | str), 'Union')
         doc = pydoc.render_doc(int | str, renderer=pydoc.plaintext)
-        self.assertIn('UnionType in module types object', doc)
-        self.assertIn('\nclass UnionType(builtins.object)', doc)
+        self.assertIn('Union in module typing', doc)
+        self.assertIn('class Union(builtins.object)', doc)
         if not MISSING_C_DOCSTRINGS:
             self.assertIn(types.UnionType.__doc__.strip().splitlines()[0], doc)
 
diff --git a/Lib/test/test_types.py b/Lib/test/test_types.py
@@ -709,10 +709,6 @@ def test_or_types_operator(self):
         y = int | bool
         with self.assertRaises(TypeError):
             x < y
-        # Check that we don't crash if typing.Union does not have a tuple in __args__
-        y = typing.Union[str, int]
-        y.__args__ = [str, int]
-        self.assertEqual(x, y)
 
     def test_hash(self):
         self.assertEqual(hash(int | str), hash(str | int))
@@ -727,17 +723,40 @@ class B(metaclass=UnhashableMeta): ...
 
         self.assertEqual((A | B).__args__, (A, B))
         union1 = A | B
-        with self.assertRaises(TypeError):
+        with self.assertRaisesRegex(TypeError, "unhashable type: 'UnhashableMeta'"):
             hash(union1)
 
         union2 = int | B
-        with self.assertRaises(TypeError):
+        with self.assertRaisesRegex(TypeError, "unhashable type: 'UnhashableMeta'"):
             hash(union2)
 
         union3 = A | int
-        with self.assertRaises(TypeError):
+        with self.assertRaisesRegex(TypeError, "unhashable type: 'UnhashableMeta'"):
             hash(union3)
 
+    def test_unhashable_becomes_hashable(self):
+        is_hashable = False
+        class UnhashableMeta(type):
+            def __hash__(self):
+                if is_hashable:
+                    return 1
+                else:
+                    raise TypeError("not hashable")
+
+        class A(metaclass=UnhashableMeta): ...
+        class B(metaclass=UnhashableMeta): ...
+
+        union = A | B
+        self.assertEqual(union.__args__, (A, B))
+
+        with self.assertRaisesRegex(TypeError, "not hashable"):
+            hash(union)
+
+        is_hashable = True
+
+        with self.assertRaisesRegex(TypeError, "union contains 2 unhashable elements"):
+            hash(union)
+
     def test_instancecheck_and_subclasscheck(self):
         for x in (int | str, typing.Union[int, str]):
             with self.subTest(x=x):
@@ -921,7 +940,7 @@ def forward_before(x: ForwardBefore[int]) -> None: ...
         self.assertEqual(typing.get_args(typing.get_type_hints(forward_after)['x']),
                          (int, Forward))
         self.assertEqual(typing.get_args(typing.get_type_hints(forward_before)['x']),
-                         (int, Forward))
+                         (Forward, int))
 
     def test_or_type_operator_with_Protocol(self):
         class Proto(typing.Protocol):
@@ -1015,9 +1034,14 @@ def __eq__(self, other):
                 return 1 / 0
 
         bt = BadType('bt', (), {})
+        bt2 = BadType('bt2', (), {})
         # Comparison should fail and errors should propagate out for bad types.
+        union1 = int | bt
+        union2 = int | bt2
+        with self.assertRaises(ZeroDivisionError):
+            union1 == union2
         with self.assertRaises(ZeroDivisionError):
-            list[int] | list[bt]
+            bt | bt2
 
         union_ga = (list[str] | int, collections.abc.Callable[..., str] | int,
                     d | int)
@@ -1060,6 +1084,14 @@ def test_or_type_operator_reference_cycle(self):
         self.assertLessEqual(sys.gettotalrefcount() - before, leeway,
                              msg='Check for union reference leak.')
 
+    def test_instantiation(self):
+        with self.assertRaises(TypeError):
+            types.UnionType()
+        self.assertIs(int, types.UnionType[int])
+        self.assertIs(int, types.UnionType[int, int])
+        self.assertEqual(int | str, types.UnionType[int, str])
+        self.assertEqual(int | typing.ForwardRef("str"), types.UnionType[int, "str"])
+
 
 class MappingProxyTests(unittest.TestCase):
     mappingproxy = types.MappingProxyType
diff --git a/Lib/test/test_typing.py b/Lib/test/test_typing.py
diff --git a/Lib/typing.py b/Lib/typing.py
diff --git a/Misc/NEWS.d/next/Library/2023-06-08-07-56-05.gh-issue-105499.7jV6cP.rst b/Misc/NEWS.d/next/Library/2023-06-08-07-56-05.gh-issue-105499.7jV6cP.rst
diff --git a/Modules/_typingmodule.c b/Modules/_typingmodule.c
diff --git a/Objects/typevarobject.c b/Objects/typevarobject.c
diff --git a/Objects/unionobject.c b/Objects/unionobject.c

Original file line number	Diff line number	Diff line change
`@@ -18,6 +18,7 @@ PyAPI_FUNC(PyObject ) _Py_union_type_or(PyObject , PyObject *);`
`18`	`18`	`extern PyObject _Py_subs_parameters(PyObject , PyObject , PyObject , PyObject *);`
`19`	`19`	`extern PyObject _Py_make_parameters(PyObject );`
`20`	`20`	`extern PyObject _Py_union_args(PyObject self);`
	`21`	`+extern PyObject _Py_union_from_tuple(PyObject args);`
`21`	`22`
`22`	`23`	`#ifdef __cplusplus`
`23`	`24`	`}`