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

Co-authored-by: Alex Waygood <[email protected]>
Co-authored-by: Ken Jin <[email protected]>
Co-authored-by: Carl Meyer <[email protected]>

Author: 4 people

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.
 

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)

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:
 

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_frame, tb_lasti, tb_lineno)
 
    The type of traceback objects such as found in ``sys.exception().__traceback__``.

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_origin(obj) is typing.Union or get_origin(obj) is types.UnionType``.
+
 .. data:: Optional
 
    ``Optional[X]`` is equivalent to ``X | None`` (or ``Union[X, None]``).

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

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

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
 }

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_origin, Union
-        return get_origin(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

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