gh-137065: Revert "gh-105499: Merge typing.Union and types.UnionType (#105511)"

This reverts commit d1db43c.
This reverts commit 0f511d8.
This reverts commit dc6d66f.

Author: serhiy-storchaka

Doc/deprecations/pending-removal-in-future.rst

@@ -122,11 +122,6 @@ 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

@@ -526,7 +526,7 @@ The :mod:`functools` module defines the following functions:
      ...     for i, elem in enumerate(arg):
      ...         print(i, elem)
 
-   :class:`typing.Union` can also be used::
+   :class:`types.UnionType` and :data:`typing.Union` can also be used::
 
     >>> @fun.register
     ... def _(arg: int | float, verbose=False):
@@ -662,8 +662,8 @@ The :mod:`functools` module defines the following functions:
       The :func:`~singledispatch.register` attribute now supports using type annotations.
 
    .. versionchanged:: 3.11
-      The :func:`~singledispatch.register` attribute now supports
-      :class:`typing.Union` as a type annotation.
+      The :func:`~singledispatch.register` attribute now supports :class:`types.UnionType`
+      and :data:`typing.Union` as type annotations.
 
 
 .. class:: singledispatchmethod(func)

Doc/library/stdtypes.rst

@@ -5649,7 +5649,7 @@ 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 subscripting :class:`typing.Union`.
+enables cleaner type hinting syntax compared to :data:`typing.Union`.
 
 .. describe:: X | Y | ...
 
@@ -5685,10 +5685,9 @@ enables cleaner type hinting syntax compared to subscripting :class:`typing.Unio
 
       int | str == str | int
 
-   * It creates instances of :class:`typing.Union`::
+   * It is compatible with :data:`typing.Union`::
 
       int | str == typing.Union[int, str]
-      type(int | str) is typing.Union
 
    * Optional types can be spelled as a union with ``None``::
 
@@ -5714,15 +5713,16 @@ enables cleaner type hinting syntax compared to subscripting :class:`typing.Unio
       TypeError: isinstance() argument 2 cannot be a parameterized generic
 
 The user-exposed type for the union object can be accessed from
-:class:`typing.Union` and used for :func:`isinstance` checks::
+:class:`types.UnionType` and used for :func:`isinstance` checks.  An object cannot be
+instantiated from the type::
 
-   >>> import typing
-   >>> isinstance(int | str, typing.Union)
+   >>> import types
+   >>> isinstance(int | str, types.UnionType)
    True
-   >>> typing.Union()
+   >>> types.UnionType()
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
-   TypeError: cannot create 'typing.Union' instances
+   TypeError: cannot create 'types.UnionType' instances
 
 .. note::
    The :meth:`!__or__` method for type objects was added to support the syntax
@@ -5749,11 +5749,6 @@ The user-exposed type for the union object can be accessed from
 
 .. 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

@@ -314,10 +314,6 @@ 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

@@ -1091,7 +1091,7 @@ Special forms
 These can be used as types in annotations. They all support subscription using
 ``[]``, but each has a unique syntax.
 
-.. class:: Union
+.. data:: Union
 
    Union type; ``Union[X, Y]`` is equivalent to ``X | Y`` and means either X or Y.
 
@@ -1132,14 +1132,6 @@ 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

@@ -723,10 +723,10 @@ 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 :class:`typing.Union`, especially in type hints.
+using :data:`typing.Union`, especially in type hints.
 
 In previous versions of Python, to apply a type hint for functions accepting
-arguments of multiple types, :class:`typing.Union` was used::
+arguments of multiple types, :data:`typing.Union` was used::
 
    def square(number: Union[int, float]) -> Union[int, float]:
        return number ** 2

Doc/whatsnew/3.11.rst

@@ -741,7 +741,7 @@ functools
 ---------
 
 * :func:`functools.singledispatch` now supports :class:`types.UnionType`
-  and :class:`typing.Union` as annotations to the dispatch argument.::
+  and :data:`typing.Union` as annotations to the dispatch argument.::
 
     >>> from functools import singledispatch
     >>> @singledispatch

Doc/whatsnew/3.14.rst

@@ -2071,56 +2071,9 @@ turtle
   (Contributed by Marie Roald and Yngve Mardal Moe in :gh:`126350`.)
 
 
-types
------
-
-* :class:`types.UnionType` is now an alias for :class:`typing.Union`.
-  See :ref:`below <whatsnew314-typing-union>` for more details.
-  (Contributed by Jelle Zijlstra in :gh:`105499`.)
-
-
 typing
 ------
 
-.. _whatsnew314-typing-union:
-
-* :class:`types.UnionType` and :class:`typing.Union` are now aliases for each other,
-  meaning that both old-style unions (created with ``Union[int, str]``) and new-style
-  unions (``int | str``) now create instances of the same runtime type. This unifies
-  the behavior between the two syntaxes, but leads to some differences in behavior that
-  may affect users who introspect types at runtime:
-
-  - Both syntaxes for creating a union now produce the same string representation in
-    ``repr()``. For example, ``repr(Union[int, str])``
-    is now ``"int | str"`` instead of ``"typing.Union[int, str]"``.
-  - Unions created using the old syntax are no longer cached. Previously, running
-    ``Union[int, str]`` multiple times would return the same object
-    (``Union[int, str] is Union[int, str]`` would be ``True``), but now it will
-    return two different objects. Users should use ``==`` to compare unions for equality, not
-    ``is``. New-style unions have never been cached this way.
-    This change could increase memory usage for some programs that use a large number of
-    unions created by subscripting ``typing.Union``. However, several factors offset this cost:
-    unions used in annotations are no longer evaluated by default in Python 3.14
-    because of :pep:`649`; an instance of :class:`types.UnionType` is
-    itself much smaller than the object returned by ``Union[]`` was on prior Python
-    versions; and removing the cache also saves some space. It is therefore
-    unlikely that this change will cause a significant increase in memory usage for most
-    users.
-  - Previously, old-style unions were implemented using the private class
-    ``typing._UnionGenericAlias``. This class is no longer needed for the implementation,
-    but it has been retained for backward compatibility, with removal scheduled for Python
-    3.17. Users should use documented introspection helpers like :func:`typing.get_origin`
-    and :func:`typing.get_args` instead of relying on private implementation details.
-  - It is now possible to use :class:`typing.Union` itself in :func:`isinstance` checks.
-    For example, ``isinstance(int | str, typing.Union)`` will return ``True``; previously
-    this raised :exc:`TypeError`.
-  - The ``__args__`` attribute of :class:`typing.Union` objects is no longer writable.
-  - It is no longer possible to set any attributes on :class:`typing.Union` objects.
-    This only ever worked for dunder attributes on previous versions, was never
-    documented to work, and was subtly broken in many cases.
-
-  (Contributed by Jelle Zijlstra in :gh:`105499`.)
-
 * :class:`typing.TypeAliasType` now supports star unpacking.
 
 
@@ -3189,11 +3142,6 @@ Changes in the Python API
   This temporary change affects other threads.
   (Contributed by Serhiy Storchaka in :gh:`69998`.)
 
-* :class:`types.UnionType` is now an alias for :class:`typing.Union`,
-  causing changes in some behaviors.
-  See :ref:`above <whatsnew314-typing-union>` for more details.
-  (Contributed by Jelle Zijlstra in :gh:`105499`.)
-
 * The runtime behavior of annotations has changed in various ways; see
   :ref:`above <whatsnew314-pep649>` for details. While most code that interacts
   with annotations should continue to work, some undocumented details may behave

Include/internal/pycore_unionobject.h

@@ -18,7 +18,6 @@ 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

@@ -929,11 +929,16 @@ 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
-        return (isinstance(cls, UnionType) and
-                all(isinstance(arg, type) for arg in cls.__args__))
+        from typing import get_args
+        return (_is_union_type(cls) and
+                all(isinstance(arg, type) for arg in get_args(cls)))
 
     def register(cls, func=None):
         """generic_func.register(cls, func) -> func
@@ -965,7 +970,7 @@ 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 isinstance(cls, UnionType):
+                if _is_union_type(cls):
                     raise TypeError(
                         f"Invalid annotation for {argname!r}. "
                         f"{cls!r} not all arguments are classes."
@@ -981,8 +986,10 @@ def register(cls, func=None):
                         f"{cls!r} is not a class."
                     )
 
-        if isinstance(cls, UnionType):
-            for arg in cls.__args__:
+        if _is_union_type(cls):
+            from typing import get_args
+
+            for arg in get_args(cls):
                 registry[arg] = func
         else:
             registry[cls] = func