gh-137065: Unmerge types.UnionType and typing.Union

Partially revert GH-105499.

types.UnionType no longer an alias of typing.Union and no longer subscriptable,
but subscribing typing.Union returns an instance of types.UnionType.
typing.Union can no longer be used in the isinstance() checks.

Author: serhiy-storchaka

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

@@ -123,7 +123,7 @@ 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
+  :data:`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`.)
 

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::
+   :data:`typing.Union` can also be used::
 
     >>> @fun.register
     ... def _(arg: int | float, verbose=False):
@@ -663,7 +663,7 @@ The :mod:`functools` module defines the following functions:
 
    .. versionchanged:: 3.11
       The :func:`~singledispatch.register` attribute now supports
-      :class:`typing.Union` as a type annotation.
+      :data:`typing.Union` as a type annotation.
 
 
 .. class:: singledispatchmethod(func)

Doc/library/stdtypes.rst

@@ -5573,7 +5573,7 @@ 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 subscripting :data:`typing.Union`.
 
 .. describe:: X | Y | ...
 
@@ -5609,7 +5609,7 @@ enables cleaner type hinting syntax compared to subscripting :class:`typing.Unio
 
       int | str == str | int
 
-   * It creates instances of :class:`typing.Union`::
+   * It creates instances of :class:`types.UnionType`::
 
       int | str == typing.Union[int, str]
       type(int | str) is typing.Union
@@ -5638,15 +5638,15 @@ 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::
 
-   >>> 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
@@ -5673,11 +5673,6 @@ 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

@@ -316,7 +316,9 @@ Standard names are defined for the following types:
 
    .. versionchanged:: 3.14
 
-      This is now an alias for :class:`typing.Union`.
+      Added read-only attributes :attr:`!__name__`, :attr:`!__qualname__`
+      and :attr:`!__origin__`.
+
 
 .. class:: TracebackType(tb_next, tb_frame, tb_lasti, tb_lineno)
 

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.
 
-.. class:: Union
+.. data:: Union
 
    Union type; ``Union[X, Y]`` is equivalent to ``X | Y`` and means either X or Y.
 
@@ -1128,10 +1128,10 @@ These can be used as types in annotations. They all support subscription using
       :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
+      Both ``Union[int, str]`` and ``int | str`` now create instances of
+      the same class, :class:`types.UnionType`.
+      To check whether an object is a union at runtime, use
+      ``isinstance(obj, types.UnionType)``. For compatibility with earlier versions of
       Python, use
       ``get_origin(obj) is typing.Union or get_origin(obj) is types.UnionType``.
 

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

@@ -2089,8 +2089,8 @@ turtle
 types
 -----
 
-* :class:`types.UnionType` is now an alias for :class:`typing.Union`.
-  See :ref:`below <whatsnew314-typing-union>` for more details.
+* Add read-only attributes :attr:`!__name__`, :attr:`!__qualname__` and
+  :attr:`!__origin__` for :class:`types.UnionType` instances.
   (Contributed by Jelle Zijlstra in :gh:`105499`.)
 
 
@@ -2099,38 +2099,22 @@ 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
+* Both old-style unions (created with ``Union[int, str]``) and new-style
+  unions (``int | str``) now create instances of the same runtime type,
+  :class:`types.UnionType`. 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.
+  - The ``__args__`` attribute of :data:`typing.Union` objects is no longer writable.
+  - It is no longer possible to set any attributes on the :data:`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.
 
@@ -2782,8 +2766,8 @@ 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.
+* Subscription of :data:`typing.Union` now returns a :class:`types.UnionType`
+  instance, causing changes in some behaviors.
   See :ref:`above <whatsnew314-typing-union>` for more details.
   (Contributed by Jelle Zijlstra in :gh:`105499`.)
 

Lib/annotationlib.py

@@ -6,6 +6,7 @@
 import keyword
 import sys
 import types
+from _typing import _make_union
 
 __all__ = [
     "Format",
@@ -292,10 +293,10 @@ def __hash__(self):
         ))
 
     def __or__(self, other):
-        return types.UnionType[self, other]
+        return _make_union(self, other)
 
     def __ror__(self, other):
-        return types.UnionType[other, self]
+        return _make_union(other, self)
 
     def __repr__(self):
         extra = []

Lib/inspect.py

@@ -1333,7 +1333,8 @@ def getargvalues(frame):
 def formatannotation(annotation, base_module=None, *, quote_annotation_strings=True):
     if not quote_annotation_strings and isinstance(annotation, str):
         return annotation
-    if getattr(annotation, '__module__', None) == 'typing':
+    if (isinstance(annotation, types.UnionType)
+        or getattr(annotation, '__module__', None) == 'typing'):
         def repl(match):
             text = match.group()
             return text.removeprefix('typing.')