bpo-41428: Documentation for PEP 604 (gh-22517)

Author: Fidget-Spinner

Doc/library/stdtypes.rst

@@ -4743,6 +4743,128 @@ define these methods must provide them as a normal Python accessible method.
 Compared to the overhead of setting up the runtime context, the overhead of a
 single class dictionary lookup is negligible.
 
+.. _types-union:
+
+Union Type
+==========
+
+.. index::
+   object: Union
+   pair: 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 type annotations. The union type expression enables cleaner
+type hinting syntax compared to :data:`typing.Union`.
+
+.. describe:: X | Y | ...
+
+   Defines a union object which holds types *X*, *Y*, and so forth. ``X | Y``
+   means either X or Y.  It is equivalent to ``typing.Union[X, Y]``.
+   Example::
+
+      def square(number: int | float) -> int | float:
+          return number ** 2
+
+.. describe:: union_object == other
+
+   Union objects can be tested for equality with other union objects.  Details:
+
+   * Unions of unions are flattened, e.g.::
+
+       (int | str) | float == int | str | float
+
+   * Redundant types are removed, e.g.::
+
+       int | str | int == int | str
+
+   * When comparing unions, the order is ignored, e.g.::
+
+      int | str == str | int
+
+   * It is compatible with :data:`typing.Union`::
+
+      int | str == typing.Union[int, str]
+
+   * Optional types can be spelled as a union with ``None``::
+
+      str | None == typing.Optional[str]
+
+.. describe:: isinstance(obj, union_object)
+
+   Calls to :func:`isinstance` are also supported with a Union object::
+
+      >>> isinstance("", int | str)
+      True
+
+   ..
+      At the time of writing this, there is no documentation for parameterized
+      generics or PEP 585. Thus the link currently points to PEP 585 itself.
+      Please change the link for parameterized generics to reference the correct
+      documentation once documentation for PEP 585 becomes available.
+
+   However, union objects containing `parameterized generics
+   <https://www.python.org/dev/peps/pep-0585/>`_ cannot be used::
+
+      >>> isinstance(1, int | list[int])
+      Traceback (most recent call last):
+        File "<stdin>", line 1, in <module>
+      TypeError: isinstance() argument 2 cannot contain a parameterized generic
+
+.. describe:: issubclass(obj, union_object)
+
+   Calls to :func:`issubclass` are also supported with a Union Object.::
+
+      >>> issubclass(bool, int | str)
+      True
+
+   ..
+      Once again, please change the link below for parameterized generics to
+      reference the correct documentation once documentation for PEP 585
+      becomes available.
+
+   However, union objects containing `parameterized generics
+   <https://www.python.org/dev/peps/pep-0585/>`_ cannot be used::
+
+      >>> issubclass(bool, bool | list[str])
+      Traceback (most recent call last):
+        File "<stdin>", line 1, in <module>
+      TypeError: issubclass() argument 2 cannot contain a parameterized generic
+
+The type for the Union object is :data:`types.Union`.  An object cannot be
+instantiated from the type::
+
+   >>> import types
+   >>> isinstance(int | str, types.Union)
+   True
+   >>> types.Union()
+   Traceback (most recent call last):
+     File "<stdin>", line 1, in <module>
+   TypeError: cannot create 'types.Union' instances
+
+.. note::
+   The :meth:`__or__` method for type objects was added to support the syntax
+   ``X | Y``.  If a metaclass implements :meth:`__or__`, the Union may
+   override it::
+
+      >>> class M(type):
+      ...     def __or__(self, other):
+      ...     return "Hello"
+      ...
+      >>> class C(metaclass=M):
+      ...     pass
+      ...
+      >>> C | int
+      'Hello'
+      >>> int | C
+      int | __main__.C
+
+.. seealso::
+
+   :pep:`604` -- PEP proposing the ``X | Y`` syntax and the Union type.
+
+.. versionadded:: 3.10
+
 
 .. _typesother:
 

Doc/library/types.rst

@@ -256,6 +256,11 @@ Standard names are defined for the following types:
 
    .. versionadded:: 3.10
 
+.. data:: Union
+
+   The type of :ref:`union type expressions<types-union>`.
+
+   .. versionadded:: 3.10
 
 .. class:: TracebackType(tb_next, tb_frame, tb_lasti, tb_lineno)
 

Doc/library/typing.rst

@@ -544,6 +544,10 @@ These can be used as types in annotations using ``[]``, each having a unique syn
    .. versionchanged:: 3.7
       Don't remove explicit subclasses from unions at runtime.
 
+   .. versionchanged:: 3.10
+      Unions can now be written as ``X | Y``. See
+      :ref:`union type expressions<types-union>`.
+
 .. data:: Optional
 
    Optional type.

Doc/whatsnew/3.10.rst

@@ -82,6 +82,29 @@ New Features
 * :pep:`618`: The :func:`zip` function now has an optional ``strict`` flag, used
   to require that all the iterables have an equal length.
 
+PEP604: New Type 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 (annotations).
+
+In previous versions of Python, to apply a type hint for functions accepting
+arguments of multiple types, :data:`typing.Union` was used::
+
+   def square(number: Union[int, float]) -> Union[int, float]:
+       return number ** 2
+
+
+Now, type hints can be written in a more succinct manner::
+
+   def square(number: int | float) -> int | float:
+       return number ** 2
+
+
+See :pep:`604` for more details.
+
+(Contributed by Maggie Moss and Philippe Prados in :issue:`41428`.)
 
 Other Language Changes
 ======================

Misc/ACKS

@@ -1257,6 +1257,7 @@ Grant Olson
 Furkan Onder
 Koray Oner
 Ethan Onstott
+Ken Jin Ooi
 Piet van Oostrum
 Tomas Oppelstrup
 Jason Orendorff

Misc/NEWS.d/next/Documentation/2020-10-03-18-20-46.bpo-41428._ju1NE.rst

@@ -0,0 +1 @@
+Add documentation for :pep:`604` (Allow writing union types as ``X | Y``).