[3.9] bpo-42010: [docs] Clarify subscription of types (GH-22822) (GH-22840)

(cherry picked from commit 7cdf30fff39ea97f403b5472096349998d190e30)

Co-authored-by: kj <28750310+Fidget-Spinner@users.noreply.github.com>
Automerge-Triggered-By: GH:gvanrossum

diff --git a/Doc/reference/expressions.rst b/Doc/reference/expressions.rst
index b68c29860cf3329ccd8020293ab60754eb45c37f..f1f19c3f02d3907c4538e7b116a9a07bf89f0c72 100644 (file)
--- a/Doc/reference/expressions.rst
+++ b/Doc/reference/expressions.rst
@@ -796,8 +796,8 @@ Subscriptions
    object: dictionary
    pair: sequence; item
 
-A subscription selects an item of a sequence (string, tuple or list) or mapping
-(dictionary) object:
+Subscription of a sequence (string, tuple or list) or mapping (dictionary)
+object usually selects an item from the collection:
 
 .. productionlist:: python-grammar
    subscription: `primary` "[" `expression_list` "]"
@@ -833,6 +833,17 @@ this method will need to explicitly add that support.
 A string's items are characters.  A character is not a separate data type but a
 string of exactly one character.
 
+..
+   At the time of writing this, there is no documentation for generic alias
+   or PEP 585.  Thus the link currently points to PEP 585 itself.
+   Please change the link for generic alias to reference the correct
+   documentation once documentation for PEP 585 becomes available.
+
+Subscription of certain :term:`classes <class>` or :term:`types <type>`
+creates a `generic alias <https://www.python.org/dev/peps/pep-0585/>`_.
+In this case, user-defined classes can support subscription by providing a
+:meth:`__class_getitem__` classmethod.
+
 
 .. _slicings:
 

diff --git a/Misc/NEWS.d/next/Documentation/2020-10-21-02-21-14.bpo-42010.76vJ0u.rst b/Misc/NEWS.d/next/Documentation/2020-10-21-02-21-14.bpo-42010.76vJ0u.rst
new file mode 100644 (file)
index 0000000..2a0cbf1
--- /dev/null
+++ b/Misc/NEWS.d/next/Documentation/2020-10-21-02-21-14.bpo-42010.76vJ0u.rst
@@ -0,0 +1,4 @@
+Clarify that subscription expressions are also valid for certain
+:term:`classes <class>` and :term:`types <type>` in the standard library, and
+for user-defined classes and types if the classmethod
+:meth:`__class_getitem__` is provided.