bpo-42010: [docs] Clarify subscription of types (GH-22822)
diff --git a/Doc/reference/expressions.rst b/Doc/reference/expressions.rst
index 81dd6fc..938a973 100644
--- a/Doc/reference/expressions.rst
+++ b/Doc/reference/expressions.rst
@@ -800,8 +800,8 @@
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` "]"
@@ -837,6 +837,17 @@
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: