#5250: document __instancecheck__ and __subclasscheck__. I hope the part about the class/metaclass distinction is understandable.
diff --git a/Doc/reference/datamodel.rst b/Doc/reference/datamodel.rst
index 2dcaed2..9b18715 100644
--- a/Doc/reference/datamodel.rst
+++ b/Doc/reference/datamodel.rst
@@ -1762,6 +1762,48 @@
locking/synchronization.
+Customizing instance and subclass checks
+----------------------------------------
+
+.. versionadded:: 2.6
+
+The following methods are used to override the default behavior of the
+:func:`isinstance` and :func:`issubclass` built-in functions.
+
+In particular, the metaclass :class:`abc.ABCMeta` implements these methods in
+order to allow the addition of Abstract Base Classes (ABCs) as "virtual base
+classes" to any class or type (including built-in types), and including to other
+ABCs.
+
+.. method:: class.__instancecheck__(self, instance)
+
+ Return true if *instance* should be considered a (direct or indirect)
+ instance of *class*. If defined, called to implement ``isinstance(instance,
+ class)``.
+
+
+.. method:: class.__subclasscheck__(self, subclass)
+
+ Return true if *subclass* should be considered a (direct or indirect)
+ subclass of *class*. If defined, called to implement ``issubclass(subclass,
+ class)``.
+
+
+Note that these methods are looked up on the type (metaclass) of a class. They
+cannot be defined as class methods in the actual class. This is consistent with
+the lookup of special methods that called on instances, only that in this case
+the instance is itself a class.
+
+.. seealso::
+
+ :pep:`3119` - Introducing Abstract Base Classes
+ Includes the specification for customizing :func:`isinstance` and
+ :func:`issubclass` behavior through :meth:`__instancecheck__` and
+ :meth:`__subclasscheck__`, with motivation for this functionality in the
+ context of adding Abstract Base Classes (see the :mod:`abc` module) to the
+ language.
+
+
.. _callable-types:
Emulating callable objects