http://www.python.org/dev/peps/pep-3119/#overloading-isinstance-and-issubclass
(GvR and Talin)
"The primary mechanism proposed here is to allow overloading the
built-in functions isinstance() and issubclass(). The overloading works
as follows: The call isinstance(x, C) first checks whether
C.__instancecheck__ exists, and if so, calls C.__instancecheck__(x)
instead of its normal implementation. Similarly, the call issubclass(D,
C) first checks whether C.__subclasscheck__ exists, and if so, calls
C.__subclasscheck__(D) instead of its normal implementation."

These two new special methods are not documented (for 3.x at least.  A
post on python-list today verifies that at least .__instancecheck__ was
implemented (version not specified).  I assume .__subclasscheck__ was
also.  This issue perhaps applies to 2.6/2.7 also.

I suggest either adding "Customizing instance and subclass checks" after
"Customizing class creation" or add to that section and rename it to
"Customizing class creation and checks" since the needed addition seems
too small for its own section.  Something like

The result of isinstance(object, class) can be modified by giving the
*class* a .__instancecheck__(object) method.  The result of
issubclass(sub,super) can be modified by giving the *superclass* a
.__subclasscheck__(sub) method.

Here's a draft of an updated version of the "Data model" docs reflecting the changes in the PEP.

Committed a similar patch in r80081.  Thanks very much!

This might need to be reopened.

There's a verb missing from he following sentence of the patch:
"This is consistent with the lookup of special methods that called on instances, only that in this case the instance is itself a class."

Secondly, and more importantly, the issubclass() and isinstance() docs don't mention the new special methods in question.

Fixed the missing verb in r80084.  I don't think the new special methods need to be mention in isinstance() or issubclass() docs; they are very rarely needed, and other builtins don't mention their special methods either (e.g. abs(), complex(), or getattr().)

One last triviality: Could the mention of __subclasscheck__() in http://docs.python.org/dev/library/abc.html#abc.ABCMeta.__subclasshook__ get linked to the newly-added docs?