What is the status of the __subclasses__ method?

[Terry Reedy]

"Michael Hudson" <mwh at python.net> wrote in message
news:m3y8qn73oo.fsf at pc150.maths.bris.ac.uk...
> I'm not sure this attitude really applies to Python.  There's no real
> definition of what "officially part of the language" means.

I think 'in the Python Reference Manual' qualifies pretty well as defining
the language.  The Python Library Reference fleshes out the current PSF
CPython distribution. Any discrepancies between these and the (C)Python
implementation are considered bugs to be fixed.  Guido has (as I remember)
occasionally omitted (and rejected patches about) items that he wants to
remain unofficial implementation details subject to possible change.

Someone using an undocumented implementation details like __subclasses__()
might especially want to test code against beta releases as they come out
to either protest a change or start adjusting.  But the OP can relex
somewhat since code-breaking changes are not made intentionally without
some clear benefit otherwise.

Terry J. Reedy