[Python-Dev] Backport PEP 3129: Class Decorators

[Eli Courtwright]

Since we're talking about class decorators, I have a question about function
and instancemethod objects.  The following code works

class Root(object):
   def index(self):
       return "Hello World!"
   index.exposed = True

but this code

class Root(object):
   def index(self):
       return "Hello World!"
index.exposed = True

gives the following exception

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'instancemethod' object has no attribute 'exposed'

I can tell that instancemethods can't have attributes added to them outside
of their class definition.  Is this part of the Python language spec, or
just an implementation detail of CPython?

I bring this up here because it makes writing certain class decorators much
more annoying.  For example, if I want to write a class decorator that will
set "exposed=True" for every method of a class, I must resort to
shenanigans.

- Eli
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