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To: python-list@python.org
From: Terry Reedy <tjreedy@udel.edu>
Subject: Re: Get classes from "self.MyClass" to improve subclassability
Date: Fri, 12 Jun 2015 19:08:43 -0400
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On 6/12/2015 7:12 AM, Thomas G=C3=BCttler wrote:
> Here is a snippet from the argparse module:
>
> {{{
>      def parse_known_args(self, args=3DNone, namespace=3DNone):
>          ...
>          # default Namespace built from parser defaults
>          if namespace is None:
>              namespace =3D Namespace() # < =3D=3D=3D=3D=3D=3D=3D my iss=
ue
> }}}
>
> I subclass from the class of the above snippet.
>
> I would like to use a different Namespace class.
>
> if the above could would use
>
>      namespace =3D self.Namespace()
>
> it would be very easy for me to inject a different Namespace class.

The default arg (None) for the namespace parameter of the=20
parse_known_args is an attribute of the function, not of the class or=20
instance thereof.  Unless the default Namespace is used elsewhere, this=20
seems sensible.

In CPython, at least, and probably in pypy, you can change this=20
attribute.  (But AFAIK, this is not guaranteed in all implementations.)

 >>> def f(n =3D 1): pass

 >>> f.__defaults__
(1,)
 >>> f.__defaults__ =3D (2,)
 >>> f.__defaults__
(2,)

So the following works

 >>> class C():
	def f(n=3D1): print(n)
=09
 >>> class D(C):
	C.f.__defaults__ =3D (2,)
=09
 >>> D.f()
2

Of course, this may or may not do more than you want.

 >>> C.f()
2

--=20
Terry Jan Reedy