keeping a ref to a non-member function in a class

Gregory Bond gnb at itga.com.au
Tue Aug 16 09:57:39 CEST 2005


I'm building a class hierarchy that needs to keep as a class variable a 
reference to a (non-member) function, so that different subclasses can 
use different generator functions.  But it seems Python is treating the 
function as a member function because the reference to it is in class 
scope....

Here's a simple case of what I'm trying (in the real code, fn is a 
function that returns a database connection relevant to that subclass):

> def foo():
>         print "foo called"
> 
> class S(object):
>         fn = foo
> 
>         def bar(cls):
>                 cls.fn()
>         bar = classmethod(bar)
> 
> def foo2():
>         print "foo2 called"
> 
> class D(S):
>         fn = foo2
> 
> D.bar()

[I'm on python 2.3.4 so no @classmethod decorator! ]

When I run this:

> Traceback (most recent call last):
>   File "t_o.py", line 19, in ?
>     D.bar()
>   File "t_o.py", line 10, in bar
>     cls.fn()
> TypeError: unbound method foo2() must be called with D instance as first argument (got nothing instead)

This shows it is trying to do what I expect (call foo2()) but somehow 
the type is all wrong.

I've tried playing with staticmethod() but I can't quite get it all 
worked out...



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