Overriding methods per-object

[Steven D'Aprano]

On Fri, 17 Apr 2009 18:22:49 -0700, Pavel Panchekha wrote:

> I've got an object which has a method, __nonzero__ The problem is, that
> method is attached to that object not that class
> 
>> a = GeneralTypeOfObject()
>> a.__nonzero__ = lambda: False
>> a.__nonzero__()
> False
> 
> But:
> 
>> bool(a)
> True
> 
> What to do?

(1) Don't do that.

(2) If you *really* have to do that, you can tell the class to look at 
the instance:

class GeneralTypeOfObject(object):
    def __nonzero__(self):
        try:
            return self.__dict__['__nonzero__']
        except KeyError:
            return something

(3) But a better solution might be to subclass the class and use an 
instance of that instead. You can even change the class on the fly.

a.__class__ = SubclassGeneralTypeOfObject  # note the lack of brackets




-- 
Steven