__call__ considered harmful or indispensable?

[Carl Banks]

On Aug 2, 2:30 pm, s... at pobox.com wrote:
> I'm wondering, are there some general cases where __call__ methods of
> a user-defined class are simply indispensable?

Indispensable's a strong word, but one thing that entails calling
syntax, and can't (reasonably) be done with closures is to override a
base class's __call__ method.

There is one notable type in Python that defines __call__ and that you
might want to subclass, namely type itself.  So using __call__ is
indispensible for some metaclasses.

Here's my little example.  I'm sure people could work out ways to do
it without __call__ or even a metaclass, but I doubt it'd be so
concise and out of the way as this:

class CachedResourceMetaclass(type):
    def __init__(self,name,bases,clsdict):
 
super(CachedResourceMetaclass,self).__init__(name,bases,clsdict)
        self._cache = {}
        self._releasefunc = self.release
        def decref(obj):
            obj._refcount -= 1
            if obj._refcount <= 0:
                del self._cache[obj._key]
                obj._releasefunc()
        self.release = decref
    def __call__(self,*args):
        obj = self._cache.get(args)
        if obj is not None:
            obj._refcount += 1
        else:
            obj = super(CachedResourceMetaclass,self).__call__(*args)
            obj._key = args
            obj._refcount = 1
            self._cache[args] = obj
        return obj


Carl Banks