watching mutables?

[Skip Montanaro]

    Anton> Variables should be 'marked' to install this function for
    Anton> them. For example 'watch(m,f)' indicates that function f is
    Anton> called if mutable variable m is changed. 

You could implement a proxy object which delegates to the real object and
catches all __setattr__ and __setitem__ calls.  Something like:

    class MutableProxy:
        def __init__(self, obj):
            self.__dict__['obj'] = obj

        def __setattr__(self, attr, val):
            print "Hey! My %s attribute got diddled!" % attr
            setattr(self.__dict__['obj'], attr, val)

        def __getattr__(self, attr):
            return getattr(self.__dict__['obj'], attr)

    if __name__ == "__main__":
        class Dummy:
            pass

        d = MutableProxy(Dummy())
        d.x = 1
        print d.x

__setitem__ and __getitem__ are similar, but you have to worry about ranges.
You'd probably have to wrap __call__ as well.

There's probably a way to do it with metaclasses, but ...  ooh! I just got a
headache, better back off before my brain expl ... <* blam! *>

-- 
Skip Montanaro - skip at pobox.com
"Airplanes don't fly until the paperwork equals the weight of the
aircraft. Same with i18N." - from the "Perl, Unicode and i18N FAQ"