python - How to properly subclass dict and override __getitem__ & __setitem__

Question

I am debugging some code and I want to find out when a particular dictionary is accessed. Well, it's actually a class that subclass dict and implements a couple extra features. Anyway, what I would like to do is subclass dict myself and add override __getitem__ and __setitem__ to produce some debugging output. Right now, I have

class DictWatch(dict):
    def __init__(self, *args):
        dict.__init__(self, args)

    def __getitem__(self, key):
        val = dict.__getitem__(self, key)
        log.info("GET %s['%s'] = %s" % str(dict.get(self, 'name_label')), str(key), str(val)))
        return val

    def __setitem__(self, key, val):
        log.info("SET %s['%s'] = %s" % str(dict.get(self, 'name_label')), str(key), str(val)))
        dict.__setitem__(self, key, val)

'name_label' is a key which will eventually be set that I want to use to identify the output. I have then changed the class I am instrumenting to subclass DictWatch instead of dict and changed the call to the superconstructor. Still, nothing seems to be happening. I thought I was being clever, but I wonder if I should be going a different direction.

Thanks for the help!

Answer 1

Another issue when subclassing dict is that the built-in __init__ doesn't call update, and the built-in update doesn't call __setitem__. So, if you want all setitem operations to go through your __setitem__ function, you should make sure that it gets called yourself:

class DictWatch(dict):
    def __init__(self, *args, **kwargs):
        self.update(*args, **kwargs)

    def __getitem__(self, key):
        val = dict.__getitem__(self, key)
        print('GET', key)
        return val

    def __setitem__(self, key, val):
        print('SET', key, val)
        dict.__setitem__(self, key, val)

    def __repr__(self):
        dictrepr = dict.__repr__(self)
        return '%s(%s)' % (type(self).__name__, dictrepr)
        
    def update(self, *args, **kwargs):
        print('update', args, kwargs)
        for k, v in dict(*args, **kwargs).iteritems():
            self[k] = v