I am building a class which subclasses dict, and overrides __setitem__. I would like to be certain that my method will be called in all instances where dictionary items could possibly be set.

I have discovered three situations where Python (in this case, 2.6.4) does not call my overridden __setitem__ method when setting values, and instead calls PyDict_SetItem directly

  1. In the constructor
  2. In the setdefault method
  3. In the update method

As a very simple test:

class MyDict(dict):
    def __setitem__(self, key, value):
        print "Here"
        super(MyDict, self).__setitem__(key, str(value).upper())

>>> a = MyDict(abc=123)
>>> a['def'] = 234
Here
>>> a.update({'ghi': 345})
>>> a.setdefault('jkl', 456)
456
>>> print a
{'jkl': 456, 'abc': 123, 'ghi': 345, 'def': '234'}

You can see that the overridden method is only called when setting the items explicitly. To get Python to always call my __setitem__ method, I have had to reimplement those three methods, like this:

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

    def __setitem__(self, key, value):
        print "Here"
        super(MyUpdateDict, self).__setitem__(key, value)

    def update(self, *args, **kwargs):
        if args:
            if len(args) > 1:
                raise TypeError("update expected at most 1 arguments, got %d" % len(args))
            other = dict(args[0])
            for key in other:
                self[key] = other[key]
        for key in kwargs:
            self[key] = kwargs[key]

    def setdefault(self, key, value=None):
        if key not in self:
            self[key] = value
        return self[key]

Are there any other methods which I need to override, in order to

python dictionary subclass
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