Reducing Dictionary in Python

reducing_dict.py

class ReducingDict(dict):

    def __init__(self, function, initial = None):
        self._function = function
        self._initial = initial
        super(ReducingDict, self).__init__()

    def __setitem__(self, key, new_value):
        if key in self:
            curr_value = self[key]
            new_value = self._function(curr_value, new_value)
            if new_value == curr_value:
                return
        elif self._initial != None:
            new_value = self._function(copy.copy(self._initial), new_value)
        super(ReducingDict, self).__setitem__(key, new_value)

This dictionary will reduce items with similar keys. For example, it can be used to count or some items by key, group items into lists by category, etc. Feedback welcome.

Here's an example of a dictionary adding numbers:

import operator
counter = ReducingDict(operator.add)
counter['x']=1
counter['y']=2
counter['x']=3

print counter  → {'x': 4, 'y': 2}