Reducing dict in Python - a dict that reduces values as they are entered

gistfile1.py

import copy

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]
            reduced_value = self._function(curr_value, new_value)
            if reduced_value == curr_value:
                return
        elif self._initial:
            new_value = self._function(copy.copy(self._initial), new_value)
        super(ReducingDict, self).__setitem__(key, new_value)