DeltaDict A dictionary for tracking the deltas between updates.

delta_dict.py

#!/usr/bin/python

"""
DeltaDict: A python dictionary for keeping track of the delta between numeric
values.

Insert a number in a key, and you'll get that number back.  Next insert a
higher number.  Now a get() will return the delta between the numbers.

Useful for monitoring applications where you're presented with a monotomically
increasing counter, but only really care about the deltas.  

Future improvement: flag into __init__() to allow absolute value deltas
instead of wrapping around at zero.
"""

import collections

class DeltaDict(collections.MutableMapping) :
    def __init__(self, *args, **kwargs):
        self.store = dict()
        self.delta_store = dict()
        self.update(dict(*args, **kwargs))  # use the free update to set keys

    def __getitem__(self, key):
        #return self.store[self.__keytransform__(key)]
        return self.delta_store[key]

    def __setitem__(self, key, value):
        if not self.store.has_key(key) or value < self.store[key] :
            self.store[key] = value
            self.delta_store[key] = value
        else :
            curval = self.store[key]
            self.delta_store[key] = value - curval
            self.store[key] = value
        #self.store[key] = value

    def __delitem__(self, key):
        del self.store[key]
        del self.delta_store[key]

    def __iter__(self):
        return iter(self.delta_store)

    def __len__(self):
        return len(self.store)