Calculating Difference from two different lists in Python

Question

I have two different sets of Lists and new list always have new value and i want to know how much the value has changed ?

previous=[ [525, 'test', 'A3'],[522, 'test', 'A4']]

current = [ [525, 'test', 'A3'],[622, 'test', 'A4'],[320, 'test', 'A5']]

for A4 the difference is 100 How can able to calculate the difference ?

Desired output:

 A4 = 100
 A5 = 320

Answer 1

This code could solve your problem (I followed the idea of Chris Charley to use a dictionary in order to map the keys A3, A4, ... to their integer values for each entry in the previous variable).

from operator import itemgetter

previous=[ [525, 'test', 'A3'],[522, 'test', 'A4']]
current = [ [525, 'test', 'A3'],[622, 'test', 'A4'],[320, 'test', 'A5']]

def differences():
    d = dict(map(itemgetter(2, 0), previous))
    for key, value in map(itemgetter(2, 0), current):
        difference = abs(d.get(key, 0) - value)
        if difference > 0:
            yield key, difference

output = dict(differences())

The output will be:

{'A4': 100, 'A5': 320}

differences() its a generator function that returns for each entry in the current list, a tuple with two items: The key A* and the amount of change between the previous and current lists (only if the difference is not 0) e.g: ('A4', 100)