How to store the total withdrawal amount for each category object?

Question

I have a Category class and there is a ledger attribute for each instance of this class. This ledger attribute is actually a list of dictionaries which contain the

Answer 1

Use a collections.defaultdict to make aggregations such as that easy as pie.

import collections

# ...

withdrawn_per_category = collections.defaultdict(int)
for i in categories:
    for p in i.ledger:
        if p["amount"] < 0:
            withdrawn_per_category[i.name] += -p["amount"]

(I've opted to use int as the default data type, but it doesn't truly matter here, so long as it's a conversible numeric type.)

Without collections

If for some reason you don't want to use the handy, built-in collections module, you can emulate the same behavior yourself with a regular dict:

withdrawn_per_category = {}
for i in categories:
    for p in i.ledger:
        if p["amount"] < 0:
            withdrawn_per_category[i.name] = withdrawn_per_category.get(i.name, 0) - p["amount"]