Accumulating values in dictionary

[Arnaud Delobelle]

Zack <zhalbrecht at gmail.com> writes:

> Given a bunch of objects that all have a certain property, I'd like to
> accumulate the totals of how many of the objects property is a certain
> value. Here's a more intelligible example:
>
> Users all have one favorite food. Let's create a dictionary of
> favorite foods as the keys and how many people have that food as their
> favorite as the value.
>
> d = {}
> for person in People:
>     fav_food = person.fav_food
>     if d.has_key(fav_food):
>         d[fav_food] = d[fav_food] + 1
>     else:
>         d[fav_food] = 1

This is fine.  If I wrote this, I'd write it like this:

d = {}
for person in people:
    fav_food = person.fav_food
    if fav_food in d:
        d[fav_food] += 1
    else:
        d[fav_food] = 1

You can use d.get() instead:

d = {}
for person in people:
    fav_food = person.fav_food
    d[fav_food] = d.get(fav_food, 0) + 1

Or you can use a defaultdict:

from collections import defaultdict
d = defaultdict(int) # That means the default value will be 0
for person in people:
    d[person.fav_food] += 1

HTH

-- 
Arnaud