Question

I find myself frequently making indexed lists from flat ones in Python. This is such a common task that I was wondering if there’s a standard utility that I should be using for it.

The context is this: given an array, I need to create a dict of smaller arrays using some key for grouping.

e.g:
["Andy","Alice","Bob","Beth","Charlie"] becomes
{"A":["Andy","Alice"],"B":["Bob","Beth"],"C":["Charlie"]}

My solution looks like this:

def make_index(data,key,value=lambda x:x):
    d={}
    for item in data:
        k = key(item)
        v = value(item)
        try: d[k].append(v)
        except KeyError: d[k]=[v]
    return d

It’s simple and all, but am I reinventing something that is implemented better elsewhere?

Answer 1

You can do the same a little simpler with a defaultdict:

from collections import defaultdict

def make_index(data,key,value=lambda x:x):
    d=defaultdict(list)
    for item in data:
        d[key(item)].append(value(item))
    return d

Using a defaultdict is faster than using .setdefault(), which would be the other option.