How to update the values in a dict cleanly?

Question

I have a piece of code which is iterating a list and then accumulate the values in a dict based on the keys, the ugly part is that I need to check if the key already exist in the dict, if not then I need to set the value to zero first.

I'm wondering if there's more elegant style to implement this:

for r in records:
    if not d.has_key(r[0]):
        d[r[0]] = 0
    d[r[0]] += r[1]

And further more, if the value is a list, then how to rewrite the following code:

for (key, value) in records:
    if not d.has_key(key):
        d[key] = []
    d[key].append(value)

Is your list containing tuples because you have used it like r[0] and r[1] ? — Tanveer Alam, Jul 02 '14 at 06:38

score 3 · Accepted Answer · edited May 23 '17 at 10:32

3

You can use the builtin dict.get method to get the default value, like this

for r in records:
    d[r[0]] = d.get(r[0], 0) + r[1]

The second argument to dict.get is the default value to be returned if the key is not there in the dictionary.

Even better, you can unpack the values, like this

for (key, value) in records:
    d[key] = d.get(key, 0) + value

edited May 23 '17 at 10:32

Community

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1

answered Jul 02 '14 at 05:58

thefourtheye

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Thanks and it really looks better! BTW, if the value is a list instead of accumulated numerical, how should I deal with that? – ciphor Jul 02 '14 at 07:07
@ciphor You mean a gathering all the values in a list? – thefourtheye Jul 02 '14 at 07:08
Yes, exactly, I want to gather all the values into lists grouping by different keys – ciphor Jul 02 '14 at 08:41
@ciphor You can do something like this `d.setdefault(key, []).append(value)` – thefourtheye Jul 02 '14 at 09:58

myildirim · Answer 2 · 2014-07-02T06:41:45.420

This is a common pattern when working with dicts in python so there is a defaultdict in the collections module. This lets you specify a default value for keys that are not in the dictionary. e.g:

from collections import defaultdict
d = defaultdict(int)
d[10] += 1

In [19]: dict(d)
Out[19]: {10: 1}

As you see, you don't have to check if the key exists in the dict if you use defaultdict.

d = defaultdict(int)
for r in records:
    d[r[0]] += r[1]

How to update the values in a dict cleanly?

2 Answers2