CODEWITHSUNDEEP
pythondictionary
ciphor
ciphor

Reputation: 8288

How to update the values in a dict cleanly?

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)

Upvotes: 1

Views: 121

Answers (2)

myildirim
myildirim

Reputation: 2428

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]

Upvotes: 3

thefourtheye
thefourtheye

Reputation: 239443

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

Upvotes: 3

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