Summing the contents of two collections.Counter() objects

Question

I am working with collections.Counter() counters. I would like to combine two of them in a meaningful manner.

Suppose I have 2 counters, say,

Counter({'menu': 20, 'good': 15, 'happy': 10, 'bar': 5})

and

Counter({'menu': 1, 'good': 1, 'bar': 3})

I am trying to end up with:

Counter({'menu': 21, 'good': 16, 'happy': 10,'bar': 8})

How can I do this?

More specifically, `sum(counters, Counter())` to make sure the sum algorithm starts with a Counter base instead of a simple numeric base. — KobeJohn, Sep 29 '16 at 04:34

score 115 · Accepted Answer · edited Feb 28 '21 at 23:10

All you need to do is add them:

>>> from collections import Counter
>>> a = Counter({'menu': 20, 'good': 15, 'happy': 10, 'bar': 5})
>>> b = Counter({'menu': 1, 'good': 1, 'bar': 3})
>>> a + b
Counter({'menu': 21, 'good': 16, 'happy': 10, 'bar': 8})

From the docs:

Several mathematical operations are provided for combining Counter objects to produce multisets (counters that have counts greater than zero). Addition and subtraction combine counters by adding or subtracting the counts of corresponding elements.

Note that if you want to save memory by modifying the Counter in-place rather than creating a new one, you can do a.update(b) or b.update(a).

Summing the contents of two collections.Counter() objects

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