Python append Counter to Counter, like Python dictionary update

Question

I have 2 Counters (Counter from collections), and I want to append one to the other, while the overlapping keys from the first counter would be ignored. Like the dic.update (python dictionaries update)

For example:

from collections import Counter
a = Counter(a=4, b=0, c=1)
b = Counter(z=1, b=2, c=3)

So something like (ignore overlapping keys from the first counter):

# a.update(b) 
Counter({'a':4, 'z':1, 'b':2, 'c':3})

I guess I could always convert it to some kind of a dictionary and then convert it back to Counter, or use a condition. But I was wondering if there is a better option, because I'm using it on a pretty large data set.

I guess you could do `for key in b: if key not in a: a[key] = b[key]` — L3viathan, Mar 07 '18 at 15:25

Aran-Fey · Accepted Answer · 2019-06-14T20:56:47.417

11

Counter is a dict subclass, so you can explicitly invoke dict.update (rather than Counter.update) and pass two counters as the arguments:

a = Counter(a=4, b=0, c=1)
b = Counter(z=1, b=2, c=3)

dict.update(a, b)

print(a)
# Counter({'a': 4, 'c': 3, 'b': 2, 'z': 1})

edited Jun 14 '19 at 20:56

answered Mar 07 '18 at 15:26

Aran-Fey

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well I tried it, but it returns a `NoneType`. Am I missing something? – sheldonzy Mar 07 '18 at 15:29
3

@sheldonzy Of course. It modifies `a` in-place, so it doesn't have to return anything. – Aran-Fey Mar 07 '18 at 15:29
@Aran-Fey Oh smart, you call `dict.update` instead of `Counter.update`. Way more subtle than I thought at first, good job. – Olivier Melançon Mar 07 '18 at 15:30
Why not use `a.update(b)`? I just checked and it works the same. – wjandrea Jul 29 '20 at 19:12

score 5 · Answer 2 · answered Mar 07 '18 at 15:35

5

You can also use dict unpacking

from collections import Counter
a = Counter(a=4, b=0, c=1)
b = Counter(z=1, b=2, c=3)
Counter({**a, **b})
Counter({'a': 4, 'c': 3, 'b': 2, 'z': 1})

answered Mar 07 '18 at 15:35

Sohaib Farooqi

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Python append Counter to Counter, like Python dictionary update

2 Answers2