Python merge two counter objects, keeping the largest sum

Question

If I have two counter objects and I would like to merge them, adding new values from one and keeping the largest count if the two counters contain the same values.

Counter a = { "apple":3, "peach":1, "pears":7, "watermelon":2, "grapes":7 }

Counter b = { "apple":12, "kiwi":9, "grapes":2, "pears":21, "pineapple":2, "oranges":2 }

#desired output
counter c = { "apple":12, "pears":21, "grapes":7 "peach":1, "watermelon":2, "pineapple":2, "oranges":2} 

Currently I've tried updating the counter but that seems to merge the two counters but sums their counts. I would just like to merge the counters and keep the largest value or add to the counter if there isn't one already.

Answer 1

After OP edited their question, the desired output can be achieved by simply using the bitwise or ('|') operator:

from collections import Counter

a = Counter({ "apple":3, "peach":1, "pears":7, "watermelon":2, "grapes":7 })

b = Counter({ "apple":12, "kiwi":9, "grapes":2, "pears":21, "pineapple":2, "oranges":2 })

c = a | b

print(c)
>> Counter({'pears': 21, 'apple': 12, 'kiwi': 9, 'grapes': 7, 'watermelon': 2, 'oranges': 2,
            'pineapple': 2, 'peach': 1})

Answer 2

You could take distinct keys from each of the Counter's using inbuilt a&b, and use them to get your desired max logic like this:

Counter({key:max(a[key], b[key]) for key in a&b})

Here is a sample run:

>>> from collections import Counter
>>> a=Counter({ "apple":3, "peach":1, "pears":7, "watermelon":2, "grapes":7 })
>>> b=Counter({ "apple":12, "kiwi":9, "grapes":2, "pears":21, "pineapple":2, "oranges":2 })
>>> Counter({key:max(a[key], b[key]) for key in a&b})
Counter({'pears': 21, 'apple': 12, 'grapes': 7})
>>> 

Note that if you wanted the minimum for common elements that is already built in python with this construct:

>>> a&b
Counter({'pears': 7, 'apple': 3, 'grapes': 2})
>>>