Percentage difference between two counter objects

I have two counter object like this:

A1 = {‘word1’: 7, ‘word2’: 5, ‘word3’: 4}
A2 = {‘word1’: 3, ‘word2’: 2, ‘word3’: 4}

I want to compute a percentage difference between the two counters, to obtain something like:

R = {‘word1’: -57.1, ‘word2’: -60, ‘word3’: 0}

I've tried this solution, but it gives me error

R = ((A2 - A1) / A1) * 100 

TypeError: unsupported operand type(s) for /: 'Counter' and 'Counter'

Any advice will be appreciated!

Solution

One approach is to use a dictionary comprehension:

from collections import Counter

A1 = Counter({'word1': 7, 'word2': 5, 'word3': 4})
A2 = Counter({'word1': 3, 'word2': 2, 'word3': 4})

res = {key: (A2.get(key, 0) / value - 1) * 100 for key, value in A1.items()}
print(res)

Output

{'word1': -57.14285714285714, 'word2': -60.0, 'word3': 0.0}

Another approach is to use the same comprehension but only give the results that are in the intersection of the Counter objects:

# A1 with an extra key
A1 = Counter({'word1': 7, 'word2': 5, 'word3': 4, 'word4': 5})
A2 = Counter({'word1': 3, 'word2': 2, 'word3': 4})

res = {key: (A2[key] / value - 1) * 100 for key, value in A1.items() if key in A2}
print(res)

Output

{'word1': -57.14285714285714, 'word2': -60.0, 'word3': 0.0}

Note that A2 - A1, will indeed subtract the counts, but only keeps results with positive values.