Comparing list of Counters in Python

Question

I have a list of counters in python:

[Counter({12.011: 30.0, 15.999: 2.0}),
Counter({12.011: 12.0, 15.999: 2.0}),... Counter({12.011: 40.0, 
15.999: 5.0, 79.904: 5.0})]

How do I find the count of each Counter element. Counter gives me an error of unhashable type. Another way I thought is to iterate through the list in a nested for-loop, checking if each counter is equal to any other counter in the list and increment a dictionary accordingly. Is there a better solution?

Answer 1

In Python <= 3.6 your best bet is probably comparing every Counter to every other Counter.

In Python >= 3.7 there is a better way. Because Counter is a subclass of dict, and dict now keep their order, it is possible to use the string representation of each Counter object as its hash.

This is a bit hacky but it works:

from collections import Counter

li = [Counter({'a': 1, 'b': 2, 'c': 3}),
      Counter({'a': 1, 'b': 2, 'c': 4}),
      Counter({'a': 1, 'b': 3, 'c': 3}),
      Counter({'a': 1, 'b': 2, 'c': 3})]


Counter.__hash__ = lambda counter: hash(str(counter))

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

Answer 2

You could convert each Counter into a frozenset of key, value (items) tuples and then use this as elements to pass to a Counter (as frozenset are hashable), for example like this:

import random

from collections import Counter

random.seed(42)

counts = [Counter([random.randint(0, 2) for _ in range(10)]) for _ in range(10)]

uniques = {frozenset(e.items()): e for e in counts}

counters_count = Counter(map(lambda e : frozenset(e.items()), counts))

for key, count in counters_count.items():
    print uniques[key], count

Output

Counter({0: 7, 1: 2, 2: 1}) 1
Counter({1: 4, 2: 4, 0: 2}) 1
Counter({0: 4, 1: 4, 2: 2}) 2
Counter({0: 4, 1: 3, 2: 3}) 1
Counter({0: 5, 2: 3, 1: 2}) 1
Counter({1: 5, 2: 5}) 1
Counter({0: 5, 1: 4, 2: 1}) 1
Counter({0: 4, 2: 4, 1: 2}) 1
Counter({2: 4, 0: 3, 1: 3}) 1