count_by

Groups the elements of a list based on the given function and returns the count of elements in each group.

Use collections.defaultdict to initialize a dictionary.
Use map() to map the values of the given list using the given function.
Iterate over the map and increase the element count each time it occurs.

from collections import defaultdict

def count_by(lst, fn = lambda x: x):
  count = defaultdict(int)
  for val in map(fn, lst):
    count[val] += 1
  return dict(count)

Examples

from math import floor

count_by([6.1, 4.2, 6.3], floor) # {6: 2, 4: 1}
count_by(['one', 'two', 'three'], len) # {3: 2, 5: 1}

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