Group list elements

Python, List, Dictionary · Nov 2, 2020

Groups the elements of a list based on the given function.

Use collections.defaultdict to initialize a dictionary.
Use fn in combination with a for loop and dict.append() to populate the dictionary.
Use dict() to convert it to a regular dictionary.

from collections import defaultdict

def group_by(lst, fn):
  d = defaultdict(list)
  for el in lst:
    d[fn(el)].append(el)
  return dict(d)

from math import floor

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