group_by

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)

Examples

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']}

Recommended snippets

count_by
Python, List
Groups the elements of a list based on the given function and returns the count of elements in each group.
bifurcate_by
Python, List
Splits values into two groups, based on the result of the given filtering function.
find_index
Python, List
Finds the index of the first element in the given list that satisfies the provided testing function.