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

Use map() and fn to map the values of the list to the keys of a dictionary.
Use list comprehension to map each element to the appropriate key.


def group_by(lst, fn):
  return {key : [el for el in lst if fn(el) == key] for key in map(fn, lst)}


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']}