30-seconds-of-code/snippets/difference_by.md

title, tags, firstSeen, lastUpdated

title	tags	firstSeen	lastUpdated
difference_by	list,function,intermediate	2018-02-08T15:59:27+02:00	2020-11-02T19:27:53+02:00

Returns the difference between two lists, after applying the provided function to each list element of both.

• Create a set, using map() to apply fn to each element in b.
• Use a list comprehension in combination with fn on a to only keep values not contained in the previously created set, _b.

def difference_by(a, b, fn):
  _b = set(map(fn, b))
  return [item for item in a if fn(item) not in _b]

from math import floor

difference_by([2.1, 1.2], [2.3, 3.4], floor) # [1.2]
difference_by([{ 'x': 2 }, { 'x': 1 }], [{ 'x': 1 }], lambda v : v['x'])
# [ { x: 2 } ]