Splits a sequence according to a boolean function.

split_seq.py

from typing import Callable, TypeVar, Sequence, Tuple


T = TypeVar('T')

def split_seq(items: Sequence[T], is_left: Callable[[T], bool]) -> Tuple[Sequence[T], Sequence[T]]:
  l, r = [], []
  for x in items:
    a = l if is_left(x) else r
    a.append(x)
  return l, r


def test_split_seq():
  is_even = lambda x: x % 2 == 0
  nums = tuple(range(10))
  evens, odds = split_seq(nums, is_even)
  assert len(evens) == len(odds)
  assert len(evens) == 5
  assert all(map(is_even, evens))
  assert all(map(lambda x: not is_even(x), odds))