strperm.py

from typing import List

def perm(input: str) -> List[str]:
  return [
    head + p    
    for i, head in enumerate(input)
    for p in perm(input[:i] + input[i+1:])
  ] if input else [input]

print(perm('abc'))

timeit(
  "perm('abcd')",
  setup="import gc; gc.enable()",
  globals=globals(),
  number=100000
)