toggle_case.py

import sys


def to_lower(s):
    return s.lower()


def to_upper(s):
    return s.upper()


def process_chr(s, fn=to_upper):
    if len(s) == 0:
        return ""

    char = s[0]
    if char.isalpha():
        return fn(char) + process_chr(s[1:], to_lower if fn == to_upper else to_upper)
    elif char in (".", "!", "?"):
        return char + process_chr(s[1:], to_upper)
    else:
        return char + process_chr(s[1:], fn)


if __name__ == '__main__':
    print(process_chr(sys.argv[1]))