Python 2 generate random string with at least one character in each charset

generate_random_string.py

def generate_random_string(length, charsets=[string.ascii_uppercase, string.ascii_lowercase, string.digits, string.punctuation]):
    system_random = random.SystemRandom()
    dividers = sorted(system_random.sample(xrange(1, length), len(charsets) - 1))
    charset_lengths = [a - b for a, b in zip(dividers + [length], [0] + dividers)]
    random_string = ''.join([''.join(system_random.choice(charset) for _ in range(charset_length)) for charset, charset_length in zip(charsets, charset_lengths)])
    return ''.join(system_random.sample(random_string, len(random_string)))