Generate a secure python password with no dependencies

simple_password_generator.py

"""
A simple password generator that produces secure passwords of a given length
without requiring the installation of additional modules.

The four valid password characters classes - lowercase, uppercase, digits and
punctuation - are inserted randomly into the password using cryptographically
strong random numbers from the secrets module until the password length
requirement is met, ensuring that each of the classes is included in the password.

Password strength verified with https://www.security.org/how-secure-is-my-password/

NOTE: I've put a lower limit of 8 characters, which is standard, but the very first
example this produced was "b`q8M1N/", which can apparently be cracked in about 12
hours. Longer passwords are better. See https://xkcd.com/936/
"""

import secrets
import string

def generate_password(desired_length):
    # don't even bother with passwords shorter than 8 characters
    if desired_length < 8:
        raise Exception("Passwords should be 8 characters or more")

    # randomly select at least enough characters from each character class
    valid_char_classes = [
        string.ascii_lowercase,
        string.ascii_uppercase,
        string.digits,
        string.punctuation ]

    # generate an array of password characters
    password = []
    # loop over the character classes until we've completed the password
    while True:
        for char_class in valid_char_classes:
            # randomly select a character from the class
            random_char = secrets.choice(char_class)
            # randomly select the insert position
            insert_position = secrets.randbelow(len(password) + 1)
            # insert the character at that position
            password.insert(insert_position, random_char)
            # return the password string when complete
            if len(password) == desired_length:
                return "".join(password)

# choose a password length between 8 and 20
desired_length = 8 + secrets.randbelow(13)
print(f"Generated password of length {desired_length}: {generate_password(desired_length)}")