generate-password.py

import string
import random

DEFAULT_POOLS = [
    string.ascii_lowercase,
    string.ascii_uppercase,
    string.digits,
    '_!@$'
]


def generate_password(length=14, pools=None):
    """ Generates a password that contains at least 1 character out of every of
    the specified character pools. The character pools should be distinct sets.
    The minimum length for the password is the number of pools provided. """

    if pools is None:
        pools = DEFAULT_POOLS
    assert length >= len(pools)
    password = ''.join(random.choice(''.join(pools)) for _ in range(length))
    counts = (
        {'count': sum(password.count(x) for x in pool), 'pool': pool}
        for pool in pools
    )
    counts = sorted(counts, key=lambda x: x['count'])
    for data in filter(lambda x: x['count'] == 0, counts):
        source = next((x for x in reversed(counts) if x['count'] > 1))
        source['count'] -= 1
        indices = (password.find(x) for x in source['pool'])
        index = next(filter(lambda i: i >= 0, indices))
        password = password[:index] + random.choice(data['pool']) + password[index+1:]
    assert all(any(x in password for x in pool) for pool in pools)
    return password