Generate password in python

I'dl like to generate some alphanumeric passwords in python. Some possible ways are:

import string
from random import sample, choice
chars = string.letters + string.digits
length = 8
''.join(sample(chars,length)) # way 1
''.join([choice(chars) for i in range(length)]) # way 2

But I don't like both because:

way 1 only unique chars selected and you can't generate passwords where length > len(chars)
way 2 we have i variable unused and I can't find good way how to avoid that

So, any other good options?

P.S. So here we are with some testing with timeit for 100000 iterations:

''.join(sample(chars,length)) # way 1; 2.5 seconds
''.join([choice(chars) for i in range(length)]) # way 2; 1.8 seconds (optimizer helps?)
''.join(choice(chars) for _ in range(length)) # way 3; 1.8 seconds
''.join(choice(chars) for _ in xrange(length)) # way 4; 1.73 seconds
''.join(map(lambda x: random.choice(chars), range(length))) # way 5; 2.27 seconds

So, the winner is ''.join(choice(chars) for _ in xrange(length)).

Solution 1:

You should use the secrets module to generate cryptographically safe passwords, which is available starting in Python 3.6. Adapted from the documentation:

import secrets
import string
alphabet = string.ascii_letters + string.digits
password = ''.join(secrets.choice(alphabet) for i in range(20))  # for a 20-character password

For more information on recipes and best practices, see this section on recipes in the Python documentation. You can also consider adding string.punctuation or even just using string.printable for a wider set of characters.

Solution 2:

For the crypto-PRNG folks out there:

def generate_temp_password(length):
    if not isinstance(length, int) or length < 8:
        raise ValueError("temp password must have positive length")

    chars = "ABCDEFGHJKLMNPQRSTUVWXYZ23456789"
    from os import urandom

    # original Python 2 (urandom returns str)
    # return "".join(chars[ord(c) % len(chars)] for c in urandom(length))

    # Python 3 (urandom returns bytes)
    return "".join(chars[c % len(chars)] for c in urandom(length))

Note that for an even distribution, the chars string length ought to be an integral divisor of 128; otherwise, you'll need a different way to choose uniformly from the space.

Solution 3:

WARNING this answer should be ignored due to critical security issues!

Option #2 seems quite reasonable except you could add a couple of improvements:

''.join(choice(chars) for _ in range(length))          # in py2k use xrange

_ is a conventional "I don't care what is in there" variable. And you don't need list comprehension there, generator expression works just fine for str.join. It is also not clear what "slow" means, if it is the only correct way.

Solution 4:

Two recipes using the builtin secrets (python 3.6+)

1. secrets.token_urlsafe

This is much faster than the accepted answer. (see timings below)

import secrets
password = secrets.token_urlsafe(32)

Example output:

4EPn9Z7RE3l6jtCxEy7CPhia2EnYDEkE6N1O3-WnntU

The argument for token_urlsafe is number of bytes. On average, one byte is 1.3 characters (base64 encoded).

2. Enforce amount of digits/upper characters etc

This is slighly modified copy from the docs of secrets. With this you have more fine grained control on how to generated passwords have to look. Of course, this is not fast option if you need to generate a lot of passwords.

Forcing length to be 20 characters
Forcing at least 4 lower case character
Forcing at least 4 upper case characters
Forcing at least 4 digits
Special characters can be added to alphabet. In this example, there are just - and _ added.

import string
import secrets
alphabet = string.ascii_letters + string.digits + '-_'
while True:
    password = ''.join(secrets.choice(alphabet) for i in range(20))
    if (sum(c.islower() for c in password) >=4
            and sum(c.isupper() for c in password) >=4
            and sum(c.isdigit() for c in password) >=4):
        break

Example output:

HlxTm2fcFE54JA1I_Yp5

3. "I don't need the finer-grained control"

If considered speed, you can also drop the while-loop. In this case, it actually simplifies to gerrit's answer (but then you loose the finer-grained control):

import string
import secrets
alphabet = string.ascii_letters + string.digits + '-_'
password = ''.join(secrets.choice(alphabet) for i in range(20))

Speed comparison

1. secrets.token_urlsafe

1.62 µs ± 96.6 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

2. Enforce amount of digits/upper characters etc

107 µs ± 11.9 µs per loop (mean ± std. dev. of 7 runs, 10000 loops each)

3. "I don't need the finer-grained control"

77.2 µs ± 9.31 µs per loop (mean ± std. dev. of 7 runs, 10000 loops each)

^{Speed comparison setup: python 3.8.5 64-bit on Win10, 43 characters in each password (=32 bytes for token_urlsafe).}

Solution 5:

I think this'll do the trick. random.SystemRandom uses the same underlying crypto random function as os.urandom but it uses the familiar random interface. This function won't be subject to the weird 128 byte thing as in Ben's answer.

import random
import string

def gen_random_string(char_set, length):
    if not hasattr(gen_random_string, "rng"):
        gen_random_string.rng = random.SystemRandom() # Create a static variable
    return ''.join([ gen_random_string.rng.choice(char_set) for _ in xrange(length) ])

password_charset = string.ascii_letters + string.digits
gen_random_string(password_charset, 32)