Python password generation: Count unique values in list, remove doubles and replace with new unique values for PW Gen

Question

Im searching for a fast and pythonic way to generate new (forgotten) user passwords with high entropy. To achieve this i use a list with 64 chars which in this code will only be named "chars = []"

The problem is that sometimes chars are more than once (sometimes even triple) in a password. As far as i found out i can remove repeating items from a list with "set". But how can i append new items with this code that dont repeat again? Maybe this could be done all at once when generating it?

I need to say that im quite new to programming. Maybe my code is bad but i was reading much about random and urandom and i am sure i want to use os.urandom. Any suggestions, snippets and hints to make the code better would be very much appreciated.

chars = []
z = map(ord, os.urandom(12))
passw = []

for i in z:
    y = int(i) % 64 # modulo each item from urandom to map it to the list
    passw.append(chars[y])

print ''.join(passw)

Thank you for taking your time!

Answer 1

Remove each char from the list of available chars after taking it.

import random
import string

def mkpwd(l=12):
    # Start with all available characters.
    chars = [c for c in string.ascii_letters + string.digits]
    # Make sure we don't take more characters than are available.
    l = min(l, len(chars))
    # Collect the picked characters.
    cs = []
    for _ in range(l):
        # Take a random character from the current list of characters.
        i = random.randint(0, len(chars) - 1)
        c = chars[i]
        cs.append(c)
        # Remove the taken character from the list.
        # This ensures that every character is only taken once.
        del chars[i]
    return("".join(cs))

print(mkpwd())
print(mkpwd(24))
print(mkpwd(62))
print(mkpwd(1024))

Output (example):

0GEmMWXYbqeQ
eL83iPTMNatK54Efr2ZhdqWn
fIBj2DTw6P1grGJKZONdv9U8CaWet7l4n5QiFczRhqmxSALHMY0Vskp3oEXbuy
v6Fpt3yrMcGwENTjXODk5W2bZYeL8AqHQ0lJ9oadfIsg7mhBzU14RVnKiuxSPC

Answer 2

Loop while you reach the desired length

passw = ''

while len(passw) <= 12:
    char = chars[ord(os.urandom(1)) % 64]
    if char not in passw:
        passw += char