Random string generation with upper case letters and digits

Question

I want to generate a string of size N.

It should be made up of numbers and uppercase English letters such as:

• 6U1S75
• 4Z4UKK
• U911K4

Answer 1

This method is slightly faster, and slightly more annoying, than the random.choice() method Ignacio posted.

It takes advantage of the nature of pseudo-random algorithms, and banks on bitwise and and shift being faster than generating a new random number for each character.

# must be length 32 -- 5 bits -- the question didn't specify using the full set
# of uppercase letters ;)
_ALPHABET = 'ABCDEFGHJKLMNPQRSTUVWXYZ23456789'

def generate_with_randbits(size=32):
    def chop(x):
        while x:
            yield x & 31
            x = x >> 5
    return  ''.join(_ALPHABET[x] for x in chop(random.getrandbits(size * 5))).ljust(size, 'A')

...create a generator that takes out 5 bit numbers at a time 0..31 until none left

...join() the results of the generator on a random number with the right bits

With Timeit, for 32-character strings, the timing was:

[('generate_with_random_choice', 28.92901611328125),
 ('generate_with_randbits', 20.0293550491333)]

...but for 64 character strings, randbits loses out ;)

I would probably never use this approach in production code unless I really disliked my co-workers.

edit: updated to suit the question (uppercase and digits only), and use bitwise operators & and >> instead of % and //