Generate random string to match a specific pattern

Question

import random
import string


def random_char(y):
    return ''.join(random.choice(string.ascii_uppercase) for x in range(y))


p1 = str(random.randrange(1, 99))
p2 = str(random_char(5))
p3 = str(random.randrange(1, 9))
p4 = str(random_char(2))
p5 = str(random.randrange(1, 9))
p6 = str(random_char(1))  # Y
p7 = str(random.randrange(1, 9))  # 7
p8 = str(random_char(3))  # AUS
result = p1 + p2+p3+p4+p5+p6+p7+p8
print(result)

I generate a specific code like "31SPZVG2CZ2R8WFU" How can i do it in a more elegant way?

Answer 1

In your random_char function, you don't use x at all. Replace it with _ (it's conventional in python to use _ for throwaway variables). y could also be renamed to something more descriptive. The name of the function could also be renamed to random_chars since you're generating one or more of them.

Also, use string formatting instead of all those extra variables:

def generate_key():
    return (f"{random.randrange(1, 9)}{random_chars(5)}{random.randrange(1, 9)}{random_chars(2)}" 
            f"{random.randrange(1, 9)}{random_chars(1)}{random.randrange(1, 9)}{random_chars(3)}")

Note that the f-strings are available for Python versions >= 3.6

As a side note, there's this nice exrex which can:

Generate all - or random - matching strings to a given regular expression and more. It's pure python, without external dependencies.

Given some pattern (similar to what you want):

r"(\d[A-Z]{1,4}){4}"

• (...){4} matches the previous token exactly 4 times.
• \d matches a digit (equivalent to [0-9])
• [A-Z]{1,4} matches the previous token between 1 and 4 times, as many times as possible, giving back as needed (greedy)
• A-Z matches a single character in the range between A (index 65) and Z (index 90) (case sensitive)

Note: I'm not a regex expert so there might be an easier / more correct version of this.

I think you can use this regex which returns exactly the pattern you want: \d{1,2}[A-Z]{5}\d[A-Z]{2}\d[A-Z]{1}\d[A-Z]{3}

Your entire code could be rewritten as:

import exrex

random_key = exrex.getone(r'(\d[A-Z]{1,4}){4}')
print(random_key)

Which would generate:

'3C2BBV3NGKJ2XYJ'

For more information about regular expressions feel free to search on the internet to get familiar with them. For tests, I usually use regex101

Answer 2

Instead of treating number and letter parts differently, you could use random.choice for both and save some code:

import random
from string import ascii_uppercase

spec = [
    range(1, 99), *[ascii_uppercase] * 5,
    range(1, 9), *[ascii_uppercase] * 2,
    range(1, 9), *[ascii_uppercase],
    range(1, 9), *[ascii_uppercase] * 3,
]
print(''.join(str(random.choice(pool)) for pool in spec))

Try it online!