A quick DnD-style dice roller

roll.py

'''
A quick DnD-style dice roller by Mia yun Ruse.

(Public domain, because it's hardly inventing the wheel.)

Allows for expressions such as:

> 5d0
> 5:d2*10+3; 2d20
23, 13, 13, 13, 13, 24

where * (or x) and + do math, (Y)dX sums Y rolls of an X-sided die (1-X),
; allows multiple values, and : repeats the expression (count on the left).

On CLI, evaluates input, or does a REPL if empty input.
(Hit up and enter in the REPL to repeat a request.)

Brackets are not supported. That's a tad much for a game to ask you to roll.
'''

from functools import reduce
from random import randint
from operator import mul

def product(x):
    return reduce(mul, x, 1)

def dice(x):
    if not x:
        return 0

    dice = 1
    if 'd' in x:
        x, dice = x.split('d')
    return sum(randint(1, int(dice or 1)) for i in range(int(x or 1)))

def roll(string):
    string = string.replace('x', '*').replace('-', '+-')
    return sum(product(dice(x) for x in mults.split('*'))
               for mults in string.split('+'))

def rolls(string):
    vals = []
    for x in string.split(';'):
        count = 1
        if ':' in x:
            count, x = x.split(':')
        vals += [roll(x) for i in range(int(count or 1))]
    return vals

def main(string):
    if not string.strip():
        return None
    print(', '.join(str(x) for x in rolls(string)))

if __name__ == '__main__':
    from sys import argv
    if argv[1:]:
        main(' '.join(argv[1:]))
    else:
        print('roll.py')
        lastval = ''
        while True:
            val = input('> ')
            if val == '\x1b[A':
                # CLI up-arrow
                val = lastval
            lastval = val
            try: main(val)
            except ValueError: print('?')