Dice roller script

roll.py

#!/usr/bin/env python3

from collections import defaultdict
from secrets import token_bytes


def roll(num_sides: int) -> int:
    assert num_sides > 0, 'cannot have no sides'
    return int.from_bytes(token_bytes(48), 'little') % num_sides + 1


def distribution(f, *, samples=100_000):
    distribution = defaultdict(lambda: 0)

    for r in (f() for _ in range(samples)):
        distribution[r] += 1

    return dict(sorted(distribution.items(), key=lambda i: i[0]))


if __name__ == '__main__':
    import sys, re

    def exit_with(msg, *, code=1):
        print(msg, file=sys.stderr)
        exit(code)

    if len(sys.argv) < 2:
        exit_with('Expected dice expression')

    regex = re.compile(
        r'^(?P<times>\d+)?'            # the number of times to roll, default 1
        r'd(?P<sides>\d+)'             # the number of sides of the dice to roll
        r'(?:\s*\+\s*(?P<add>\d+))?$'  # an amount to add to the total rolled
    )

    result = regex.match(' '.join(sys.argv[1:]))

    if not result:
        exit_with('Invalid dice expression')

    sides, times, add = result.group('sides', 'times', 'add')
    sides, times, add = int(sides), int(times or 1), int(add or 0)

    total = sum((roll(sides) for _ in range(times)), add)

    print(total)
    exit(0)