A Python script that uses Parsley to roll dice in the usual D&D notation

dice_roll.py

import random
import parsley

def roll(x, y, k=None):
    k = k or x
    if k > 0:
       s = slice(0, k)
    else:
      s = slice(k, x)
    return sum(sorted((random.randrange(1, y+1) for _ in range(x)), reverse=True )[s])

grammar = parsley.makeGrammar("""
dice = number:x 'd' number:y keep?:k -> roll(x, y, k)
keep = 'k' '-'?:sign number:k -> int("%s%s" % (sign or "", k))
die = 'd' number:y -> roll(1, y)
number = <digit+>:ds -> int(ds)
value = die | dice | number
op = '+' | '-'
expr2 = value:left ws op:op ws expr:right -> left + right if op == '+' else left - right
expr = expr2 | value
ws = ' '?
""", {'roll': roll})


if __name__ == "__main__":
    import sys
    if sys.argv[1:]:
        print(grammar(" ".join(sys.argv[1:])).expr())
    else:
        while True:
            print(grammar(input("Dice: ")).expr())