Rolling the dice ...

#!/usr/bin/env python3
import random
import re

PATTERN = re.compile(r'''(\d+) d (\d+)              # count, "d", eyes
                         (?:(\+|-|\*|/?) (\d+))?    # [operator, value]
                     ''', re.X)


def Add(summand):
    return lambda a: a + summand

def Sub(subtrahend):
    return lambda a: a - subtrahend

def Mul(factor):
    return lambda a: a * factor

def Div(divisor):
    return lambda a: a / divisor

MODIFICATOR_MAPPING = {
    '+': Add,
    '-': Sub,
    '*': Mul,
    '/': Div,
    }

def Dice(count, eyes, mod=lambda a: a):
    return lambda: mod(sum([random.randint(1, eyes) for _ in range(count)]))

def create_from_string(string):
    count, eyes, operator, value = PATTERN.match(string).groups()
        # AttributeError if something went wrong here (pattern-mismatch)
    if operator and value:
        return Dice(int(count), int(eyes),
                    MODIFICATOR_MAPPING[operator](int(value)))
    else:
        return Dice(int(count), int(eyes))