Dependancy injection demonstration for making random stuff deterministic

dicegames.py

import random


def default_diceroller():
    return random.randint(1, 6)


class CrapsGame(object):
    _diceroller = default_diceroller
    
    def __init__(self, nplayers):
        self.nplayers = nplayers

    def roll(self):
        return self._diceroller(), self._diceroller()
        

test_dicegames.py

from dicegames import CrapsGame


def make_deterministic_diceroller(outputs):
    outputs_iter = iter(outputs)
    def diceroller():
        roll = next(outputs_iter, None)
        if roll is None:
            raise AssertionError('Dice roller expended unexpectedly')
        return roll
    return diceroller


def test_craps_game():
    game = CrapsGame(5)
    game._diceroller = make_deterministic_diceroller([1, 2, 3])
    assert game.roll() == (1, 2)