semi-functional rewrite of Norvig monopoly simulator to avoid global state

monopoly.py

# Rewrite of Norvig's monopoly simulator to avoid global state
# except for `board`, which is used immutably.
# http://nbviewer.ipython.org/url/norvig.com/ipython/Probability.ipynb

import random
split = str.split

# The board: a list of the names of the 40 squares
board = split("""GO   A1 CC1 A2  T1 R1 B1  CH1 B2 B3
                 JAIL C1 U1  C2  C3 R2 D1  CC2 D2 D3
                 FP   E1 CH2 E2  E3 R3 F1  F2  U2 F3
                 G2J  G1 G2  CC3 G3 R4 CH3 H1  T2 H2""")

# The deck of 16 community chest cards. See do_card.
def get_cc_deck():
    deck = split('GO JAIL' + 14 * ' ?')
    random.shuffle(deck)
    return deck

# The deck of 16 chance cards. See do_card.
def get_ch_deck():
    deck = split('GO JAIL C1 E3 H2 R1 R R U -3' + 6 * ' ?')
    random.shuffle(deck)
    return deck

def monopoly(steps):
    """Simulate given number of steps of monopoly game, yielding the name of the
    current square after each step."""
    here = 0
    CC = get_cc_deck()
    CH = get_ch_deck()
    doubles = 0
    for _ in range(steps):
        d1, d2 = random.randint(1, 6), random.randint(1, 6)
        here = goto(here + d1 + d2)
        doubles = (doubles + 1) if (d1 == d2) else 0
        if doubles == 3 or board[here] == 'G2J':
            here = goto('JAIL')
        elif board[here].startswith('CC'):
            here = do_card(CC, here)
        elif board[here].startswith('CH'):
            here = do_card(CH, here)
        yield board[here]

def goto(square):
    """Go to destination square, which can be either a square number or a square
    name. Return the index of the resulting square."""
    if isinstance(square, int):
        return square % len(board)
    else:
        return board.index(square)

def do_card(deck, square):
    """Take the top card from deck and do what it says. Return the index of the
    resulting square."""
    card = deck.pop()        # Remove card from deck
    deck[0:0] = [card]       # Put card back on bottom of deck
    if card == 'R' or card == 'U':
        while not board[square].startswith(card):
            # Advance to next railroad or utility
            square = goto(square + 1)
        return square
    elif card == '-3':
        # Go back 3 spaces
        return goto(square - 3)
    elif card == '?':
        # Card is about money, not about movement
        return square
    else:
        # Go to destination named on card
        return goto(card)