CORN, WOLF, DUCK

#!/usr/bin/env python3
# -*-coding: utf8-*-

# constants
R = '-->'
L = '<--'
CORN = 'corn'
DUCK = 'duck'
WOLF = 'wolf'
BOAT = 'boat'
ALL = [CORN, WOLF, DUCK]


# move duck from bank if conditions are met
def force_move_duck(bank):
    if DUCK in bank and CORN in bank:
        return True
    if DUCK in bank and WOLF in bank:
        return True


# just pretty print all from both river banks
def print_banks(left_bank, right_bank, direction):
    l = '({})'.format(', '.join(left_bank))
    r = '({})'.format(', '.join(right_bank))
    print('{:>30s} {} {:s}'.format(l, direction, r))


# move to right_bank
def to_right_bank(left_bank, right_bank):
    if force_move_duck(left_bank) and len(left_bank) == 3:
        left_bank.remove(DUCK)
        right_bank.append(DUCK)
    else:
        boat = left_bank.pop(0)
        right_bank.append(boat)

    print_banks(left_bank, right_bank, R)
    return left_bank, right_bank


# move to left_bank
def to_left_bank(left_bank, right_bank):
    if force_move_duck(right_bank):
        right_bank.remove(DUCK)
        left_bank.append(DUCK)

    print_banks(left_bank, right_bank, L)
    return left_bank, right_bank


def loop():
    # Initial status
    left_bank = ALL
    right_bank = []
    print_banks(left_bank, right_bank, '~~~')

    # main loop
    while True:
        left_bank, right_bank = to_right_bank(left_bank, right_bank)

        # we break the loop if ALL are in right_bank
        if len(set(ALL) - set(right_bank)) == 0:
            break
        left_bank, right_bank = to_left_bank(left_bank, right_bank)


if __name__ == '__main__':
    loop()