Sample code fordiscussion in: http://www.python-forum.de/viewtopic.php?f=1&t=29274

shop.py

#!/usr/bin/env python

ITEMS = {
    'Werkzeug': 20, 
    'Öl': 4, 
    'Holzbalken': 6, 
    'Korb': 3, 
    'Stoff': 8, 
    'Kreide': 1, 
    'Seil': 4, 
    'Holzeimer':7, 
    'Harke': 5, 
    'Axt': 8, 
    'Lederbeutel': 3
}

class Shop:

    def __init__(self, items):
        self.items = items

    def __str__(self):
        return "\nShop:\n{}".format("\n".join("{}: {}".format(name, price) for 
                                    name, price in self.items.items()))

    def append(self, name, price):
        self.items[name] = price

    def sell(self, name):
        return self.items.pop(name)


class Player:

    def __init__(self, name):
        self.name = name
        self.items = {}

    def __str__(self):
        return "\n{}:\n{}".format(self.name, 
                                  "\n".join("{}: {}".format(name, price) for 
                                            name, price in self.items.items()))

    def buy(self, name, price):
        self.items[name] = price


def handle_purchasing(name, player, shop):
    price = shop.sell(name)
    player.buy(name, price)


def main():
    p = Player("Hyperion")
    print(p)
    s = Shop(ITEMS)
    s.append("Python", 42)
    print(s)
    for name in ("Öl", "Seil", "Axt"):
        handle_purchasing(name, p, s)
    print(s)
    print(p)


if __name__ == "__main__":
    main()