example.py

from abc import ABCMeta, abstractmethod


class Payment(object):

    __metaclass__ = ABCMeta

    @abstractmethod
    def do_pay(self):
        pass


class Bank(Payment):

    def __init__(self):
        self.card = None
        self.account = None

    def __get_account(self):
        self.account = self.card

        return self.account

    def __has_founds(self):
        print("Bank:: Checking if Account %d has enough funds" % self.__get_account())
        return True

    def set_card(self, card):
        self.card = card

    def do_pay(self):
        if self.__has_founds():
            print("Bank:: Paying the merchant")
            return True
        else:
            print("Bank:: Sorry, not enough funds")
            return False


class DebitCard(Payment):

    def __init__(self):
        self.bank = Bank()

    def do_pay(self):
        card = input("Proxy:: Punch in Card Number: ")
        self.bank.set_card(card)

        return self.bank.do_pay()


class You(object):

    def __init__(self):
        print("You:: Lets buy the Denim shirt!")

        self.debit_card = DebitCard()
        self.is_purchased = None

    def make_payment(self):
        self.is_purchased = self.debit_card.do_pay()

    def __del__(self):

        if self.is_purchased:
            print("You:: Wow! Denim shirt is mine :-)")
        else:
            print("You:: I should earn more :(")

if __name__ == "__main__":
    you = You()
    you.make_payment()