First, complete the following class definition:

class BankAccount:

    def __init__(self, initial_balance):

        """Creates an account with the given balance."""

        …

    def deposit(self, amount):

        """Deposits the amount into the account."""

        …

    def withdraw(self, amount):

        """

        Withdraws the amount from the account.  Each withdrawal resulting in a

        negative balance also deducts a penalty fee of 5 dollars from the balance.

        """

        …

    def get_balance(self):

        """Returns the current balance in the account."""

        …

    def get_fees(self):

        """Returns the total fees ever deducted from the account."""

        …

The deposit and withdraw methods
each change the account balance. The withdraw method
also deducts a fee of 5 dollars from the balance if the withdrawal (before any fees) results in a negative balance. Since we also have the method get_fees,
you will need to have a variable to keep track of the fees paid.

Here's one possible test of the class. It should print the values 10 and 5, respectively, since the withdrawal incurs a fee of 5 dollars.

my_account = BankAccount(10)

my_account.withdraw(15)

my_account.deposit(20)

print my_account.get_balance(), my_account.get_fees()

这里用了一个列表来储存数据。其他没什么特别的。

class BankAccount:

    def __init__(self, initial_balance):

        self.balance = initial_balance

        self.fee = [0]

        """Creates an account with the given balance."""

    def deposit(self, amount):

        """Deposits the amount into the account."""

        self.balance = self.balance + amount

        return self.balance

    def withdraw(self, amount):

        """

        Withdraws the amount from the account.  Each withdrawal resulting in a

        negative balance also deducts a penalty fee of 5 dollars from the balance.

        """

        self.balance = self.balance - amount

        if self.balance < 0:

            self.balance = self.balance - 5

            self.fee[0] += 1

    def get_balance(self):

        """Returns the current balance in the account."""

        return self.balance

    def get_fees(self):

        """Returns the total fees ever deducted from the account."""

        return self.fee[0]*5