lab07.py

class Account:
    """An account has a balance and a holder.
    >>> a = Account('John')
    >>> a.deposit(10)
    10
    >>> a.balance
    10
    >>> a.interest
    0.02
    >>> a.time_to_retire(10.25) # 10 -> 10.2 -> 10.404
    2
    >>> a.balance               # balance should not change
    10
    >>> a.time_to_retire(11)    # 10 -> 10.2 -> ... -> 11.040808032
    5
    >>> a.time_to_retire(100)
    117
    """
    max_withdrawal = 10
    interest = 0.02

    def __init__(self, account_holder):
        self.balance = 0
        self.holder = account_holder

    def deposit(self, amount):
        self.balance = self.balance + amount
        return self.balance

    def withdraw(self, amount):
        if amount > self.balance:
            return "Insufficient funds"
        if amount > self.max_withdrawal:
            return "Can't withdraw that amount"
        self.balance = self.balance - amount
        return self.balance

    def time_to_retire(self, amount):
        """Return the number of years until balance would grow to amount."""
        assert self.balance > 0 and amount > 0 and self.interest > 0
        year = 0
        now = self.balance
        while now < amount:
            year+=1
            now *= (self.interest + 1)
            if now>amount:
                return year


class FreeChecking(Account):
    """A bank account that charges for withdrawals, but the first two are free!
    >>> ch = FreeChecking('Jack')
    >>> ch.balance = 20
    >>> ch.withdraw(100)  # First one's free
    'Insufficient funds'
    >>> ch.withdraw(3)    # And the second
    17
    >>> ch.balance
    17
    >>> ch.withdraw(3)    # Ok, two free withdrawals is enough
    13
    >>> ch.withdraw(3)
    9
    >>> ch2 = FreeChecking('John')
    >>> ch2.balance = 10
    >>> ch2.withdraw(3) # No fee
    7
    >>> ch.withdraw(3)  # ch still charges a fee
    5
    >>> ch.withdraw(5)  # Not enough to cover fee + withdraw
    'Insufficient funds'
    """
    withdraw_fee = 1
    free_withdrawals = 2

    def withdraw(self, money):
        if self.free_withdrawals>0:
            self.free_withdrawals -= 1
            return Account.withdraw(self, money)
        else:
            return Account.withdraw(self,money+self.withdraw_fee)