04 Class and Linked List

这篇笔记包含第七周后两次lecture到第九章的内容。第十章跳过。

类

class Account:
    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'
        self.balance = self.balance - amount
        return self.balance

类成员

class Account:
    interest = 0.02  # a class attribute

    # ...

继承

class CheckingAccount(Account):
    withdraw_fee = 1
    interest = 0.01

    def withdraw(self, amount):
        return Account.withdraw(self, amount + self.withdraw_fee)
        # or super().withdraw(self.amount + self.withdraw_fee)

python允许继承多个类。

class SavingAccount(Account):
    deposit_fee = 2
    
    def deposit(self, amount):
        return Account.deposit(self, amount - self.deposit_fee)

class AsSeenOnTVAccount(CheckingAccount, SavingAccount):
    def __init__(self, account_holder):
        self.holder = account_holder
        self.balance = 1

__str__() 与 __repr__()

在python中， __str__() 生成对人类易懂的字符串， __repr__() 为编译器生成字符串。

>>> from fractions import Fraction
>>> half = Fraction(1, 2)
>>> str(half)
'1/2'
>>> repr(half)
'Fraction(1, 2)'

链表

class Link:
    """A linked list.

    >>> s = Link(1)
    >>> s.first
    1
    >>> s.rest is Link.empty
    True
    >>> s = Link(2, Link(3, Link(4)))
    >>> s.first = 5
    >>> s.rest.first = 6
    >>> s.rest.rest = Link.empty
    >>> s                                    # Displays the contents of repr(s)
    Link(5, Link(6))
    >>> s.rest = Link(7, Link(Link(8, Link(9))))
    >>> s
    Link(5, Link(7, Link(Link(8, Link(9)))))
    >>> print(s)                             # Prints str(s)
    <5 7 <8 9>>
    """
    empty = ()

    def __init__(self, first, rest=empty):
        assert rest is Link.empty or isinstance(rest, Link)
        self.first = first
        self.rest = rest

    def __repr__(self):
        if self.rest is not Link.empty:
            rest_repr = ', ' + repr(self.rest)
        else:
            rest_repr = ''
        return 'Link(' + repr(self.first) + rest_repr + ')'

    def __str__(self):
        string = '<'
        while self.rest is not Link.empty:
            string += str(self.first) + ' '
            self = self.rest
        return string + str(self.first) + '>'