python嵌套类

python nested classes

提问by Daquicker

First of all, here's my test code, I'm using python 3.2.x:

首先，这是我的测试代码，我使用的是 python 3.2.x：

class account:
    def __init__(self):
        pass

    class bank:
        def __init__(self):
            self.balance = 100000

        def balance(self):
            self.balance

        def whitdraw(self, amount):
            self.balance -= amount

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

when I do:

当我做：

a = account()
a.bank.balance

I expected to get the value of balance returned, instead I get the function "balance", why is this? It returns the value of balance when I do:

我希望得到返回的 balance 值，而不是得到函数“balance”，这是为什么？当我这样做时，它返回 balance 的值：

class bank:
    def __init__(self):
        self.balance = 100000

    def balance(self):
        self.balance

    def whitdraw(self, amount):
        self.balance -= amount

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

a = bank()
a.balance

So I want to know why this is and it would be great if someone could come up with a way to give me the value of balance in the nested version.

所以我想知道这是为什么，如果有人能想出一种方法来给我嵌套版本中的平衡值，那就太好了。

采纳答案by codeape

My version of your code, with comments:

我的代码版本，带有注释：

#
# 1. CamelCasing for classes
#
class Account:
    def __init__(self):
        # 2. to refer to the inner class, you must use self.Bank
        # 3. no need to use an inner class here
        self.bank = self.Bank()

    class Bank:
        def __init__(self):
            self.balance = 100000

        # 4. in your original code, you had a method with the same name as 
        #    the attribute you set in the constructor. That meant that the 
        #    method was replaced with a value every time the constructor was 
        #    called. No need for a method to do a simple attribute lookup. This
        #    is Python, not Java.

        def withdraw(self, amount):
            self.balance -= amount

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

a = Account()
print(a.bank.balance)

回答by NPE

There are several problems:

有几个问题：

1. You're using the name balancefor both the data member and for the function.
2. You're missing a returnstatement in balance().
3. balance()operates on an instanceof bank. There is no instance in a.bank.balance: here, a.bankrefers to the inner class itself.

1. 您正在balance为数据成员和函数使用名称。
2. 您return在balance().
3. balance()对 的实例进行操作bank。a.bank.balance: 这里没有实例，a.bank指的是内部类本身。

回答by mgilson

a.bankis the class(not instance) since you've never created an instanceof the bank on a. So if a.bankis a class, a.bank.balanceis a method bound to that class.

a.bank是类（不是实例），因为您从未在 上创建银行的实例a。所以如果a.bank是一个类，a.bank.balance是一个绑定到该类的方法。

This works however:

然而，这有效：

class account:
    def __init__(self):
        self.bank = account.bank()

    class bank:
        def __init__(self):
            self.balance = 100000

        def whitdraw(self, amount):
            self.balance -= amount

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

a = account()
print a.bank.balance

Of course, as you show working code without nested classes, It really begs the question about whyyou want to use nested classes for this. I would argue that the non-nested version is much cleaner.

当然，当您展示没有嵌套类的工作代码时，它确实回避了为什么要为此使用嵌套类的问题。我认为非嵌套版本要干净得多。