Python 类中的变量范围 [英] Variable scopes in Python classes

问题描述

在类中声明变量(函数外):所有类函数都可以访问它(基本上是公共变量)

Declaring a variable in a class (outside of a function): all class functions can access it (basically a public variable)

在类内的函数内声明变量:只有该函数可以访问它(它在该函数的范围内)

Declaring a variable inside a function inside a class: only that function can access it (it's in that function's scope)

在类内的函数内用self.(variable name)声明变量:所有类函数都可以访问它(这与global(变量名)?)

Declaring a variable with self.(variable name) inside a function inside a class: all class functions can access it (how is this different from global (variable name)?)

而且由于没有私有/受保护，所以一切都是公共的，因此可以从类内部访问的所有内容都可以从类外部访问.

And since there is no private/protected, everything is public, so everything accessible from inside a class is accessible from outside the class.

还有什么其他的细微差别我应该知道，或者我已经基本掌握了吗?

Are there any other nuances I should know, or have I pretty much got it?

推荐答案

由于您问题中的清单不是 100% 清楚，我决定用一个简单的例子来解释它.它还包括一些诸如 __something 变量之类的东西，您没有在列表中提及.

Since the listing in your question is not 100% clear, I've decided to explain it with a simple example. It also includes some things like __something variables you did not mention in your list.

class Test:
    a = None
    b = None

    def __init__(self, a):
        print self.a
        self.a = a
        self._x = 123
        self.__y = 123
        b = 'meow'

一开始，a 和 b 只是为类本身定义的变量 - 可以通过 Test.a 和 Test 访问.b 而不是特定于任何实例.

At the beginning, a and b are only variables defined for the class itself - accessible via Test.a and Test.b and not specific to any instance.

创建该类的实例时(这会导致 __init__ 被执行):

When creating an instance of that class (which results in __init__ being executed):

• print self.a 没有找到实例变量，因此返回类变量
• self.a = a:创建一个新的实例变量a.这会隐藏类变量，因此 self.a 现在将引用实例变量；要访问类变量，您现在必须使用 Test.a
• 对 self._x 的赋值创建了一个新的实例变量.它被视为不是公共 API 的一部分"(也就是受保护的)，但从技术上讲，它没有不同的行为.
• 对 self.__y 的赋值创建了一个名为 _Test__y 的新实例变量，即它的名字被破坏了，所以除非你使用被破坏的名字，否则它不能从外部访问班级.这可用于私有"变量.
• 对 b 的赋值创建了一个局部变量.除了 __init__ 函数之外，它在任何地方都不可用，因为它没有保存在实例、类或全局范围内.

• print self.a doesn't find an instance variable and thus returns the class variable
• self.a = a: a new instance variable a is created. This shadows the class variable so self.a will now reference the instance variable; to access the class variable you now have to use Test.a
• The assignment to self._x creates a new instance variable. It's considered "not part of the public API" (aka protected) but technically it has no different behaviour.
• The assignment to self.__y creates a new instance variable named _Test__y, i.e. its name is mangled so unless you use the mangled name it cannot be accessed from outside the class. This could be used for "private" variables.
• The assignment to b creates a local variable. It is not available from anywhere but the __init__ function as it's not saved in the instance, class or global scope.

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