Python 中的类方法差异:绑定、未绑定和静态 [英] Class method differences in Python: bound, unbound and static

问题描述

以下类方法有什么区别?

一个是静态的，另一个不是吗?

类测试(对象):def method_one(self):打印调用的method_one"def method_two():打印调用的method_two"a_test = 测试()a_test.method_one()a_test.method_two()

解决方案

在 Python 中，bound 和 unbound 方法是有区别的.

基本上，调用成员函数(如method_one)，绑定函数

a_test.method_one()

被翻译成

Test.method_one(a_test)

即对未绑定方法的调用.因此，调用您的 method_two 版本将失败并返回 TypeError

<预><代码>>>>a_test = 测试()>>>a_test.method_two()回溯(最近一次调用最后一次):文件<stdin>"，第 1 行，在 <module> 中类型错误:method_two() 不带参数(给定 1)

您可以使用装饰器更改方法的行为

类测试(对象):def method_one(self):打印调用的method_one"@静态方法def method_two():打印调用的方法二"

装饰器告诉内置的默认元类 type(类的类，参见 this question) 不为 method_two 创建绑定方法.

现在，您可以直接在实例或类上调用静态方法:

<预><代码>>>>a_test = 测试()>>>a_test.method_one()调用method_one>>>a_test.method_two()调用方法二>>>Test.method_two()调用方法二

What is the difference between the following class methods?

Is it that one is static and the other is not?

class Test(object):
  def method_one(self):
    print "Called method_one"

  def method_two():
    print "Called method_two"

a_test = Test()
a_test.method_one()
a_test.method_two()

解决方案

In Python, there is a distinction between bound and unbound methods.

Basically, a call to a member function (like method_one), a bound function

a_test.method_one()

is translated to

Test.method_one(a_test)

i.e. a call to an unbound method. Because of that, a call to your version of method_two will fail with a TypeError

>>> a_test = Test() 
>>> a_test.method_two()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: method_two() takes no arguments (1 given) 

You can change the behavior of a method using a decorator

class Test(object):
    def method_one(self):
        print "Called method_one"

    @staticmethod
    def method_two():
        print "Called method two"

The decorator tells the built-in default metaclass type (the class of a class, cf. this question) to not create bound methods for method_two.

Now, you can invoke static method both on an instance or on the class directly:

>>> a_test = Test()
>>> a_test.method_one()
Called method_one
>>> a_test.method_two()
Called method_two
>>> Test.method_two()
Called method_two

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