Python中staticmethod和classmethod的差别

草莓猪

Class vs static methods in Python

这篇文章试图表明：什么是staticmethod/classmethod,而且这两者之间的差别.

staticmethod和classmethod均被作为装饰器，用作界说一个函数为”staticmethod”照旧”classmethod”

假如想要相识Python装饰器的底子，可以看 这篇文章（http://www.pythoncentral.io/python-decorators-overview/）

Simple, static and class methods

类中最常用到的方法是 实例方法(instance methods), 即，实例对象作为第一个参数通报给函数

比方，下面是一个根本的实例方法

[color=#800080 !important]class[color=#006FE0 !important] [color=teal !important]Kls[color=#333333 !important]([color=#800080 !important]object[color=#333333 !important])[color=#006FE0 !important]:
[color=#006FE0 !important]    [color=teal !important]def __init__[color=#333333 !important](self[color=#333333 !important],[color=#006FE0 !important] [color=#002D7A !important]data[color=#333333 !important])[color=#006FE0 !important]:
[color=#006FE0 !important]        self[color=#333333 !important].[color=#002D7A !important]data[color=#006FE0 !important] = [color=teal !important]data

[color=teal !important]    def printd[color=#333333 !important](self[color=#333333 !important])[color=#006FE0 !important]:
[color=#006FE0 !important]        [color=teal !important]print[color=#333333 !important](self[color=#333333 !important].[color=#002D7A !important]data[color=#333333 !important])

[color=#002D7A !important]ik1[color=#006FE0 !important] = [color=teal !important]Kls[color=#333333 !important]([color=#DD1144 !important]'arun'[color=#333333 !important])
[color=#002D7A !important]ik2[color=#006FE0 !important] = [color=teal !important]Kls[color=#333333 !important]([color=#DD1144 !important]'seema'[color=#333333 !important])

[color=#002D7A !important]ik1[color=#333333 !important].[color=teal !important]printd[color=#333333 !important]()
[color=#002D7A !important]ik2[color=#333333 !important].[color=teal !important]printd[color=#333333 !important]()

得到的输出:

[color=teal !important]arun
[color=#002D7A !important]seema

调用关系图:

检察代码和图解:

[color=#009999 !important]1[color=#006FE0 !important]/[color=#009999 !important]2[color=#006FE0 !important] 参数通报给函数
[color=#009999 !important]3[color=#006FE0 !important]   self参数指向实例自己[color=#006FE0 !important]
[color=#009999 !important]4[color=#006FE0 !important]   我们不必要显式提供实例，表明器自己会处置惩罚

如果我们想仅实现类之间交互而不是通过实例？我们可以在类之外创建一个简朴的函数来实现这个功能，但是将会使代码扩散到类之外，这个大概对将来代码维护带来题目。


比方：

[color=teal !important]def get_no_of_instances[color=#333333 !important]([color=#002D7A !important]cls_obj[color=#333333 !important])[color=#006FE0 !important]:
[color=#006FE0 !important]    return[color=#006FE0 !important] [color=#002D7A !important]cls_obj[color=#333333 !important].[color=teal !important]no_inst

[color=#800080 !important]class[color=#006FE0 !important] [color=teal !important]Kls[color=#333333 !important]([color=#800080 !important]object[color=#333333 !important])[color=#006FE0 !important]:
[color=#006FE0 !important]    [color=#002D7A !important]no_inst[color=#006FE0 !important] = [color=#009999 !important]0

[color=#006FE0 !important]    [color=teal !important]def __init__[color=#333333 !important](self[color=#333333 !important])[color=#006FE0 !important]:
[color=#006FE0 !important]        [color=#002D7A !important]Kls[color=#333333 !important].[color=#002D7A !important]no_inst[color=#006FE0 !important] = [color=#002D7A !important]Kls[color=#333333 !important].[color=#002D7A !important]no_inst[color=#006FE0 !important] + [color=#009999 !important]1

[color=#002D7A !important]ik1[color=#006FE0 !important] = [color=teal !important]Kls[color=#333333 !important]()
[color=#002D7A !important]ik2[color=#006FE0 !important] = [color=teal !important]Kls[color=#333333 !important]()

[color=teal !important]print[color=#333333 !important]([color=teal !important]get_no_of_instances[color=#333333 !important]([color=#002D7A !important]Kls[color=#333333 !important]))

效果:


2


The Python @classmethod


如今我们要做的是在类里创建一个函数，这个函数参数是类对象而不是实例对象.


在上面谁人实现中，假如要实现不获取实例,必要修改如下:

[color=teal !important]def iget_no_of_instance[color=#333333 !important]([color=#002D7A !important]ins_obj[color=#333333 !important])[color=#006FE0 !important]:
[color=#006FE0 !important]    return[color=#006FE0 !important] [color=#002D7A !important]ins_obj[color=#333333 !important].[color=#002D7A !important]__class__[color=#333333 !important].[color=teal !important]no_inst

[color=#800080 !important]class[color=#006FE0 !important] [color=teal !important]Kls[color=#333333 !important]([color=#800080 !important]object[color=#333333 !important])[color=#006FE0 !important]:
[color=#006FE0 !important]    [color=#002D7A !important]no_inst[color=#006FE0 !important] = [color=#009999 !important]0

[color=#006FE0 !important]    [color=teal !important]def __init__[color=#333333 !important](self[color=#333333 !important])[color=#006FE0 !important]:
[color=#006FE0 !important]        [color=#002D7A !important]Kls[color=#333333 !important].[color=#002D7A !important]no_inst[color=#006FE0 !important] = [color=#002D7A !important]Kls[color=#333333 !important].[color=#002D7A !important]no_inst[color=#006FE0 !important] + [color=#009999 !important]1

[color=#002D7A !important]ik1[color=#006FE0 !important] = [color=teal !important]Kls[color=#333333 !important]()
[color=#002D7A !important]ik2[color=#006FE0 !important] = [color=teal !important]Kls[color=#333333 !important]()
[color=teal !important]print iget_no_of_instance[color=#333333 !important]([color=#002D7A !important]ik1[color=#333333 !important])

效果
[color=#009999 !important]2

可以利用Python2.2引入的新特性，利用@classmethod在类代码中创建一个函数

[color=#800080 !important]class[color=#006FE0 !important] [color=teal !important]Kls[color=#333333 !important]([color=#800080 !important]object[color=#333333 !important])[color=#006FE0 !important]:
[color=#006FE0 !important]    [color=#002D7A !important]no_inst[color=#006FE0 !important] = [color=#009999 !important]0

[color=#006FE0 !important]    [color=teal !important]def __init__[color=#333333 !important](self[color=#333333 !important])[color=#006FE0 !important]:
[color=#006FE0 !important]        [color=#002D7A !important]Kls[color=#333333 !important].[color=#002D7A !important]no_inst[color=#006FE0 !important] = [color=#002D7A !important]Kls[color=#333333 !important].[color=#002D7A !important]no_inst[color=#006FE0 !important] + [color=#009999 !important]1

[color=#006FE0 !important]    [color=#333333 !important]@[color=teal !important]classmethod
[color=teal !important]    def get_no_of_instance[color=#333333 !important]([color=#002D7A !important]cls_obj[color=#333333 !important])[color=#006FE0 !important]:
[color=#006FE0 !important]        return[color=#006FE0 !important] [color=#002D7A !important]cls_obj[color=#333333 !important].[color=teal !important]no_inst

[color=#002D7A !important]ik1[color=#006FE0 !important] = [color=teal !important]Kls[color=#333333 !important]()
[color=#002D7A !important]ik2[color=#006FE0 !important] = [color=teal !important]Kls[color=#333333 !important]()

[color=teal !important]print [color=#002D7A !important]ik1[color=#333333 !important].[color=teal !important]get_no_of_instance[color=#333333 !important]()
[color=teal !important]print [color=#002D7A !important]Kls[color=#333333 !important].[color=teal !important]get_no_of_instance[color=#333333 !important]()

We get the following output:

[color=#009999 !important]2
[color=#009999 !important]2

The Python @staticmethod


通常，有许多环境下一些函数与类相干，但不必要任何类或实例变量就可以实现一些功能.


好比设置情况变量，修改另一个类的属性等等.这种环境下，我们也可以利用一个函数，一样会将代码扩散到类之外（难以维护）


下面是一个例子:

[color=#002D7A !important]IND[color=#006FE0 !important] = [color=#DD1144 !important]'ON'

[color=teal !important]def checkind[color=#333333 !important]()[color=#006FE0 !important]:
[color=#006FE0 !important]    return[color=#006FE0 !important] [color=#333333 !important]([color=#002D7A !important]IND[color=#006FE0 !important] == [color=#DD1144 !important]'ON'[color=#333333 !important])

[color=#800080 !important]class[color=#006FE0 !important] [color=teal !important]Kls[color=#333333 !important]([color=#800080 !important]object[color=#333333 !important])[color=#006FE0 !important]:
[color=#006FE0 !important]    [color=teal !important]def __init__[color=#333333 !important](self[color=#333333 !important],[color=#002D7A !important]data[color=#333333 !important])[color=#006FE0 !important]:
[color=#006FE0 !important]        self[color=#333333 !important].[color=#002D7A !important]data[color=#006FE0 !important] = [color=teal !important]data

[color=teal !important]    def do_reset[color=#333333 !important](self[color=#333333 !important])[color=#006FE0 !important]:
[color=#006FE0 !important]        if[color=#006FE0 !important] [color=teal !important]checkind[color=#333333 !important]()[color=#006FE0 !important]:
[color=#006FE0 !important]            [color=teal !important]print[color=#333333 !important]([color=#DD1144 !important]'Reset done for:'[color=#333333 !important],[color=#006FE0 !important] self[color=#333333 !important].[color=#002D7A !important]data[color=#333333 !important])

[color=#006FE0 !important]    [color=teal !important]def set_db[color=#333333 !important](self[color=#333333 !important])[color=#006FE0 !important]:
[color=#006FE0 !important]        if[color=#006FE0 !important] [color=teal !important]checkind[color=#333333 !important]()[color=#006FE0 !important]:
[color=#006FE0 !important]            self[color=#333333 !important].[color=#002D7A !important]db[color=#006FE0 !important] = [color=#DD1144 !important]'new db connection'
[color=#006FE0 !important]            [color=teal !important]print[color=#333333 !important]([color=#DD1144 !important]'DB connection made for:'[color=#333333 !important],self[color=#333333 !important].[color=#002D7A !important]data[color=#333333 !important])

[color=#002D7A !important]ik1[color=#006FE0 !important] = [color=teal !important]Kls[color=#333333 !important]([color=#009999 !important]12[color=#333333 !important])

[color=#002D7A !important]ik1[color=#333333 !important].[color=teal !important]do_reset[color=#333333 !important]()
[color=#002D7A !important]ik1[color=#333333 !important].[color=teal !important]set_db[color=#333333 !important]()

效果:

[color=teal !important]Reset done for[color=#006FE0 !important]: [color=#009999 !important]12
[color=teal !important]DB connection made for[color=#006FE0 !important]: [color=#009999 !important]12

如今我们利用@staticmethod, 我们可以将全部代码放到类中

[color=#002D7A !important]IND[color=#006FE0 !important] = [color=#DD1144 !important]'ON'

[color=#800080 !important]class[color=#006FE0 !important] [color=teal !important]Kls[color=#333333 !important]([color=#800080 !important]object[color=#333333 !important])[color=#006FE0 !important]:
[color=#006FE0 !important]    [color=teal !important]def __init__[color=#333333 !important](self[color=#333333 !important],[color=#006FE0 !important] [color=#002D7A !important]data[color=#333333 !important])[color=#006FE0 !important]:
[color=#006FE0 !important]        self[color=#333333 !important].[color=#002D7A !important]data[color=#006FE0 !important] = data

[color=#006FE0 !important]    [color=#333333 !important]@[color=teal !important]staticmethod
[color=teal !important]    def checkind[color=#333333 !important]()[color=#006FE0 !important]:
[color=#006FE0 !important]        return[color=#006FE0 !important] [color=#333333 !important]([color=#002D7A !important]IND[color=#006FE0 !important] == [color=#DD1144 !important]'ON'[color=#333333 !important])

[color=#006FE0 !important]    [color=teal !important]def do_reset[color=#333333 !important](self[color=#333333 !important])[color=#006FE0 !important]:
[color=#006FE0 !important]        if[color=#006FE0 !important] self[color=#333333 !important].[color=teal !important]checkind[color=#333333 !important]()[color=#006FE0 !important]:
[color=#006FE0 !important]            [color=teal !important]print[color=#333333 !important]([color=#DD1144 !important]'Reset done for:'[color=#333333 !important],[color=#006FE0 !important] self[color=#333333 !important].[color=#002D7A !important]data[color=#333333 !important])

[color=#006FE0 !important]    [color=teal !important]def set_db[color=#333333 !important](self[color=#333333 !important])[color=#006FE0 !important]:
[color=#006FE0 !important]        if[color=#006FE0 !important] self[color=#333333 !important].[color=teal !important]checkind[color=#333333 !important]()[color=#006FE0 !important]:
[color=#006FE0 !important]            self[color=#333333 !important].[color=#002D7A !important]db[color=#006FE0 !important] = [color=#DD1144 !important]'New db connection'
[color=#006FE0 !important]        [color=teal !important]print[color=#333333 !important]([color=#DD1144 !important]'DB connection made for: '[color=#333333 !important],[color=#006FE0 !important] self[color=#333333 !important].[color=#002D7A !important]data[color=#333333 !important])

[color=#002D7A !important]ik1[color=#006FE0 !important] = [color=teal !important]Kls[color=#333333 !important]([color=#009999 !important]12[color=#333333 !important])
[color=#002D7A !important]ik1[color=#333333 !important].[color=teal !important]do_reset[color=#333333 !important]()
[color=#002D7A !important]ik1[color=#333333 !important].[color=teal !important]set_db[color=#333333 !important]()

得到的效果:

[color=teal !important]Reset done for[color=#006FE0 !important]: [color=#009999 !important]12
[color=teal !important]DB connection made for[color=#006FE0 !important]: [color=#009999 !important]12

How @staticmethod and @classmethod are different

[color=#800080 !important]class[color=#006FE0 !important] [color=teal !important]Kls[color=#333333 !important]([color=#800080 !important]object[color=#333333 !important])[color=#006FE0 !important]:
[color=#006FE0 !important]    [color=teal !important]def __init__[color=#333333 !important](self[color=#333333 !important],[color=#006FE0 !important] [color=#002D7A !important]data[color=#333333 !important])[color=#006FE0 !important]:
[color=#006FE0 !important]        self[color=#333333 !important].[color=#002D7A !important]data[color=#006FE0 !important] = [color=teal !important]data

[color=teal !important]    def printd[color=#333333 !important](self[color=#333333 !important])[color=#006FE0 !important]:
[color=#006FE0 !important]        [color=teal !important]print[color=#333333 !important](self[color=#333333 !important].[color=#002D7A !important]data[color=#333333 !important])

[color=#006FE0 !important]    [color=#333333 !important]@[color=teal !important]staticmethod
[color=teal !important]    def smethod[color=#333333 !important]([color=#006FE0 !important]*[color=#002D7A !important]arg[color=#333333 !important])[color=#006FE0 !important]:
[color=#006FE0 !important]        [color=teal !important]print[color=#333333 !important]([color=#DD1144 !important]'Static:'[color=#333333 !important],[color=#006FE0 !important] [color=#002D7A !important]arg[color=#333333 !important])

[color=#006FE0 !important]    [color=#333333 !important]@[color=teal !important]classmethod
[color=teal !important]    def cmethod[color=#333333 !important]([color=#006FE0 !important]*[color=#002D7A !important]arg[color=#333333 !important])[color=#006FE0 !important]:
[color=#006FE0 !important]        [color=teal !important]print[color=#333333 !important]([color=#DD1144 !important]'Class:'[color=#333333 !important],[color=#006FE0 !important] [color=#002D7A !important]arg[color=#333333 !important])

调用

[color=#006FE0 !important]>>> [color=#002D7A !important]ik[color=#006FE0 !important] = [color=teal !important]Kls[color=#333333 !important]([color=#009999 !important]23[color=#333333 !important])
[color=#006FE0 !important]>>> [color=#002D7A !important]ik[color=#333333 !important].[color=teal !important]printd[color=#333333 !important]()
[color=#009999 !important]23
[color=#006FE0 !important]>>> [color=#002D7A !important]ik[color=#333333 !important].[color=teal !important]smethod[color=#333333 !important]()
[color=#800080 !important]Static[color=#006FE0 !important]: [color=#333333 !important]()
[color=#006FE0 !important]>>> [color=#002D7A !important]ik[color=#333333 !important].[color=teal !important]cmethod[color=#333333 !important]()
[color=#800080 !important]Class[color=#006FE0 !important]: [color=#333333 !important](,)
[color=#006FE0 !important]>>> [color=#002D7A !important]Kls[color=#333333 !important].[color=teal !important]printd[color=#333333 !important]()
[color=#002D7A !important]TypeError[color=#006FE0 !important]: [color=teal !important]unbound method printd[color=#333333 !important]()[color=#006FE0 !important] [color=teal !important]must be called with Kls instance as[color=#006FE0 !important] [color=teal !important]first argument[color=#006FE0 !important] [color=#333333 !important]([color=teal !important]got nothing [color=#002D7A !important]instead[color=#333333 !important])
[color=#006FE0 !important]>>> [color=#002D7A !important]Kls[color=#333333 !important].[color=teal !important]smethod[color=#333333 !important]()
[color=#800080 !important]Static[color=#006FE0 !important]: [color=#333333 !important]()
[color=#006FE0 !important]>>> [color=#002D7A !important]Kls[color=#333333 !important].[color=teal !important]cmethod[color=#333333 !important]()
[color=#800080 !important]Class[color=#006FE0 !important]: [color=#333333 !important](,)

图解

gxs1916

论坛有你更精彩！

草莓猪

真是难得给力的帖子啊。

japan2046

党的好公民，人民的好公仆。。。

啊哈肆

论坛有你更精彩！

mini0427

淡定，淡定，淡定……

@Xizi_BCjesikh

我顶，我顶，我顶顶顶

ssegg

加油，加油，不要沉下去，我是最热贴

mini042701

楼主您的技术水准，我最服你，其他都是浮云

阳光新天地

激动人心，无法言表！