Think about it this way: normal methods are useful to hide the details of dispatch: you can type myobj.foo() without worrying about whether the foo() method is implemented by the myobj object's class or one of its parent classes. Class methods are exactly analogous to this, but with the class object instead: they let you call MyClass.foo() without having to worry about whether foo() is implemented specially by MyClass because it needed its own specialized version, or whether it is letting its parent class handle the call.

当您在创建实际实例之前进行设置或计算时，类方法是必不可少的，因为在实例存在之前，您显然不能将实例用作方法调用的调度点。在SQLAlchemy源代码中可以看到一个很好的例子;在下面的链接中查看dbapi()类方法:

https://github.com/zzzeek/sqlalchemy/blob/ab6946769742602e40fb9ed9dde5f642885d1906/lib/sqlalchemy/dialects/mssql/pymssql.py#L47

You can see that the dbapi() method, which a database backend uses to import the vendor-specific database library it needs on-demand, is a class method because it needs to run before instances of a particular database connection start getting created — but that it cannot be a simple function or static function, because they want it to be able to call other, supporting methods that might similarly need to be written more specifically in subclasses than in their parent class. And if you dispatch to a function or static class, then you "forget" and lose the knowledge about which class is doing the initializing.

Think about it this way: normal methods are useful to hide the details of dispatch: you can type myobj.foo() without worrying about whether the foo() method is implemented by the myobj object's class or one of its parent classes. Class methods are exactly analogous to this, but with the class object instead: they let you call MyClass.foo() without having to worry about whether foo() is implemented specially by MyClass because it needed its own specialized version, or whether it is letting its parent class handle the call.

当您在创建实际实例之前进行设置或计算时，类方法是必不可少的，因为在实例存在之前，您显然不能将实例用作方法调用的调度点。在SQLAlchemy源代码中可以看到一个很好的例子;在下面的链接中查看dbapi()类方法:

https://github.com/zzzeek/sqlalchemy/blob/ab6946769742602e40fb9ed9dde5f642885d1906/lib/sqlalchemy/dialects/mssql/pymssql.py#L47

You can see that the dbapi() method, which a database backend uses to import the vendor-specific database library it needs on-demand, is a class method because it needs to run before instances of a particular database connection start getting created — but that it cannot be a simple function or static function, because they want it to be able to call other, supporting methods that might similarly need to be written more specifically in subclasses than in their parent class. And if you dispatch to a function or static class, then you "forget" and lose the knowledge about which class is doing the initializing.

我以前用过PHP，最近我在问自己，这个类方法是怎么回事?Python手册是非常技术性的，非常简短的文字，所以它不会帮助理解这个功能。我一直在谷歌上搜索，然后我找到了答案——> http://code.anjanesh.net/2007/12/python-classmethods.html。

如果你懒得点击它。我的解释很简短。：）

在PHP中(也许不是所有人都知道PHP，但是这个语言非常直接，每个人都应该明白我在说什么)，我们有这样的静态变量:

class A
{

    static protected $inner_var = null;

    static public function echoInnerVar()
    {
        echo self::$inner_var."\n";
    }

    static public function setInnerVar($v)
    {
        self::$inner_var = $v;
    }

}

class B extends A
{
}

A::setInnerVar(10);
B::setInnerVar(20);

A::echoInnerVar();
B::echoInnerVar();

在这两种情况下，输出都是20。

但是在python中，我们可以添加@classmethod装饰器，这样就可以分别输出10和20。例子:

class A(object):
    inner_var = 0

    @classmethod
    def setInnerVar(cls, value):
        cls.inner_var = value

    @classmethod
    def echoInnerVar(cls):
        print cls.inner_var


class B(A):
    pass


A.setInnerVar(10)
B.setInnerVar(20)

A.echoInnerVar()
B.echoInnerVar()

聪明,不是吗?

当用户登录我的网站时，user()对象从用户名和密码实例化。

如果我需要一个没有用户在那里登录的用户对象(例如，一个管理用户可能想要删除另一个用户帐户，所以我需要实例化该用户并调用它的delete方法):

我有类方法来获取用户对象。

class User():
    #lots of code
    #...
    # more code

    @classmethod
    def get_by_username(cls, username):
        return cls.query(cls.username == username).get()

    @classmethod
    def get_by_auth_id(cls, auth_id):
        return cls.query(cls.auth_id == auth_id).get()

工厂方法(替代构造函数)确实是类方法的经典例子。

基本上，类方法适用于任何您希望有一个方法自然地适合类的名称空间，但不与类的特定实例相关联的时候。

例如，在excellent unipath模块中:

当前目录

Path.cwd () 返回实际的当前目录;例如,路径(“/ tmp / my_temp_dir”)。这是一个类方法。 .chdir () 使self为当前目录。

由于当前目录是进程范围的，cwd方法没有应该与之关联的特定实例。但是，将cwd更改为给定Path实例的目录确实应该是一个实例方法。

嗯…因为Path.cwd()确实返回了一个Path实例，我猜它可以被认为是一个工厂方法…

它允许您编写可与任何兼容类一起使用的泛型类方法。

例如:

@classmethod
def get_name(cls):
    print cls.name

class C:
    name = "tester"

C.get_name = get_name

#call it:
C.get_name()

如果你不使用@classmethod，你可以用self关键字来做，但它需要一个Class的实例:

def get_name(self):
    print self.name

class C:
    name = "tester"

C.get_name = get_name

#call it:
C().get_name() #<-note the its an instance of class C