python内置函數2-classmethod()

Help on class classmethod in module __builtin__:

class classmethod(object)

 |  classmethod(function) -> method

 |  

 |  Convert a function to be a class method.

 |  A class method receives the class as implicit first argument,

 |  just like an instance method receives the instance.

 |  To declare a class method, use this idiom:

 |    class C:

 |        def f(cls, arg1, arg2, ...): ...

 |        f = classmethod(f)

 |  It can be called either on the class (e.g. C.f()) or on an instance

 |  (e.g. C().f()).  The instance is ignored except for its class.

 |  If a class method is called for a derived class, the derived class

 |  object is passed as the implied first argument.

 |  Class methods are different than C++ or Java static methods.

 |  If you want those, see the staticmethod builtin.

 |  Methods defined here:

 |  __get__(...)

 |      descr.__get__(obj[, type]) -> value

 |  __getattribute__(...)

 |      x.__getattribute__('name') <==> x.name

 |  __init__(...)

 |      x.__init__(...) initializes x; see help(type(x)) for signature

 |  ----------------------------------------------------------------------

 |  Data descriptors defined here:

 |  __func__

 |  Data and other attributes defined here:

 |  __new__ = <built-in method __new__ of type object>

 |      T.__new__(S, ...) -> a new object with type S, a subtype of T

classmethod(function)

Return a class method for function.

A class method receives the class as implicit first argument, just like an instance method receives the instance. To declare a class method, use this idiom:

class C(object):

    @classmethod

    def f(cls, arg1, arg2, ...):

        ...

The @classmethod form is a function decorator – see the description of function definitions in Function definitions for details.

It can be called either on the class (such as C.f()) or on an instance (such as C().f()). The instance is ignored except for its class. If a class method is called for a derived class, the derived class object is passed as the implied first argument.

Class methods are different than C++ or Java static methods. If you want those, see staticmethod() in this section.

For more information on class methods, consult the documentation on the standard type hierarchy in The standard type hierarchy.

Changed in version 2.4: Function decorator syntax added.

中文說明：

classmethod是用來指定一個類的方法為類方法，沒有此參數指定的類的方法為執行個體方法，使用方法如下：

class C: 

    def f(cls, arg1, arg2, ...): ... 

類方法既可以直接類調用(C.f())，也可以進行執行個體調用(C().f())。

>>> class C:

...     @classmethod

...     def f(self):

...         print "This is a class method"

... 

>>> C.f()

This is a class method

>>> c=C()

>>> c.f()

>>> class D:

...         print "This is not a class method"

>>> D.f()

Traceback (most recent call last):

  File "<stdin>", line 1, in <module>

TypeError: unbound method f() must be called with D instance as first argument (got nothing instead)

>>> d=D()