关于python：除了将self改为cls之外，classmethod还能做什么？

What does classmethod do except changing self to cls?

有一个关于classmethod和property组合在一起的回答问题：在类方法上使用property()。

我还是不明白问题的原因，请帮忙。

我对classmethod的理解是，它只是用cls取代了self。考虑到这一点，在过去的几年里我写了几个类方法，现在我发现我一直都是错的。

那么，@classmethod和@cm与下面的代码有什么区别？

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def cm(func):
def decorated(self, *args, **kwargs):
return func(self.__class__, *args, **kwargs)
return decorated

class C:
V = 0

@property
@classmethod
def inc1(cls):
cls.V += 1
print("V1 =", cls.V)

@property
@cm
def inc3(cls):
cls.V += 3
print("V3 =", cls.V)

c = C()
#c.inc1 # fails with: TypeError: 'classmethod' object is not callable
c.inc3 # works

inc3和cm起作用，但inc1和classmethod起作用。

what is the difference between @classmethod and @cm from the code below?

在类创建期间，在创建实例之前，decorator正在调用。

在您的例子中，@cm返回func(self.__class__, *args, **kwargs)，这依赖于self，因此它应该用作实例方法。

另一方面，@classmethod可以在创建实例之前使用。

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def cm(func):
def decorated(self, *args, **kwargs):
return func(self.__class__, *args, **kwargs)
return decorated

class C:
@classmethod
def inc1(cls):
(blablabla)
@cm
def inc3(cls):
(blablabla)

C().inc1() # works as a instance method
C.inc1() # works as a classmethod
C().inc3() # works as a instance method
C.inc3() # TypeError: unbound method decorated() must be called with C instance as first argument (got nothing instead)

对于ClassMethod和属性的组合，可以通过返回自定义对象来完成。参考文献

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class ClassPropertyDescriptor(object):
def __init__(self, f):
self.f = f
def __get__(self, obj, klass=None):
if klass is None:
klass = type(obj)
return self.f.__get__(obj, klass)()

def classproperty(func):
if not isinstance(func, (classmethod, staticmethod)):
func = classmethod(func)
return ClassPropertyDescriptor(func)

class C:
@classproperty
def inc1(cls):
(blablabla)

C.inc1 # works as a classmethod property

[编辑]

Q. What does the classmethod() call do with the method it decorates to achieve that?

可以通过使用描述符来实现

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class ClassMethodDescriptor(object):
def __init__(self, f):
self.f = f
def __get__(self, obj, klass=None):
if klass is None:
klass = type(obj)
def newfunc(*args):
return self.f(klass, *args)
return newfunc

def myclassmethod(func):
return ClassMethodDescriptor(func)

class C:
@myclassmethod
def inc1(cls):
(blablabla)

C.inc1() # works as a classmethod

Q. Why is the result not callable?

因为ClassMethodDescriptor的实现没有定义__call__函数。一旦使用@property，它将返回不可调用的ClassMethodDescriptor。