Python>=3.9

只需同时使用两个装饰器。看见 this answer

Python<3.9

属性是在类上创建的,但会影响实例。因此,如果需要classmethod属性,请在元类上创建该属性。

>>> class foo(object):
...     _var = 5
...     class __metaclass__(type):  # Python 2 syntax for metaclasses
...         pass
...     @classmethod
...     def getvar(cls):
...         return cls._var
...     @classmethod
...     def setvar(cls, value):
...         cls._var = value
...     
>>> foo.__metaclass__.var = property(foo.getvar.im_func, foo.setvar.im_func)
>>> foo.var
5
>>> foo.var = 3
>>> foo.var
3

>>> class foo(object):
...     _var = 5
...     class __metaclass__(type):  # Python 2 syntax for metaclasses
...         @property
...         def var(cls):
...             return cls._var
...         @var.setter
...         def var(cls, value):
...             cls._var = value
... 
>>> foo.var
5
>>> foo.var = 3
>>> foo.var
3

或者,使用Python3的 metaclass=... 语法,以及在 foo 类主体,以及负责设置 _var :

>>> class foo_meta(type):
...     def __init__(cls, *args, **kwargs):
...         cls._var = 5
...     @property
...     def var(cls):
...         return cls._var
...     @var.setter
...     def var(cls, value):
...         cls._var = value
...
>>> class foo(metaclass=foo_meta):
...     pass
...
>>> foo.var
5
>>> foo.var = 3
>>> foo.var
3

阅读 Python 2.2 release

在以下情况下不会调用[属性的]get方法: 该属性作为类进行访问 属性(C.x),而不是作为 实例属性(C().x)。如果你 要覆盖uuu get uuu操作吗 用于用作类时的属性 属性,则可以将属性子类化- 它本身就是一种新型的 扩展它的_uget _uu方法,或者您可以 从头定义描述符类型 通过创建一个新样式的类 定义获取、设置和 __删除方法。

因此,我认为规定的解决方案是创建一个ClassProperty作为property的子类。

class ClassProperty(property):
    def __get__(self, cls, owner):
        return self.fget.__get__(None, owner)()

class foo(object):
    _var=5
    def getvar(cls):
        return cls._var
    getvar=classmethod(getvar)
    def setvar(cls,value):
        cls._var=value
    setvar=classmethod(setvar)
    var=ClassProperty(getvar,setvar)

assert foo.getvar() == 5
foo.setvar(4)
assert foo.getvar() == 4
assert foo.var == 4
foo.var = 3
assert foo.var == 3

但是,二传手实际上不起作用:

foo.var = 4
assert foo.var == foo._var # raises AssertionError

foo._var 如果未更改,则只需使用新值覆盖属性。

你也可以使用 ClassProperty

class foo(object):
    _var = 5

    @ClassProperty
    @classmethod
    def var(cls):
        return cls._var

    @var.setter
    @classmethod
    def var(cls, value):
        cls._var = value

assert foo.var == 5

我希望这个死简单只读 @classproperty decorator会帮助寻找classproperties的人。

class classproperty(object):

    def __init__(self, fget):
        self.fget = fget

    def __get__(self, owner_self, owner_cls):
        return self.fget(owner_cls)

class C(object):

    @classproperty
    def x(cls):
        return 1

assert C.x == 1
assert C().x == 1

Python 3.9 2020更新

您可以将它们一起使用:

class G:
    @classmethod
    @property
    def __doc__(cls):
        return f'A doc for {cls.__name__!r}'

顺序问题-由于描述符如何相互作用, @classmethod 必须在上面。

看见 https://docs.python.org/3.9/library/functions.html#classmethod

可以将property()函数与classmethod修饰函数一起使用吗?

不

然而,classmethod只是一个绑定方法(部分函数),它位于一个可从该类的实例访问的类上。

property :

class Example(object):
    _class_property = None
    @property
    def class_property(self):
        return self._class_property
    @class_property.setter
    def class_property(self, value):
        type(self)._class_property = value
    @class_property.deleter
    def class_property(self):
        del type(self)._class_property

此代码可用于测试-它应在不引发任何错误的情况下通过:

ex1 = Example()
ex2 = Example()
ex1.class_property = None
ex2.class_property = 'Example'
assert ex1.class_property is ex2.class_property
del ex2.class_property
assert not hasattr(ex1, 'class_property')

请注意,我们根本不需要元类——而且您也不能通过元类的实例直接访问元类。

写一篇 @classproperty 室内装修设计师

实际上,您可以创建一个 classproperty 通过子类化,只需几行代码即可实现装饰器 所有物 (它是用C实现的,但您可以看到与之相当的Python here ):

class classproperty(property):
    def __get__(self, obj, objtype=None):
        return super(classproperty, self).__get__(objtype)
    def __set__(self, obj, value):
        super(classproperty, self).__set__(type(obj), value)
    def __delete__(self, obj):
        super(classproperty, self).__delete__(type(obj))

class Foo(object):
    _bar = 5
    @classproperty
    def bar(cls):
        """this is the bar attribute - each subclass of Foo gets its own.
        Lookups should follow the method resolution order.
        """
        return cls._bar
    @bar.setter
    def bar(cls, value):
        cls._bar = value
    @bar.deleter
    def bar(cls):
        del cls._bar

这段代码应该可以正常工作:

def main():
    f = Foo()
    print(f.bar)
    f.bar = 4
    print(f.bar)
    del f.bar
    try:
        f.bar
    except AttributeError:
        pass
    else:
        raise RuntimeError('f.bar must have worked - inconceivable!')
    help(f)  # includes the Foo.bar help.
    f.bar = 5

    class Bar(Foo):
        "a subclass of Foo, nothing more"
    help(Bar) # includes the Foo.bar help!
    b = Bar()
    b.bar = 'baz'
    print(b.bar) # prints baz
    del b.bar
    print(b.bar) # prints 5 - looked up from Foo!

    
if __name__ == '__main__':
    main()

但我不确定这是否明智。旧邮件列表 article 表明它不应该起作用。

__dict__ .

__口述__ . 例如:

class MetaWithFooClassProperty(type):
    @property
    def foo(cls):
        """The foo property is a function of the class -
        in this case, the trivial case of the identity function.
        """
        return cls

然后元类的类实例可以有一个属性,该属性使用前面几节中已经演示的原理访问类的属性:

class FooClassProperty(metaclass=MetaWithFooClassProperty):
    @property
    def foo(self):
        """access the class's property"""
        return type(self).foo

现在我们看到两个实例

>>> FooClassProperty().foo
<class '__main__.FooClassProperty'>

班级呢

>>> FooClassProperty.foo
<class '__main__.FooClassProperty'>

Python 3!

[更新:见 @Amit Portnoy's answer 对于版本中更干净的方法>=3.9]

老问题,很多视图,非常需要一个真正的Python 3方式。

幸运的是,这很容易和 metaclass 夸格:

class FooProperties(type):
    
    @property
    def var(cls):
        return cls._var

class Foo(object, metaclass=FooProperties):
    _var = 'FOO!'

>>> Foo.var

“福!”

没有合理的方法使这个“类属性”系统在Python中工作。

class ClassProperty(object):
    def __init__(self, getter, setter):
        self.getter = getter
        self.setter = setter
    def __get__(self, cls, owner):
        return getattr(cls, self.getter)()
    def __set__(self, cls, value):
        getattr(cls, self.setter)(value)

class MetaFoo(type):
    var = ClassProperty('getvar', 'setvar')

class Foo(object):
    __metaclass__ = MetaFoo
    _var = 5
    @classmethod
    def getvar(cls):
        print "Getting var =", cls._var
        return cls._var
    @classmethod
    def setvar(cls, value):
        print "Setting var =", value
        cls._var = value

x = Foo.var
print "Foo.var = ", x
Foo.var = 42
x = Foo.var
print "Foo.var = ", x

问题的症结在于属性是Python所称的“描述符”。没有简单明了的方法来解释这种元编程是如何工作的,所以我必须指出 descriptor howto

只有在实现一个相当高级的框架时,才需要理解这类事情。比如透明对象持久化或RPC系统,或者一种特定于域的语言。

然而,在对之前答案的评论中,你说你

需要修改一个属性,该属性以一个类的所有实例都可以看到的方式,并且在调用这些类方法的作用域中没有对该类的所有实例的引用。

在我看来,你真正想要的是 Observer

如果您希望通过实例化对象访问class属性,那么仅在meta类上设置它是没有帮助的,在这种情况下,您还需要在对象上安装一个普通属性(它将分派给class属性)。我认为以下几点更为明确:

#!/usr/bin/python

class classproperty(property):
    def __get__(self, obj, type_):
        return self.fget.__get__(None, type_)()

    def __set__(self, obj, value):
        cls = type(obj)
        return self.fset.__get__(None, cls)(value)

class A (object):

    _foo = 1

    @classproperty
    @classmethod
    def foo(cls):
        return cls._foo

    @foo.setter
    @classmethod
    def foo(cls, value):
        cls.foo = value

a = A()

print a.foo

b = A()

print b.foo

b.foo = 5

print a.foo

A.foo = 10

print b.foo

print A.foo

class ClassProperty(object):
    def __init__(self, fget, fset):
        self.fget = fget
        self.fset = fset

    def __get__(self, instance, owner):
        return self.fget()

    def __set__(self, instance, value):
        self.fset(value)

class Foo(object):
    _bar = 1
    def get_bar():
        print 'getting'
        return Foo._bar

    def set_bar(value):
        print 'setting'
        Foo._bar = value

    bar = ClassProperty(get_bar, set_bar)

f = Foo()
#__get__ works
f.bar
Foo.bar

f.bar = 2
Foo.bar = 3 #__set__ does not

因为我需要修改一个属性,该属性可以被类的所有实例看到,并且在调用这些类方法的作用域中没有对类的所有实例的引用。

您是否至少有权访问该类的一个实例?我可以想出一个办法:

class MyClass (object):
    __var = None

    def _set_var (self, value):
        type (self).__var = value

    def _get_var (self):
        return self.__var

    var = property (_get_var, _set_var)

a = MyClass ()
b = MyClass ()
a.var = "foo"
print b.var

尝试一下,它可以在不必更改/添加大量现有代码的情况下完成任务。

>>> class foo(object):
...     _var = 5
...     def getvar(cls):
...         return cls._var
...     getvar = classmethod(getvar)
...     def setvar(cls, value):
...         cls._var = value
...     setvar = classmethod(setvar)
...     var = property(lambda self: self.getvar(), lambda self, val: self.setvar(val))
...
>>> f = foo()
>>> f.var
5
>>> f.var = 3
>>> f.var
3

这个 property callable 论据。给他们lambda包装器(它将实例作为第一个参数传递给他们),一切正常。

这里有一个解决方案,它既可以通过类进行访问,也可以通过使用元类的实例进行访问。

In [1]: class ClassPropertyMeta(type):
   ...:     @property
   ...:     def prop(cls):
   ...:         return cls._prop
   ...:     def __new__(cls, name, parents, dct):
   ...:         # This makes overriding __getattr__ and __setattr__ in the class impossible, but should be fixable
   ...:         dct['__getattr__'] = classmethod(lambda cls, attr: getattr(cls, attr))
   ...:         dct['__setattr__'] = classmethod(lambda cls, attr, val: setattr(cls, attr, val))
   ...:         return super(ClassPropertyMeta, cls).__new__(cls, name, parents, dct)
   ...:

In [2]: class ClassProperty(object):
   ...:     __metaclass__ = ClassPropertyMeta
   ...:     _prop = 42
   ...:     def __getattr__(self, attr):
   ...:         raise Exception('Never gets called')
   ...:

In [3]: ClassProperty.prop
Out[3]: 42

In [4]: ClassProperty.prop = 1
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-4-e2e8b423818a> in <module>()
----> 1 ClassProperty.prop = 1

AttributeError: can't set attribute

In [5]: cp = ClassProperty()

In [6]: cp.prop
Out[6]: 42

In [7]: cp.prop = 1
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-7-e8284a3ee950> in <module>()
----> 1 cp.prop = 1

<ipython-input-1-16b7c320d521> in <lambda>(cls, attr, val)
      6         # This makes overriding __getattr__ and __setattr__ in the class impossible, but should be fixable
      7         dct['__getattr__'] = classmethod(lambda cls, attr: getattr(cls, attr))
----> 8         dct['__setattr__'] = classmethod(lambda cls, attr, val: setattr(cls, attr, val))
      9         return super(ClassPropertyMeta, cls).__new__(cls, name, parents, dct)

AttributeError: can't set attribute

这也适用于元类中定义的setter。

我找到了一个解决这个问题的干净办法。这是一个叫做 类别实用程序 ( pip install classutilities ),请参阅文档 here on PyPi .

考虑例子:

import classutilities

class SomeClass(classutilities.ClassPropertiesMixin):
    _some_variable = 8  # Some encapsulated class variable

    @classutilities.classproperty
    def some_variable(cls):  # class property getter
        return cls._some_variable

    @some_variable.setter
    def some_variable(cls, value):  # class property setter
        cls._some_variable = value

您可以在类级别和实例级别上使用它:

# Getter on class level:
value = SomeClass.some_variable
print(value)  # >>> 8
# Getter on instance level
inst = SomeClass()
value = inst.some_variable
print(value)  # >>> 8

# Setter on class level:
new_value = 9
SomeClass.some_variable = new_value
print(SomeClass.some_variable)   # >>> 9
print(SomeClass._some_variable)  # >>> 9
# Setter on instance level
inst = SomeClass()
inst.some_variable = new_value
print(SomeClass.some_variable)   # >>> 9
print(SomeClass._some_variable)  # >>> 9
print(inst.some_variable)        # >>> 9
print(inst._some_variable)       # >>> 9

https://stackoverflow.com/a/1800999/2290820

class MetaProperty(type):

    def __init__(cls, *args, **kwargs):
        super()

    @property
    def praparty(cls):
        return cls._var

    @praparty.setter
    def praparty(cls, val):
        cls._var = val


class A(metaclass=MetaProperty):
    _var = 5


print(A.praparty)
A.praparty = 6
print(A.praparty)

对Python2和Python3有效。

from future.utils import with_metaclass

class BuilderMetaClass(type):
    @property
    def load_namespaces(self):
        return (self.__sourcepath__)

class BuilderMixin(with_metaclass(BuilderMetaClass, object)):
    __sourcepath__ = 'sp'        

print(BuilderMixin.load_namespaces)

这是我的解决方案,它也缓存类属性

class class_property(object):
    # this caches the result of the function call for fn with cls input
    # use this as a decorator on function methods that you want converted
    # into cached properties

    def __init__(self, fn):
        self._fn_name = fn.__name__
        if not isinstance(fn, (classmethod, staticmethod)):
            fn = classmethod(fn)
        self._fn = fn

    def __get__(self, obj, cls=None):
        if cls is None:
            cls = type(obj)
        if (
            self._fn_name in vars(cls) and
            type(vars(cls)[self._fn_name]).__name__ != "class_property"
        ):
            return vars(cls)[self._fn_name]
        else:
            value = self._fn.__get__(obj, cls)()
            setattr(cls, self._fn_name, value)
            return value

这是我的建议。不要使用类方法。

在这种情况下使用类方法的原因是什么?为什么没有一个普通类的普通对象呢?

只有在有需要隐藏的内容时才应该使用属性,这些内容可能在将来的实现中发生更改。

也许你的例子很简单,还有一些地狱般的计算。但它看起来并没有带来显著的价值。

受Java影响的“隐私”技术(在Python中,属性名称以_开头)实际上没有多大帮助。谁的私人信件?当您拥有源代码时,private的意义有点模糊(就像在Python中一样)

受Java影响的EJB风格的getter和setter(通常在Python中作为属性完成)用于促进Java的基本自省,以及通过静态语言编译器。所有这些getter和setter在Python中都没有那么有用。