In Python 3.9 You could use them together, but (as noted in @xgt's comment) it was removed in Python 3.11, so it is not recommended to use it.

Check the version remarks here:

https://docs.python.org/3.11/library/functions.html#classmethod

However, it used to work like so:

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

Order matters - due to how the descriptors interact, @classmethod has to be on top.

Python 3!

See @Amit Portnoy's answer for an even cleaner method in python >= 3.9

Old question, lots of views, sorely in need of a one-true Python 3 way.

Luckily, it's easy with the metaclass kwarg:

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

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

Then, >>> Foo.var

'FOO!'

There is no reasonable way to make this "class property" system to work in Python.

Here is one unreasonable way to make it work. You can certainly make it more seamless with increasing amounts of metaclass magic.

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

The knot of the issue is that properties are what Python calls "descriptors". There is no short and easy way to explain how this sort of metaprogramming works, so I must point you to the descriptor howto.

You only ever need to understand this sort of things if you are implementing a fairly advanced framework. Like a transparent object persistence or RPC system, or a kind of domain-specific language.

However, in a comment to a previous answer, you say that you

need to modify an attribute that in such a way that is seen by all instances of a class, and in the scope from which these class methods are called does not have references to all instances of the class.

It seems to me, what you really want is an Observer design pattern.

Half a solution, __set__ on the class does not work, still. The solution is a custom property class implementing both a property and a staticmethod

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

Because I need to modify an attribute that in such a way that is seen by all instances of a class, and in the scope from which these class methods are called does not have references to all instances of the class.

Do you have access to at least one instance of the class? I can think of a way to do it then:

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

Give this a try, it gets the job done without having to change/add a lot of existing code.

>>> 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

The property function needs two callable arguments. give them lambda wrappers (which it passes the instance as its first argument) and all is well.

I found one clean solution to this problem. It's a package called classutilities (pip install classutilities), see the documentation here on PyPi.

Consider example:

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

You can use it on both class level and instance level:

# 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

As you can see, it works correctly under all circumstances.

Based on 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)

For a functional approach pre Python 3.9 you can use this:

def classproperty(fget):
  return type(
      'classproperty',
      (),
      {'__get__': lambda self, _, cls: fget(cls), '__module__': None}
  )()
  
class Item:
  a = 47

  @classproperty
  def x(cls):
    return cls.a

Item.x

After searching different places, I found a method to define a classproperty valid with Python 2 and 3.

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)

Hope this can help somebody :)

A code completion friendly solution for Python < 3.9

from typing import (
    Callable,
    Generic,
    TypeVar,
)


T = TypeVar('T')


class classproperty(Generic[T]):
    """Converts a method to a class property.
    """

    def __init__(self, f: Callable[..., T]):
        self.fget = f

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

Here is my solution that also caches the class property

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

Here's my suggestion. Don't use class methods.

Seriously.

What's the reason for using class methods in this case? Why not have an ordinary object of an ordinary class?

If you simply want to change the value, a property isn't really very helpful is it? Just set the attribute value and be done with it.

A property should only be used if there's something to conceal -- something that might change in a future implementation.

Maybe your example is way stripped down, and there is some hellish calculation you've left off. But it doesn't look like the property adds significant value.

The Java-influenced "privacy" techniques (in Python, attribute names that begin with _) aren't really very helpful. Private from whom? The point of private is a little nebulous when you have the source (as you do in Python.)

The Java-influenced EJB-style getters and setters (often done as properties in Python) are there to facilitate Java's primitive introspection as well as to pass muster with the static language compiler. All those getters and setters aren't as helpful in Python.