Using property() on classmethods

Python >= 3.9

Just use both decorators together. See this answer.

Python < 3.9

A property is created on a class but affects an instance. So if you want a classmethod property, create the property on the metaclass.

>>> 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

But since you're using a metaclass anyway, it will read better if you just move the classmethods in there.

>>> 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

or, using Python 3's metaclass=... syntax, and the metaclass defined outside of the foo class body, and the metaclass responsible for setting the initial value of _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

Reading the Python 2.2 release notes, I find the following.

The get method [of a property] won't be called when the property is accessed as a class attribute (C.x) instead of as an instance attribute (C().x). If you want to override the __get__ operation for properties when used as a class attribute, you can subclass property - it is a new-style type itself - to extend its __get__ method, or you can define a descriptor type from scratch by creating a new-style class that defines __get__, __set__ and __delete__ methods.

NOTE: The below method doesn't actually work for setters, only getters.

Therefore, I believe the prescribed solution is to create a ClassProperty as a subclass of property.

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

However, the setters don't actually work:

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

foo._var is unchanged, you've simply overwritten the property with a new value.

You can also use ClassProperty as a decorator:

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