Why Does Using `property()` with `@classmethod` Decorated Methods Cause Errors in Python?

Using property() with Class Methods

Problem:

Using the property() function with class methods decorated with the @classmethod decorator results in an error.

Example Reproduction:

class Foo(object):
    _var = 5

    @classmethod
    def getvar(cls):
        return cls._var

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

    var = property(getvar, setvar)

Error:

>>> f = Foo()
>>> f.getvar()
5
>>> f.setvar(4)
>>> f.getvar()
4
>>> f.var
Traceback (most recent call last):
  File <stdin>, line 1, in ?
TypeError: 'classmethod' object is not callable
>>> f.var=5
Traceback (most recent call last):
  File <stdin>, line 1, in ?
TypeError: 'classmethod' object is not callable</stdin></stdin>

Solution:

Python 3.8 and Later:

In Python 3.8 and later, it is possible to use the property() function with @classmethod decorated functions. Simply apply both decorators to the method.

Python 2 and Python 3 (Works in 3.9-3.10 too)

A property is created on a class but affects an instance. To create 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

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