Why Can\'t I Use `property()` with Classmethods in Python, and What Are the Workarounds?

Using property() with classmethod: An Exploration

In Python, the property() function plays a crucial role in creating getters and setters for class attributes. However, attempting to use property() with classmethods, which are decorated with the @classmethod decorator, can lead to unexpected errors.

Let's consider the following code snippet that illustrates the issue:

class Foo(object):
    _var = 5

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

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

    var = property(getvar, setvar)

When trying to access or set the var attribute on an instance of the Foo class, we encounter an error:

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

As explained in the documentation and in stack Overflow discussions [Python Class Method as Setter and Getter](https://stackoverflow.com/questions/27906399/python-class-method-as-setter-and-getter), it's not possible to use the property() function with classmethods in Python 3.8 and earlier.

However, there are alternative approaches to achieving the desired functionality:

For Python 2 and Python 3 (including 3.9-3.10):

Since a property affects an instance, but a class property is created on the class, it's necessary to 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)

For Python versions from 3.9 onwards:

A cleaner solution involves moving the classmethods to the metaclass and declaring the property directly on the metaclass:

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

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

class Foo(metaclass=Foo_meta):
    _var = 5

These alternative methods effectively allow for the use of class-level properties.

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