How to create class properties in Python without a dedicated decorator?

How to Make a Class Property

A class property is a convenient way to access attributes through the class itself rather than through instances of the class. In Python, methods can be added to a class using the @classmethod decorator, but there is no such decorator for properties. However, a similar effect can be achieved through a custom descriptor class.

Custom Descriptor Class

The following code defines a custom descriptor class, ClassPropertyDescriptor, that enables the creation of class properties:

class ClassPropertyDescriptor(object):

    def __init__(self, fget, fset=None):
        self.fget = fget
        self.fset = fset

    def __get__(self, obj, klass=None):
        if klass is None:
            klass = type(obj)
        return self.fget.__get__(obj, klass)()

    def __set__(self, obj, value):
        if not self.fset:
            raise AttributeError("can't set attribute")
        type_ = type(obj)
        return self.fset.__get__(obj, type_)(value)

    def setter(self, func):
        if not isinstance(func, (classmethod, staticmethod)):
            func = classmethod(func)
        self.fset = func
        return self

Class Property Decorator

To simplify the creation of class properties, a decorator function classproperty is defined:

def classproperty(func):
    if not isinstance(func, (classmethod, staticmethod)):
        func = classmethod(func)

    return ClassPropertyDescriptor(func)

Example Usage

The following code demonstrates how to use the classproperty decorator to define a class property named bar in the Bar class:

class Bar(object):

    _bar = 1

    @classproperty
    def bar(cls):
        return cls._bar

    @bar.setter
    def bar(cls, value):
        cls._bar = value

Metaclass

To allow direct setting of class properties through the class, a metaclass definition, ClassPropertyMetaClass, is used:

class ClassPropertyMetaClass(type):
    def __setattr__(self, key, value):
        if key in self.__dict__:
            obj = self.__dict__.get(key)
        if obj and type(obj) is ClassPropertyDescriptor:
            return obj.__set__(self, value)

        return super(ClassPropertyMetaClass, self).__setattr__(key, value)

By adding __metaclass__ to the Bar class definition and updating the __set__ method of ClassPropertyDescriptor, the ability to set class properties through the class itself is enabled.

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