Is there a decorator equivalent to `@property` in Python for defining a class property?

How to Define a Class Property in Python

In Python, the @classmethod decorator can be used to add a method to a class. However, is there an equivalent decorator to define a property for a class?

Class Property Decorator

To answer this question, let's consider the following example:

class Example(object):
    the_I = 10

    def __init__(self):
        self.an_i = 20

    @property
    def i(self):
        return self.an_i

    def inc_i(self):
        self.an_i += 1

    # is this even possible?
    @classproperty
    def I(cls):
        return cls.the_I

    @classmethod
    def inc_I(cls):
        cls.the_I += 1

e = Example()
assert e.i == 20
e.inc_i()
assert e.i == 21

assert Example.I == 10
Example.inc_I()
assert Example.I == 11

In this example, the intent is to define a class property I using the @classproperty decorator. However, the error "NameError: name 'classproperty' is not defined" is raised.

Solution

To create a class property, a custom descriptor class called ClassPropertyDescriptor can be used:

import inspect

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")

        if inspect.isclass(obj):
            type_ = obj
            obj = None
        else:
            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

The ClassPropertyDescriptor class defines the behavior of the class property. It implements the __get__ and __set__ methods to handle getting and setting the property value.

Class Property Decorator

To make using the class property descriptor easier, a decorator called @classproperty can be created:

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

    return ClassPropertyDescriptor(func)

Example Usage

With the custom class property descriptor and decorator in place, the example can be rewritten as:

class Bar(object):

    _bar = 1

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

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

This code defines a class property bar that can be accessed and modified as expected. Modifying the property value on the class instance or on the class itself will update the value for all instances of the class.

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