Is there a way to create class properties in Python similar to the `@classmethod` decorator?

An Investigation into Implementing Class Properties in Python

In Python, using the @classmethod decorator allows developers to easily add methods to classes. However, the question arises: is there a comparable decorator that can be employed to add class-level properties?

The existing syntax used for class methods can serve as a guide. Consider the following code sample:

class Example(object):
   the_I = 10  # Class-level attribute
   ...  # Class methods and instance properties

The goal is to define a syntax analogous to @classmethod that can be used to create class properties, such as:

   @classproperty
   def I( cls ):
      return cls.the_I  # Defined as a property getter

Unfortunately, this approach is not supported by the Python language. However, there are alternative techniques that can be implemented to achieve the desired functionality.

Custom Property Descriptor

One such technique involves creating a custom property descriptor class, which can be implemented as follows:

class ClassPropertyDescriptor(object):

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

    def __get__(self, obj, klass=None):
        ...  # Handle property getter

    def __set__(self, obj, value):
        ...  # Handle property setter

    def setter(self, func):
        self.fset = func
        return self

def classproperty(func):
    return ClassPropertyDescriptor(func)

Using this class, a class property can be defined like so:

class Bar(object):

    _bar = 1

    @classproperty
    def bar(cls):
        return cls._bar  # Defined as a property getter

    @bar.setter
    def bar(cls, value):
        cls._bar = value  # Defined as a property setter

This approach offers the same functionality as the hypothetical @classproperty decorator.

Metaclass-Based Approach

Another option is to utilize a metaclass that handles the creation of class properties. The following code demonstrates this approach:

class ClassPropertyMetaClass(type):

    def __setattr__(self, key, value):
        ...  # Handle property definition

# Use the metaclass in the class definition
class Bar(object):
    __metaclass__ = ClassPropertyMetaClass
    _bar = 1

    @classproperty
    def bar(cls):
        return cls._bar  # Defined as a property getter

    @bar.setter
    def bar(cls, value):
        cls._bar = value  # Defined as a property setter

This approach ensures that class properties are created and handled correctly, without the need for a separate decorator.

In conclusion, while Python does not provide a built-in decorator specifically for class properties, there are custom approaches and metaclass techniques that can effectively achieve the desired functionality, enabling developers to manage class-level attributes as properties.

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