Using property() on classmethods

Question

I have a class with two class methods (using the classmethod() function) for getting and setting what is essentially a static variable. I tried to use the property() functi

Answer 1

Is it possible to use the property() function with classmethod decorated functions?

No.

However, a classmethod is simply a bound method (a partial function) on a class accessible from instances of that class.

Since the instance is a function of the class and you can derive the class from the instance, you can can get whatever desired behavior you might want from a class-property with property:

class Example(object):
    _class_property = None
    @property
    def class_property(self):
        return self._class_property
    @class_property.setter
    def class_property(self, value):
        type(self)._class_property = value
    @class_property.deleter
    def class_property(self):
        del type(self)._class_property

This code can be used to test - it should pass without raising any errors:

ex1 = Example()
ex2 = Example()
ex1.class_property = None
ex2.class_property = 'Example'
assert ex1.class_property is ex2.class_property
del ex2.class_property
assert not hasattr(ex1, 'class_property')

And note that we didn't need metaclasses at all - and you don't directly access a metaclass through its classes' instances anyways.

writing a @classproperty decorator

You can actually create a classproperty decorator in just a few lines of code by subclassing property (it's implemented in C, but you can see equivalent Python here):

class classproperty(property):
    def __get__(self, obj, objtype=None):
        return super(classproperty, self).__get__(objtype)
    def __set__(self, obj, value):
        super(classproperty, self).__set__(type(obj), value)
    def __delete__(self, obj):
        super(classproperty, self).__delete__(type(obj))

Then treat the decorator as if it were a classmethod combined with property:

class Foo(object):
    _bar = 5
    @classproperty
    def bar(cls):
        """this is the bar attribute - each subclass of Foo gets its own.
        Lookups should follow the method resolution order.
        """
        return cls._bar
    @bar.setter
    def bar(cls, value):
        cls._bar = value
    @bar.deleter
    def bar(cls):
        del cls._bar

And this code should work without errors:

def main():
    f = Foo()
    print(f.bar)
    f.bar = 4
    print(f.bar)
    del f.bar
    try:
        f.bar
    except AttributeError:
        pass
    else:
        raise RuntimeError('f.bar must have worked - inconceivable!')
    help(f)  # includes the Foo.bar help.
    f.bar = 5

    class Bar(Foo):
        "a subclass of Foo, nothing more"
    help(Bar) # includes the Foo.bar help!
    b = Bar()
    b.bar = 'baz'
    print(b.bar) # prints baz
    del b.bar
    print(b.bar) # prints 5 - looked up from Foo!

    
if __name__ == '__main__':
    main()

But I'm not sure how well-advised this would be. An old mailing list article suggests it shouldn't work.

Getting the property to work on the class:

The downside of the above is that the "class property" isn't accessible from the class, because it would simply overwrite the data descriptor from the class __dict__.

However, we can override this with a property defined in the metaclass __dict__. For example:

class MetaWithFooClassProperty(type):
    @property
    def foo(cls):
        """The foo property is a function of the class -
        in this case, the trivial case of the identity function.
        """
        return cls

And then a class instance of the metaclass could have a property that accesses the class's property using the principle already demonstrated in the prior sections:

class FooClassProperty(metaclass=MetaWithFooClassProperty):
    @property
    def foo(self):
        """access the class's property"""
        return type(self).foo

And now we see both the instance

>>> FooClassProperty().foo

and the class

>>> FooClassProperty.foo

have access to the class property.