Using property() on classmethods

Tags: oop python
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() function with these, but it results in an error. I was able to reproduce the error with the following in the interpreter:

>>> class foo(object):
...     _var=5
...     def getvar(cls):
...             return cls._var
...     getvar=classmethod(getvar)
...     def setvar(cls,value):
...             cls._var=value
...     setvar=classmethod(setvar)
...     var=property(getvar,setvar)
...
>>> f.getvar()
5
>>> f.setvar(4)
>>> f.getvar()
4
>>> f.var
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: 'classmethod' object is not callable
>>> f.var=5
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: 'classmethod' object is not callable

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



Answers

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.



I hope this dead-simple read-only @classproperty decorator would help somebody looking for classproperties.

class classproperty(object):

    def __init__(self, fget):
        self.fget = fget

    def __get__(self, owner_self, owner_cls):
        return self.fget(owner_cls)

class C(object):

    @classproperty
    def x(cls):
        return 1

assert C.x == 1
assert C().x == 1


Reading the Python 2.2 release notes, I find the following.

The get method [of a property] won't be called when the property is accessed as a class attribute (C.x) instead of as an instance attribute (C().x). If you want to override the __get__ operation for properties when used as a class attribute, you can subclass property - it is a new-style type itself - to extend its __get__ method, or you can define a descriptor type from scratch by creating a new-style class that defines __get__, __set__ and __delete__ methods.

NOTE: The below method doesn't actually work for setters, only getters.

Therefore, I believe the prescribed solution is to create a ClassProperty as a subclass of property.

class ClassProperty(property):
    def __get__(self, cls, owner):
        return self.fget.__get__(None, owner)()

class foo(object):
    _var=5
    def getvar(cls):
        return cls._var
    getvar=classmethod(getvar)
    def setvar(cls,value):
        cls._var=value
    setvar=classmethod(setvar)
    var=ClassProperty(getvar,setvar)

assert foo.getvar() == 5
foo.setvar(4)
assert foo.getvar() == 4
assert foo.var == 4
foo.var = 3
assert foo.var == 3

However, the setters don't actually work:

foo.var = 4
assert foo.var == foo._var # raises AssertionError

foo._var is unchanged, you've simply overwritten the property with a new value.

You can also use ClassProperty as a decorator:

class Foo(object):
    _var = 5

    @ClassProperty
    @classmethod
    def var(cls):
        return cls._var

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

assert foo.var == 5


This video can help you solving your question :)
By: admin