Python property inheritance

Question

I am new to python and I wasn't sure what I was doing was correct. I have a base class A and an inherited class B.

class A(object):

    def __init__(self, name):
        self.__name = name

    @property
    def name(self):
        return self.__name

    @name.setter
    def name(self, name):
        self.__name = name

class B(A):
    def __init__(self, name):
        super(NominalValue, self).__init__(name)

    @property
    def name2(self):
        return self.__name2

    @name2.setter
    def name2(self, n):
        self.__name2 = n

    def toString():
        print self.__name + self.__name2

if __name__ == "__main__":
    instance = B('name');
    instance.toString()

When I run this it complains that class B does not have any attribute __name.

AttributeError: 'B' object has no attribute '_B__name'

I am clearly not doing the inheritance correctly. How do you treat properties to be correctly inherited and avoid repeating attributes in the inherited class?

The double leading underscore has special meaning which interferes with your inheritiance. To access the superclass's __name attribute I think you need to refer to it as self._A__name. — Steven Rumbalski
– Steven Rumbalski, Commented Jan 28, 2015 at 19:47
As 6502 mentions in his answer, you shouldn't be necessarily PLANNING on having to write a property for your class. This is one of the strengths of properties in Python: it's very easy to convert an attribute to a property on the fly. Unless it's demonstrable that you need to have custom logic for getting, setting, or deleting an attribute -- don't use a property to do it. — Adam Smith
– Adam Smith, Commented Jan 28, 2015 at 19:58

Adam Smith · Accepted Answer · 2015-01-28 19:47:35Z

Attributes that begin with two underscores like your __name are signified as being private variables. There's no actual PROTECTION done to these (that is to say: if you can find it, you can access it), but they are name-mangled to be less easy to access. More information about that can be found on the docs page

Any identifier of the form __spam (at least two leading underscores, at most one trailing underscore) is textually replaced with _classname__spam, where classname is the current class name with leading underscore(s) stripped.

Because of this, even though you're inheriting A's __name attribute, you don't have an attribute __name in any object of type B. What you have is a _A__name attribute. That's how the name-mangling works: __attributename becomes _classname__attributename

You could use _name instead, which is just a flag to future users that this isn't supposed to be public. Then your whole code should work.

6502 · Accepted Answer · 2015-01-28 19:54:16Z

Attribute names prefixed with a double underscore are treated specially by the compiler and you should avoid them until you understand Python better.

Most Python programmers don't know why they are special and they can still live an happy and productive programming life. They're also objectively ugly... don't you think?

Note also that in Python you don't start using getter and setter just for the fun of it. Simply start with a data member and only migrate to properties when (and only if) needed. Writing code for a getter that simply returns the member and for a setter that simply mutates it is nonsense. When (and if) you will need to do special processing on member access change them to properties, and there will be no need to change the code using your class.

On the technical explanation side Python simply adds the class name to attributes starting with a double underscore to avoid unwanted clashes with identically named attributes in other classes. This kind of problem is however not so common and that's why most Python programmers never had the necessity of this feature.

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