error when using method defined parent class

Question

I have the following classes:

class A:
    def name(self):
        return self.__label

class B(A):
    def __init__(self, name)
        self.__label = name

ex1 = B('Tom')
print ex1.name()

What I get is:

AttributeError: B instance has no attribute '_A__label'

What's wrong and how to correct it?

What's wrong: you're using double-underscore variables. How to fix it: don't use double-underscore variables. — Daniel Roseman
– Daniel Roseman, Commented Nov 12, 2015 at 22:10
why isn't it allowed? It is hidden from the outside world, but I am using a method defined inside a parent class. Why is there a problem? — Physicist
– Physicist, Commented Nov 12, 2015 at 22:14
Because double-underscores are there precisely to stop access (or overwriting) by parent or child classes. They are almost never what you want; don't use them. — Daniel Roseman
– Daniel Roseman, Commented Nov 12, 2015 at 22:14
then how can I stop access from the outside world (e.g. unintentional access by users) but allow access from parent / child classes? — Physicist
– Physicist, Commented Nov 12, 2015 at 22:16
Don't. Just: don't. The Python principle is "we're all consenting adults here". By all means, give your attributes a single underscore, to make it clear that they shouldn't be accessed unless you know what you're doing, but you can't stop people from doing it anyway and you shouldn't try. — Daniel Roseman
– Daniel Roseman, Commented Nov 12, 2015 at 22:18

Barry Rogerson · Accepted Answer · 2015-12-01 18:48:01Z

1

When you prefix an attribute with a double underscore, Python uses 'name mangling' to access the attribute. This means it will store the attribute on the class in the format: _<class name>__<attr name>. In your example self.__label will be stored as self._B__label because you set it in the B class method. But when you try to use the attribute in the A class it converts self.__label into self._A__label and finds that it isn't set.

The use case for double underscores is when you want to ensure that your variable is always on your class even if a subclass derives your class. Because what could happen is that the subclass redefines your variable to something else, using double underscored variables makes this that much harder.

edited Dec 1, 2015 at 18:48

answered Nov 12, 2015 at 23:35

Barry Rogerson

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