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Name mangling vs qualified access to class attributes

paolieri at gmail.com paolieri at gmail.com
Tue Dec 13 15:27:26 EST 2016 
The official Python tutorial at

https://docs.python.org/3/tutorial/classes.html#private-variables

says that "name mangling is helpful for letting subclasses override methods without breaking intraclass method calls" and makes an interesting example:

class Mapping:
    def __init__(self, iterable):
        self.items_list = []
        self.__update(iterable)

    def update(self, iterable):
        for item in iterable:
            self.items_list.append(item)

    __update = update   # private copy of original update() method

class MappingSubclass(Mapping):

    def update(self, keys, values):
        # provides new signature for update()
        # but does not break __init__()
        for item in zip(keys, values):
            self.items_list.append(item)


It seems to me that, in this example, one could just have:

class Mapping:
    def __init__(self, iterable):
        self.items_list = []
        Mapping.update(self, iterable)

    def update(self, iterable):
        for item in iterable:
            self.items_list.append(item)

and avoid copying 'Mapping.update' into 'Mapping.__update'. More generally, any time one needs to "let subclasses override methods without breaking intraclass method calls" (the goal stated in the tutorial), using qualified access to class attributes/methods should suffice.

Am I missing something? Is 'self.__update(iterable)' in 'Mapping.__init__' preferable to 'Mapping.update(self, iterable)'?

I think that, instead, name mangling is helpful to avoid accidental overrides of methods/attributes by the *current* class (rather than its subclasses). Given the way that C3 linearization works, you can't know in advance who will follow your class A in B.__mro__ when B extends A. Name mangling allows you to avoid overriding methods/attributes of classes that might follow.

Any thoughts?

Best,
-- Marco
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