Name mangling vs qualified access to class attributes

Tue Dec 13 16:52:23 EST 2016

On 13Dec2016 12:27, paolieri at gmail.com <paolieri at gmail.com> wrote:
>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)'?

IMO, mostly in that "Mapping.update" hardwires the class name, whereas 
"self.__update" will survive a class rename.
I confess I've never used name mangling in the manner shown in the example.
Hoping for more insightful comments...
Cheers,
Cameron Simpson <cs at zip.com.au>