Inheriting class attribute with double underscore

Question

I think I understand the concept of \"name mangling\" in python, but there\'s something that I probably missed. Take a look at the following code:

#!/usr/bin/env         


        

Answer 1

You are not "missing", to the contrary, you just "found out" what name mangling does: it is made to ensure variables with double underscores inside a method will always see the attribute defined in the same class as that method, and in none of its subclasses.

If you simply want to use the attribute as it is overriden in each subclass, that is the normal behavior for all other attributes, but for the ones prefixed by two underscores.

So, what happens is that the .__data name used inside func is itself mangled, at compile time, to _base__data.

OrderedDict

Python's collections.OrderedDict have an extra trap: Python offers both a pure-python implementation, which uses the __ for its "private attributes", as explained above, but also have a native code implementation in C, and the private structures of that are not exposed to Python.

And the collections module ends the OrderedDict code block with these lines:

try:
    from _collections import OrderedDict
except ImportError:
    # Leave the pure Python version in place.
    pass

That is: the normal "collections.OrderedDict" is written in C, with a lot of opaque structures, that can't be tapped in by subclasses.

The only way to have access to the Python defined OrderedDict is by deleting the _collections.OrderedDict attribute (in the _collections, not collections module), and reload the collections module.

If you do that, and instantiate one ordered dict, the private data structures can be seem:

from imp import reload
import _collections, collections
backup = _collections.OrderedDict
del _collections.OrderedDict
collections = reload(collections)
PyOrderedDict = collections.OrderedDict
_collections.OrderedDict = backup
a  = PyOrderedDict()
dir(a)

Out[xx]: 
['_OrderedDict__hardroot',
 '_OrderedDict__map',
 '_OrderedDict__marker',
 '_OrderedDict__root',
 '_OrderedDict__update',
 '__class__',
 '__contains__',
 '__delattr__',
 ...
]