Seems like it may take a little more than one line, this errors for me:

  >>> a = tree()
  >>> a.__getattr__ = a.__getitem__

Yea, sure it becomes more than one line (or at least I don't know a good one-line-way) but I wouldn't count that as an issue.

It errors for you because it is a defaultdict instance and doesn't allow attribute overwrites.

This should work:

    class tree(defaultdict):
        def __init__(self): defaultdict.__init__(self, tree)
        __getattr__ = defaultdict.__getitem__
        __setattr__ = defaultdict.__setitem__

Edit: It doesn't work with the simple assignment because of the mentioned bug. Ofc the fix is trivial. See the code from beagle3.