Example of defining a custom data structure for arbitrarily-nestable trees.

tree.py

class Tree(dict):
  """A Tree is a dict-like object that nests arbitrarily.

  It supports both `tree["key"]` and `tree.key` syntax for lookups.
  A lookup for a missing key, by either syntax, results in creation
  of a new nested tree, e.g.

  >>> tree = Tree()
  >>> tree.a.b.c = "d"
  >>> tree
  Tree({'a': Tree({'b': Tree({'c': 'd'})})})
  >>> tree["a"]["b"]["c"]
  'd'
  """
  def __getitem__(self, item):
      if not item.startswith("_") and item not in self:
          self[item] = Tree()
      return super(Tree, self).__getitem__(item)

  def __repr__(self):
      return "%s(%s)" % (
          self.__class__.__name__,
          super(Tree, self).__repr__())

  def __getattr__(self, item):
      if not item.startswith("_") and item not in self:
          self[item] = Tree()
      return self[item]

  def __setattr__(self, name, value):
      self[name] = value

  def __delattr__(self, name):
      del self[name]

if __name__ == "__main__":
    import doctest
    doctest.testmod()