How do I document :rtype: for a function that returns multiple possible data types?
<p>In a Python docstring how should one document the <code>:rtype:</code> for a function that can return multiple possible data types?</p>

<p>For example, if a function can return <code>defaultdict</code> OR <code>dict</code> OR <code>list</code>, based on the functions parameters, how do you document this?</p>

<p>Code example:</p>

<pre><code>from collections import defaultdict

def read_state(state_file, state_file_type='defaultdict'):
    """Deserialize state file or create empty state and return it.

    :param state_file: The path and file name of state file to read.
    :type state_file: str
    :param state_file_type: Data type in which state is stored.
    :type state_file_type: str
    :return: Current or new empty state.
    :rtype: ????? 
    """
    if os.path.exists(state_file):
        with open(state_file) as conf_fo:
            state = json.load(conf_fo)
    elif state_file_type == 'defaultdict':
        state = defaultdict(lambda: defaultdict(list))
    elif state_file_type == 'dict':
        state = {}
    elif state_file_type == 'list':
        state = []
    else:
        raise TypeError("State file type not defined.")
    return state
</code></pre>