def _prettify_list(items):
    """Return a human readable format of a list.

    Example:

    Available Keys:
      - my_first_key
      - my_second_key
    """
    assert isinstance(items, list)
    keys_list = 'Available Keys:'
    for item in items:
        keys_list += '\n  - {0}'.format(item)
    return keys_list
