Get a single broadcast.

    Broadcasts are stored data about how to do a Pandas join.
    A Broadcast object is a namedtuple with these attributes:

        - cast: the name of the table being broadcast
        - onto: the name of the table onto which "cast" is broadcast
        - cast_on: The optional name of a column on which to join.
          None if the table index will be used instead.
        - onto_on: The optional name of a column on which to join.
          None if the table index will be used instead.
        - cast_index: True if the table index should be used for the join.
        - onto_index: True if the table index should be used for the join.

    Parameters
    ----------
    cast_name : str
        The name of the table being braodcast.
    onto_name : str
        The name of the table onto which `cast_name` is broadcast.

    Returns
    -------
    broadcast : Broadcast

def get_broadcast(cast_name, onto_name):
    """
    Get a single broadcast.

    Broadcasts are stored data about how to do a Pandas join.
    A Broadcast object is a namedtuple with these attributes:

        - cast: the name of the table being broadcast
        - onto: the name of the table onto which "cast" is broadcast
        - cast_on: The optional name of a column on which to join.
          None if the table index will be used instead.
        - onto_on: The optional name of a column on which to join.
          None if the table index will be used instead.
        - cast_index: True if the table index should be used for the join.
        - onto_index: True if the table index should be used for the join.

    Parameters
    ----------
    cast_name : str
        The name of the table being braodcast.
    onto_name : str
        The name of the table onto which `cast_name` is broadcast.

    Returns
    -------
    broadcast : Broadcast

    """
    if is_broadcast(cast_name, onto_name):
        return _BROADCASTS[(cast_name, onto_name)]
    else:
        raise KeyError(
            'no rule found for broadcasting {!r} onto {!r}'.format(
                cast_name, onto_name))