def makedocstr(parent_class, subclass_method):
    """
    Compose a string (to be used as doc string) from a method's
    doc string in a parent class and an additional doc string
    in a subclass version of the method.

    @param parent_class: class object for parent class.
    @param subclass_method: method object for subclass.
    @return: parent_class.method.__doc__ + subclass_method.__doc__
    """
    parent_method = getattr(parent_class, subclass_method.__name__)
    docstr = parent_method.__doc__
    if subclass_method.__doc__ is not None and \
           subclass_method is not parent_method:
        docstr += subclass_method.__doc__
    return docstr