def get_default_shape_type(current_type):
    """If all shapes in current_type are of identical shape type,
       return this shape type, else "polygon" as lowest common
       denominator type.

    Parameters
    ----------
    current_type : list of str
        list of current shape types

    Returns
    -------
    default_type : str
        default shape type
    """
    default = "polygon"
    if not current_type:
        return default
    first_type = current_type[0]
    if all(shape_type == first_type for shape_type in current_type):
        return first_type
    return default