Optional attributes return
None on access to indicate that the attribute is missing. However, when one tries to assign
None to an optional attribute, this can result in:
Module pyxb.binding.basis, line 62, in __setattr__ Module pyxb.binding.content, line 410, in set AssertionError
This can make assigning attributes quite complicated, especially if one creates one binding object from another one. Instead of "dest.attr = "src.attr" one must use code "if src.attr is not None: dest.attr = src.attr".