def clear_value(self):
self.set_value(None)<EOL>
Set the value to None

        This can be useful for sub-classes that use None
        to indicate an un-initialized value.

        Note that this invokes hooks for type-checking and
        bounds-checking that may be implemented by sub-classes, so it
        should will need to be re-implemented if those checks do note
        accept None as a valid value.