<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def add_media_description(self, media_description):
 """Adds a media_description. arg: media_description (displayText):
 the new media_description raise: InvalidArgument - ``media_description`` is invalid raise: NoAccess - ``Metadata.isReadOnly()`` is ``true`` raise: NullArgument - ``media_description`` is ``null`` *compliance: mandatory -- This method must be implemented.* """

        if self.get_media_descriptions_metadata().is_read_only():
            raise NoAccess()
        self.add_or_replace_value('mediaDescriptions', media_description)