code: def _from_string(cls, serialized):
          
          # Allow access to _from_string protected method
          parsed_parts = cls.parse_url(serialized)
          course_key = CourseLocator(
              parsed_parts.get('org'), parsed_parts.get('course'), parsed_parts.get('run'),
              # specifically not saying deprecated=True b/c that would lose the run on serialization
          )
          block_id = parsed_parts.get('block_id')
          return cls(course_key, parsed_parts.get('block_type'), block_id)
docs: see super