docs: def update_activity(self, activity_form=None):
          """Updates an existing activity,.
  
          arg:    activityForm (osid.learning.ActivityForm): the form
                  containing the elements to be updated
          raise:  IllegalState - activityForm already used in an update
                  transaction
          raise:  InvalidArgument - the form contains an invalid value
          raise:  NullArgument - activityForm is null
          raise:  OperationFailed - unable to complete request
          raise:  PermissionDenied - authorization failure
          raise:  Unsupported - activityForm did not originate from
                  get_activity_form_for_update()
          compliance: mandatory - This method must be implemented.
  
          """
          if activity_form is None:
              raise NullArgument()
          if not isinstance(activity_form, abc_learning_objects.ActivityForm):
              raise InvalidArgument('argument type is not an ActivityForm')
          if not activity_form.is_for_update():
              raise InvalidArgument('form is for create only, not update')
          try:
              if self._forms[activity_form.get_id().get_identifier()] == UPDATED:
                  raise IllegalState('form already used in an update transaction')
          except KeyError:
              raise Unsupported('form did not originate from this session')
          if not activity_form.is_valid():
              raise InvalidArgument('one or more of the form elements is invalid')
  
          url_path = construct_url('activities',
                                   bank_id=self._catalog_idstr)
          try:
              result = self._put_request(url_path, activity_form._my_map)
          except Exception:
              raise  # OperationFailed
          self._forms[activity_form.get_id().get_identifier()] = UPDATED
          return objects.Activity(result)