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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def update_data(self, update_data_finished_cb):
 """Request an update of the anchor data"""

        if not self._update_data_finished_cb and self.nr_of_anchors > 0:
            self._update_data_finished_cb = update_data_finished_cb
            self.anchor_data = {}

            self.data_valid = False
            self._nr_of_anchors_to_fetch = self.nr_of_anchors

            logger.debug('Updating anchor data of memory {}'.format(self.id))

            # Start reading the first anchor
            self._currently_fetching_index = 0
            self._request_page(self.anchor_ids[self._currently_fetching_index])