code: def update(self, fields, cond=None, doc_ids=None, eids=None):
          
          doc_ids = _get_doc_ids(doc_ids, eids)
  
          if callable(fields):
              return self.process_elements(
                  lambda data, doc_id: fields(data[doc_id]),
                  cond, doc_ids
              )
          else:
              return self.process_elements(
                  lambda data, doc_id: data[doc_id].update(fields),
                  cond, doc_ids
              )
docs: Update all matching documents to have a given set of fields.
  
          :param fields: the fields that the matching documents will have
                         or a method that will update the documents
          :type fields: dict | dict -> None
          :param cond: which documents to update
          :type cond: query
          :param doc_ids: a list of document IDs
          :type doc_ids: list
          :returns: a list containing the updated document's ID