def update(self):
        if self.instance is None:
            raise CQLEngineException("DML Query intance attribute is None")
        assert type(self.instance) == self.model
        null_clustering_key = False if len(self.instance._clustering_keys) == 0 else True
        static_changed_only = True
        statement = UpdateStatement(self.column_family_name, ttl=self._ttl, timestamp=self._timestamp,
                                    conditionals=self._conditional, if_exists=self._if_exists)
        for name, col in self.instance._clustering_keys.items():
            null_clustering_key = null_clustering_key and col._val_is_null(getattr(self.instance, name, None))
        # get defined fields and their column names
        for name, col in self.model._columns.items():
            # if clustering key is null, don't include non static columns
            if null_clustering_key and not col.static and not col.partition_key:
                continue
            if not col.is_primary_key:
                val = getattr(self.instance, name, None)
                val_mgr = self.instance._values[name]

                if val is None:
                    continue

                if not val_mgr.changed and not isinstance(col, columns.Counter):
                    continue

                static_changed_only = static_changed_only and col.static
                statement.add_update(col, val, previous=val_mgr.previous_value)

        if statement.assignments:
            for name, col in self.model._primary_keys.items():
                # only include clustering key if clustering key is not null, and non static columns are changed to avoid cql error
                if (null_clustering_key or static_changed_only) and (not col.partition_key):
                    continue
                statement.add_where(col, EqualsOperator(), getattr(self.instance, name))
            self._execute(statement)

        if not null_clustering_key:
            self._delete_null_columns()
updates a row.
dataset/ETHPy150Open datastax/python-driver/cassandra/cqlengine/query.py/DMLQuery.update