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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 _assign_database_backend(self, db):
 """Assign Trace instance to stochastics and deterministics and Database instance to self. :Parameters: - `db` : string, Database instance The name of the database module (see below), or a Database instance. Available databases: - `no_trace` : Traces are not stored at all. - `ram` : Traces stored in memory. - `txt` : Traces stored in memory and saved in txt files at end of sampling. - `sqlite` : Traces stored in sqlite database. - `hdf5` : Traces stored in an HDF5 file. """

        # Objects that are not to be tallied are assigned a no_trace.Trace
        # Tallyable objects are listed in the _nodes_to_tally set.

        no_trace = getattr(database, 'no_trace')
        self._variables_to_tally = set()
        for object in self.stochastics | self.deterministics:

            if object.keep_trace:
                self._variables_to_tally.add(object)
                try:
                    if object.mask is None:
                        # Standard stochastic
                        self._funs_to_tally[object.__name__] = object.get_value
                    else:
                        # Has missing values, so only fetch stochastic elements
                        # using mask
                        self._funs_to_tally[
                            object.__name__] = object.get_stoch_value
                except AttributeError:
                    # Not a stochastic object, so no mask
                    self._funs_to_tally[object.__name__] = object.get_value
            else:
                object.trace = no_trace.Trace(object.__name__)

        check_valid_object_name(self._variables_to_tally)

        # If not already done, load the trace backend from the database
        # module, and assign a database instance to Model.
        if isinstance(db, str):
            if db in dir(database):
                module = getattr(database, db)

                # Assign a default name for the database output file.
                if self._db_args.get('dbname') is None:
                    self._db_args['dbname'] = self.__name__ + '.' + db

                self.db = module.Database(**self._db_args)
            elif db in database.__modules__:
                raise ImportError(
                    'Database backend `%s` is not properly installed. Please see the documentation for instructions.' % db)
            else:
                raise AttributeError(
                    'Database backend `%s` is not defined in pymc.database.' % db)
        elif isinstance(db, database.base.Database):
            self.db = db
            self.restore_sampler_state()
        else:   # What is this for? DH.
            self.db = db.Database(**self._db_args)