def dereference(self, dbref, session=None, **kwargs):
        """Dereference a :class:`~bson.dbref.DBRef`, getting the
        document it points to.

        Raises :class:`TypeError` if `dbref` is not an instance of
        :class:`~bson.dbref.DBRef`. Returns a document, or ``None`` if
        the reference does not point to a valid document.  Raises
        :class:`ValueError` if `dbref` has a database specified that
        is different from the current database.

        :Parameters:
          - `dbref`: the reference
          - `session` (optional): a
            :class:`~pymongo.client_session.ClientSession`.
          - `**kwargs` (optional): any additional keyword arguments
            are the same as the arguments to
            :meth:`~pymongo.collection.Collection.find`.

        .. versionchanged:: 3.6
           Added ``session`` parameter.
        """
        if not isinstance(dbref, DBRef):
            raise TypeError("cannot dereference a %s" % type(dbref))
        if dbref.database is not None and dbref.database != self.__name:
            raise ValueError("trying to dereference a DBRef that points to "
                             "another database (%r not %r)" % (dbref.database,
                                                               self.__name))
        return self[dbref.collection].find_one(
            {"_id": dbref.id}, session=session, **kwargs)
Dereference a :class:`~bson.dbref.DBRef`, getting the
        document it points to.

        Raises :class:`TypeError` if `dbref` is not an instance of
        :class:`~bson.dbref.DBRef`. Returns a document, or ``None`` if
        the reference does not point to a valid document.  Raises
        :class:`ValueError` if `dbref` has a database specified that
        is different from the current database.

        :Parameters:
          - `dbref`: the reference
          - `session` (optional): a
            :class:`~pymongo.client_session.ClientSession`.
          - `**kwargs` (optional): any additional keyword arguments
            are the same as the arguments to
            :meth:`~pymongo.collection.Collection.find`.

        .. versionchanged:: 3.6
           Added ``session`` parameter.
        
stackoverflow/venv/lib/python3.6/site-packages/pymongo/database.py