def _open_db(self) -> None:
        """Open an existing db file, and read its metadata."""
        if self._debug.should("dataio"):
            self._debug.write(f"Opening data file {self._filename!r}")
        self._dbs[threading.get_ident()] = SqliteDb(self._filename, self._debug)
        self._read_db()
Open an existing db file, and read its metadata.
coverage/sqldata.py