docs: def _getFileNumber(self, filename):
          """
          Given a file name, get its file number (if any).
  
          @param filename: A C{str} file name.
          @return: An C{int} file number or C{None} if no file with that name
              has been added.
          """
          cur = self._connection.cursor()
          cur.execute('SELECT id FROM files WHERE name = ?', (filename,))
          row = cur.fetchone()
          if row is None:
              return None
          else:
              return row[0]