code: def is_file_opened(self, filename=None):
          
          if filename is None:
              # Is there any file opened?
              return len(self.data) > 0
          else:
              return self.has_filename(filename)
docs: Return if filename is in the editor stack.
  
          Args:
              filename: Name of the file to search for.  If filename is None,
                  then checks if any file is open.
  
          Returns:
              True: If filename is None and a file is open.
              False: If filename is None and no files are open.
              None: If filename is not None and the file isn't found.
              integer: Index of file name in editor stack.