code: def get_files(cls, folder, session_id=''):
          
          filelist = []
          if folder is None or not os.path.isdir(folder):
              return filelist
          if session_id is None:
              session_id = ''
          for root, dirs, files in os.walk(folder):
              for filename in files:
                  if filename.startswith('Session %s' % session_id) \
                          and filename.endswith('.mqo'):
                      filename = os.path.join(root, filename)
                      filelist.append(filename)
          return filelist
docs: Retrieve the list of files the plugin can work on.
          Find this list based on the files name, files extension or even
          actually by reading in the file.
          If a session identifier is specified it will restrict the list
          of files returned to those with this session identifier in their
          name.
  
          :arg folder: the path to the folder containing the files to
              check. This folder may contain sub-folders.
          :kwarg session_id: the session identifier of the MapQTL output
              to process.