code: def ls(self, folder=''):
          
          current_folder = self._ftp.pwd()
          self.cd(folder)
          contents = []
          self._ftp.retrlines('LIST', lambda a: contents.append(a))
          files = filter(lambda a: a.split()[0].startswith('-'), contents)
          folders = filter(lambda a: a.split()[0].startswith('d'), contents)
          files = map(lambda a: ' '.join(a.split()[8:]), files)
          folders = map(lambda a: ' '.join(a.split()[8:]), folders)
          self._ftp.cwd(current_folder)
          return files, folders
docs: Lists the files and folders of a specific directory
          default is the current working directory.
  
          :param folder: the folder to be listed.
          :type folder: string
  
          :returns: a tuple with the list of files in the folder
                    and the list of subfolders in the folder.