code: def open(self, file, mode='r', perm=0o0644):
          
          args = {
              'file': file,
              'mode': mode,
              'perm': perm,
          }
  
          return self._client.json('filesystem.open', args)
docs: Opens a file on the node
  
          :param file: file path to open
          :param mode: open mode
          :param perm: file permission in octet form
  
          mode:
            'r' read only
            'w' write only (truncate)
            '+' read/write
            'x' create if not exist
            'a' append
          :return: a file descriptor