code: def readline(self):
          
          try:
              return self._descriptor.read_queue.pop()
          except IndexError:
              # No lines in queue
              if self.has_error():
                  raise RuntimeError("Errors reading PIPE")
          return None
docs: Readline implementation.
  
          :return: popped line from descriptor queue. None if nothing found
          :raises: RuntimeError if errors happened while reading PIPE