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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def complete(self, default_output=None):
 """Marks this asynchronous Pipeline as complete. Args: default_output: What value the 'default' output slot should be assigned. Raises: UnexpectedPipelineError if the slot no longer exists or this method was called for a pipeline that is not async. """

    # TODO: Enforce that all outputs expected by this async pipeline were
    # filled before this complete() function was called. May required all
    # async functions to declare their outputs upfront.
    if not self.async:
      raise UnexpectedPipelineError(
          'May only call complete() method for asynchronous pipelines.')
    self._context.fill_slot(
        self._pipeline_key, self.outputs.default, default_output)