code: def writer(self, outputs, stdout, stderr=sys.stderr):
          
          for output in outputs:
              try:
                  output = flatten(output)
                  if self.data_interchange_format == "json":
                      # Only dump one json string, and skip another one, maybe key or value.
                      output = filter(lambda x: x, output)
                  else:
                      # JSON is already serialized, so we put `self.serialize` in a else statement.
                      output = map(self.serialize, output)
                  print("\t".join(output), file=stdout)
              except BaseException:
                  print(output, file=stderr)
                  raise
docs: Writer format is a method which iterates over the output records
          from the reducer and formats them for output.
  
          The default implementation outputs tab separated items.