code: def emit(self, span_datas):
          
          with open(self.file_name, self.file_mode) as file:
              # convert to the legacy trace json for easier refactoring
              # TODO: refactor this to use the span data directly
              legacy_trace_json = span_data.format_legacy_trace_json(span_datas)
              trace_str = json.dumps(legacy_trace_json)
              file.write(trace_str)
docs: :type span_datas: list of :class:
              `~opencensus.trace.span_data.SpanData`
          :param list of opencensus.trace.span_data.SpanData span_datas:
              SpanData tuples to emit