code: def start_span(self, name='span'):
          
          parent_span = self.current_span()
  
          # If a span has remote parent span, then the parent_span.span_id
          # should be the span_id from the request header.
          if parent_span is None:
              parent_span = base.NullContextManager(
                  span_id=self.span_context.span_id)
  
          span = trace_span.Span(
              name,
              parent_span=parent_span,
              context_tracer=self)
          with self._spans_list_condition:
              self._spans_list.append(span)
          self.span_context.span_id = span.span_id
          execution_context.set_current_span(span)
          span.start()
          return span
docs: Start a span.
  
          :type name: str
          :param name: The name of the span.
  
          :rtype: :class:`~opencensus.trace.span.Span`
          :returns: The Span object.