code: def from_carrier(self, carrier):
          
          trace_id = None
          span_id = None
          trace_options = None
  
          for key in carrier:
              key = key.lower()
              if key == _TRACE_ID_KEY:
                  trace_id = carrier[key]
              if key == _SPAN_ID_KEY:
                  span_id = carrier[key]
              if key == _TRACE_OPTIONS_KEY:
                  trace_options = bool(carrier[key])
  
          if trace_options is None:
              trace_options = DEFAULT_TRACE_OPTIONS
  
          return SpanContext(
              trace_id=trace_id,
              span_id=span_id,
              trace_options=TraceOptions(trace_options),
              from_header=True)
docs: Generate a SpanContext object using the information in the carrier.
  
          :type carrier: dict
          :param carrier: The carrier which has the trace_id, span_id, options
                          information for creating a SpanContext.
  
          :rtype: :class:`~opencensus.trace.span_context.SpanContext`
          :returns: SpanContext generated from the carrier.