code: def from_headers(self, headers):
          
          if headers is None:
              return SpanContext()
          header = headers.get(_TRACE_CONTEXT_HEADER_NAME)
          if header is None:
              return SpanContext()
          header = str(header.encode('utf-8'))
          return self.from_header(header)
docs: Generate a SpanContext object using the trace context header.
  
          :type headers: dict
          :param headers: HTTP request headers.
  
          :rtype: :class:`~opencensus.trace.span_context.SpanContext`
          :returns: SpanContext generated from the trace context header.