code: def apply_trace_flag(span, trace, default_trace):
      
      if trace is None:
          trace = default_trace
      trace = trace() if callable(trace) else trace
      if trace is False and span:
          span.set_tag(tags.SAMPLING_PRIORITY, 0)
docs: If ``trace`` (or ``default_trace``) is False, disables tracing on ``span``.
      :param span:
      :param trace:
      :param default_trace:
      :return: