code: def extract(self, format, carrier):
          
          if format in Tracer._supported_formats:
              return self._noop_span_context
          raise UnsupportedFormatException(format)
docs: Returns a :class:`SpanContext` instance extracted from a `carrier` of the
          given `format`, or ``None`` if no such :class:`SpanContext` could be
          found.
  
          The type of `carrier` is determined by `format`. See the
          :class:`Format` class/namespace for the built-in OpenTracing formats.
  
          Implementations *must* raise :exc:`UnsupportedFormatException` if
          `format` is unknown or disallowed.
  
          Implementations may raise :exc:`InvalidCarrierException`,
          :exc:`SpanContextCorruptedException`, or implementation-specific errors
          if there are problems with `carrier`.
  
  
          :param format: a python object instance that represents a given
              carrier format. `format` may be of any type, and `format` equality
              is defined by python ``==`` equality.
  
          :param carrier: the format-specific carrier object to extract from
  
          :rtype: SpanContext
          :return: a :class:`SpanContext` extracted from `carrier` or ``None`` if
              no such :class:`SpanContext` could be found.