def extract(self, format, carrier):
        """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.
        """
        if format in Tracer._supported_formats:
            return self._noop_span_context
        raise UnsupportedFormatException(format)