def METHOD_NAME(self, instance, format):
    if format not in self.checkers:
        return
    func, raises = self.checkers[format]
    result, cause = None, None
    try:
        result = func(instance)
    except raises as e:
        cause = e
    if not result:
        msg = "%r is not a %r" % (instance, format)
        raise jsonschema_exc.FormatError(msg, cause=cause)