How to use the pyyaml.composer.ComposerError function in PyYAML

def compose_node(self, parent, index):
        if self.check_event(AliasEvent):
            event = self.get_event()
            anchor = event.anchor
            if anchor not in self.anchors:
                raise ComposerError(None, None, "found undefined alias %r"
                        % anchor, event.start_mark)
            return self.anchors[anchor]
        event = self.peek_event()
        anchor = event.anchor
        if anchor is not None:
            if anchor in self.anchors:
                raise ComposerError("found duplicate anchor %r; first occurence"
                        % anchor, self.anchors[anchor].start_mark,
                        "second occurence", event.start_mark)
        self.descend_resolver(parent, index)
        if self.check_event(ScalarEvent):
            node = self.compose_scalar_node(anchor)
        elif self.check_event(SequenceStartEvent):
            node = self.compose_sequence_node(anchor)
        elif self.check_event(MappingStartEvent):
            node = self.compose_mapping_node(anchor)
        self.ascend_resolver()
        return node

def compose_node(self, parent, index):
        if self.check_event(AliasEvent):
            event = self.get_event()
            anchor = event.anchor
            if anchor not in self.anchors:
                raise ComposerError(None, None, "found undefined alias %r"
                        % anchor, event.start_mark)
            return self.anchors[anchor]
        event = self.peek_event()
        anchor = event.anchor
        if anchor is not None:
            if anchor in self.anchors:
                raise ComposerError("found duplicate anchor %r; first occurence"
                        % anchor, self.anchors[anchor].start_mark,
                        "second occurence", event.start_mark)
        self.descend_resolver(parent, index)
        if self.check_event(ScalarEvent):
            node = self.compose_scalar_node(anchor)
        elif self.check_event(SequenceStartEvent):
            node = self.compose_sequence_node(anchor)
        elif self.check_event(MappingStartEvent):
            node = self.compose_mapping_node(anchor)

def get_single_node(self):
        # Drop the STREAM-START event.
        self.get_event()

        # Compose a document if the stream is not empty.
        document = None
        if not self.check_event(StreamEndEvent):
            document = self.compose_document()

        # Ensure that the stream contains no more documents.
        if not self.check_event(StreamEndEvent):
            event = self.get_event()
            raise ComposerError("expected a single document in the stream",
                    document.start_mark, "but found another document",
                    event.start_mark)

        # Drop the STREAM-END event.
        self.get_event()

        return document