How to use the svglib.svglib.ExternalSVG function in svglib

)
                return None
            path = os.path.normpath(os.path.join(os.path.dirname(self.source_path), iri))
            if not os.access(path, os.R_OK):
                return None
            if path == self.source_path:
                # Self-referencing, ignore the IRI part
                iri = None

        if iri:
            if path.endswith('.svg'):
                if path in self._parent_chain:
                    logger.error("Circular reference detected in file.")
                    raise CircularRefError()
                if path not in self._external_svgs:
                    self._external_svgs[path] = ExternalSVG(path, self)
                ext_svg = self._external_svgs[path]
                if ext_svg.root_node is not None:
                    if fragment:
                        ext_frag = ext_svg.get_fragment(fragment)
                        if ext_frag is not None:
                            return ext_svg.renderer, ext_frag
                    else:
                        return ext_svg.renderer, ext_svg.root_node
            else:
                # A raster image path
                try:
                    # This will catch invalid images
                    PDFImage(path, 0, 0)
                except IOError:
                    logger.error("Unable to read the image %s. Skipping..." % path)
                    return None