How to use the nbconvert.exporters.export function in nbconvert

def display_as_html(filename, html_outputfile=None):
    """opens the file as html in the web browser"""
    import nbconvert
    import webbrowser

    output = nbconvert.exporters.export(nbconvert.HTMLExporter(), filename)[0]
    if html_outputfile == None:
        import tempfile
        # create temporary file
        temp = tempfile.NamedTemporaryFile(mode="w+t", suffix=".html", delete=False)
        temp.write(output)
        temp.seek(0)
        webbrowser.open(temp.name)

    else:
        with open(html_outputfile, 'w+') as f:
            f.write(output)
        webbrowser.open(html_outputfile)

if mode == 'python':
            data = self.pretty
        else:
            cleared = (mode == 'cleared')
            nb = nbformat.v4.new_notebook()
            if self.nbformat_minor is not None:
                nb.nbformat_minor = self.nbformat_minor
            nb['cells'] = [cell.node(cleared=cleared) for cell in self.cells]
            if self.metadata is not None:
                nb['metadata'] = self.metadata

        if mode == 'html':
            if nbconvert is None:
                logging.info("nbconvert not available for HTML conversion")
                return
            (data, resources) = nbconvert.exporters.export(nbconvert.HTMLExporter, nb)
        with open(filename, 'w') as f:
            if mode in ['cleared', 'full-notebook']:
                nbformat.write(nb, f, version=4)
            else:
                f.write(data)
        return {'cmd':'write_complete', 'data':self.name}