Write a notebook to a string

def writes(notebook, fmt, version=nbformat.NO_CONVERT, **kwargs):
    """Write a notebook to a string"""
    metadata = deepcopy(notebook.metadata)
    rearrange_jupytext_metadata(metadata)
    fmt = copy(fmt)
    fmt = long_form_one_format(fmt, metadata)
    ext = fmt['extension']
    format_name = fmt.get('format_name')

    jupytext_metadata = metadata.get('jupytext', {})

    if ext == '.ipynb':
        # Remove jupytext section if empty
        jupytext_metadata.pop('text_representation', {})
        if not jupytext_metadata:
            metadata.pop('jupytext', {})
        return nbformat.writes(new_notebook(cells=notebook.cells, metadata=metadata), version, **kwargs)

    if not format_name:
        format_name = format_name_for_ext(metadata, ext, explicit_default=False)

    if format_name:
        fmt['format_name'] = format_name
        update_jupytext_formats_metadata(metadata, fmt)

    writer = TextNotebookConverter(fmt)
    return writer.writes(notebook, metadata)