Return the implementation for the desired format

def get_format_implementation(ext, format_name=None):
    """Return the implementation for the desired format"""
    # remove pre-extension if any
    ext = '.' + ext.split('.')[-1]

    formats_for_extension = []
    for fmt in JUPYTEXT_FORMATS:
        if fmt.extension == ext:
            if fmt.format_name == format_name or not format_name:
                return fmt
            formats_for_extension.append(fmt.format_name)

    if formats_for_extension:
        if ext == '.md' and format_name == 'pandoc':
            raise JupytextFormatError('Please install pandoc>=2.7.2')

        raise JupytextFormatError("Format '{}' is not associated to extension '{}'. "
                                  "Please choose one of: {}.".format(format_name, ext,
                                                                     ', '.join(formats_for_extension)))
    raise JupytextFormatError("No format associated to extension '{}'".format(ext))