'Get the stylesheet for transformation of the import
@param file_format: the import source file format'

def _get_stylesheet(self, file_format='csv'):

    if (file_format == 'csv'):
        xslt_path = os.path.join(self.xslt_path, 's3csv')
    else:
        xslt_path = os.path.join(self.xslt_path, file_format, 'import.xsl')
        return xslt_path
    if self.csv_stylesheet:
        if isinstance(self.csv_stylesheet, (tuple, list)):
            stylesheet = os.path.join(xslt_path, *self.csv_stylesheet)
        else:
            stylesheet = os.path.join(xslt_path, self.controller, self.csv_stylesheet)
    else:
        xslt_filename = ('%s.%s' % (self.function, self.xslt_extension))
        stylesheet = os.path.join(xslt_path, self.controller, xslt_filename)
    if (os.path.exists(stylesheet) is False):
        msg = (self.messages.stylesheet_not_found % stylesheet)
        self.error = msg
        current.log.debug(msg)
        return None
    return stylesheet
