def urljoin_safe(base_url, url, allow_fragments=True):
    '''urljoin with warning log on error.

    Returns:
        str, None'''
    try:
        return wpull.url.urljoin(
            base_url, url, allow_fragments=allow_fragments
        )
    except ValueError as error:
        _logger.warning(__(
            _('Unable to parse URL ‘{url}’: {error}.'),
            url=url, error=error
        ))
urljoin with warning log on error.

    Returns:
        str, None