add a parameter to the url. Overwrite param if it is already there

add_param_to_url.py

def _merge_new_params_with_referring_url(url, param_name=None, param_value=None):
    if not url:
        return '/'

    if not param_name or not param_value:
        return url

    scheme, network, path, query, fragment = urlparse.urlsplit(url)

    query_map = urlparse.parse_qs(query)
    query_map[param_name] = str(param_value)
    new_query = urllib.urlencode(query_map, doseq=True)

    return urlparse.urlunsplit((scheme, network, path, new_query, fragment))