def _add_param(params, value, key):
    """ If value is not None then return dict params with added (key, value) pair

    :param params: dictionary of parameters
    :type: dict
    :param value: Value
    :param key: Key
    :return: if value not ``None`` then a copy of params with (key, value) added, otherwise returns params
    """
    if value:
        params[key] = value
    return params