def _get_list_keys(nob):
    """Get the keys of the dictionnaries contained
    by a list

    Inputs :
    --------
    nob :  a list of dictionnaries

    Outputs :
    --------
    keys_list : a list of the dictionnaries keys
    """
    keys_list = []
    for item in nob:
        keys_list.extend(list(item.keys()))
    return keys_list
