def update_not_existing_kwargs(to_update, update_from):
    """
    This function updates the keyword aguments from update_from in
    to_update, only if the keys are not set in to_update.

    This is used for updated kwargs from the default dicts.
    """
    if to_update is None:
        to_update = {}
    to_update.update({k:v for k,v in update_from.items() if k not in to_update})
    return to_update