def copy_keys(source, destination, keys=None):
    """
    Add keys in source to destination

    Parameters
    ----------
    source : dict

    destination: dict

    keys : None | iterable
        The keys in source to be copied into destination. If
        None, then `keys = destination.keys()`
    """
    if keys is None:
        keys = destination.keys()
    for k in (set(source) & set(keys)):
        destination[k] = source[k]
    return destination
