def _field_update(op, path, value):
    """Return a dictionary for a field operation

    :param op: *add*, *replace* or *test*
    :param path: Path of field
    :param value: Field value
    :return: dict
    """
    return {
        "op": op,
        "path": "/fields/{}".format(path) if "/fields/" not in path else path,
        "value": value
    }