def has_vals_within_set(df, items=None):
    """Asserts that `df` is a subset of items.

    Args:
        df (pd.DataFrame): Any pd.DataFrame.
        items (dict): Mapping of columns (col) to array-like of values (v) that
                      ``df[col]`` is expected to be a subset of.

    Returns:
        Original `df`.

    """
    for col, v in items.items():
        if not df[col].isin(v).all():
            bad = df.loc[~df[col].isin(v), col]
            raise AssertionError('Not in set', bad)
    return df
