union.py

def _pad_table(table: Set[Record], with_cols: List):
    padding_row = {col: None for col in with_cols}
    padded_table = {Record({**row, **padding_row}) for row in table}
    return padded_table


def union(left: Set[Record], right: Set[Record]) -> Set[Record]:
    # padding
    left_cols = _columns_in_table(left)
    right_cols = _columns_in_table(right)

    left = _pad_table(left, right_cols.difference(left_cols))
    right = _pad_table(right, left_cols.difference(right_cols))

    table_out = left.union(right)

    return table_out