Two columns satisfy a "one-to-one" constraint if each value in column 1 is associated with exactly one value in column 2, and vice versa. E.g. in this data, the person column and the email column satisfy a one-to-one constraint:
>>> table = [('pablo', 'ppicasso@gmail.com'),
('georgia', 'gokeefe@hotmail.com'),
('mark', 'mrothko@aol.com')]
>>> is_one_to_one(table)