How to enforce a unique index constraint in Python

class MockIndex:
    def __init__(self, columns):
        self.columns = columns
        self._values = set()

    def insert(self, row):
        key = tuple(row[col] for col in self.columns)
        if key in self._values:
            raise ValueError(f"Duplicate key {key} for columns {self.columns}")
        self._values.add(key)

    def delete(self, row):
        key = tuple(row[col] for col in self.columns)
        if key not in self._values:
            raise KeyError(f"Key {key} not found")
        self._values.remove(key)

    def contains(self, row):
        key = tuple(row[col] for col in self.columns)
        return key in self._values


if __name__ == "__main__":
    index = MockIndex(["email"])
    row1 = {"id": 1, "email": "a@b.com", "name": "Alice"}
    row2 = {"id": 2, "email": "c@d.com", "name": "Bob"}
    row3 = {"id": 3, "email": "a@b.com", "name": "Eve"}

    index.insert(row1)
    index.insert(row2)
    try:
        index.insert(row3)
    except ValueError as e:
        print(f"Error: {e}")

    print(f"Contains row2: {index.contains(row2)}")
    index.delete(row2)
    print(f"Contains row2 after delete: {index.contains(row2)}")