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How to enforce a unique index constraint in Python

Mock a database unique index in Python that rejects duplicate rows based on one or more columns.

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._v…

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