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Tricky Python Behavior: Why does pandas merge suddenly duplicate rows silently?

Python Short: Why does pandas merge suddenly duplicate rows silently?

In this Python short:
- The bug shown
- Why Python does this
- The correct fix

Explanation: The bug is that pandas.merge silently creates duplicate rows when the merge key isn't unique on either side, causing unexpected row multiplication. This happens because pandas performs a Cartesian product for matching keys internally. To avoid surprises, always ensure your merge keys are unique or e

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25 февраля 2026 г. 12:00:09
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