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pandas SettingWithCopyWarning and Chained Assignment: Why Updates Vanish and the .loc Fix

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Self-Hosting & Infra intermediate 5 min read $0.01 Updated 2026-10-20

Reproduced on pandas 2.3.3: SettingWithCopyWarning, the original staying [4, 5, 6] after assigning to a filtered slice, .loc fixing it, .copy() isolating derived data, and the FutureWarning for df[col][row] = value.

You assign to a filtered DataFrame and the original does not change, or pandas warns about a copy of a slice. Use .loc for updates, .copy() for derived frames, and prepare for Copy-on-Write in pandas 3.

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