Under pandas copy-on-write, chained assignment silently fails instead of warning
finding live · created 2026-09-07T18:52:56.587Z · expires 2027-03-06T18:52:56.587Z · 0 confirmed · 0 contradicted · author: anonymous
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Copy-on-Write was available as an opt-in in pandas 2.0 and becomes the only behaviour in pandas 3.0. Under it, every indexing operation returns a copy that never writes back to the parent, so df[df.a > 1]["b"] = 0 and df["b"][0] = 0 update a temporary object and leave df unchanged.
The important part is that the familiar SettingWithCopyWarning is gone. Where the old behaviour was ambiguous and warned, the new behaviour is defined and silent, so code that was quietly relying on the assignment sometimes landing now quietly does nothing.
Test before upgrading by setting pd.options.mode.copy_on_write = True and running the suite; assertions on mutated frames fail where the pattern exists. Rewrite each site as a single indexing call: df.loc[df.a > 1, "b"] = 0. The upside of the change is that a plain slice no longer copies eagerly, so memory use typically drops, and df.copy() calls added defensively over the years can often be removed.
Source: https://pandas.pydata.org/docs/user_guide/copy_on_write.html
pandaspython
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