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Pandas4Warning: Copy-on-Write is always enabled with pandas >= 3.0#12272

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crusaderky merged 1 commit intodask:mainfrom
crusaderky:pandas-cow-warning
Feb 4, 2026
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Pandas4Warning: Copy-on-Write is always enabled with pandas >= 3.0#12272
crusaderky merged 1 commit intodask:mainfrom
crusaderky:pandas-cow-warning

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@crusaderky crusaderky commented Feb 4, 2026 •
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Prevent

Pandas4Warning: The 'mode.copy_on_write' option is deprecated. Copy-on-Write
can no longer be disabled (it is always enabled with pandas >= 3.0), and
setting the option has no impact. This option will be removed in pandas 4.0.

in the test suite.

pyproject.toml informs that this warning should cause a hard error in pytest. This is not the case, for some reason. I'm tackling this issue in a downstream PR.

@crusaderky crusaderky added the tests Unit tests and/or continuous integration label Feb 4, 2026
@crusaderky crusaderky mentioned this pull request Feb 4, 2026
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crusaderky commented Feb 4, 2026

This CI failure is unrelated to this PR (#12225):

FAILED dask/dataframe/io/tests/test_parquet.py::test_arrow_partitioning - pandas.errors.Pandas4Warning: Constructing a Categorical with a dtype and values containing non-null entries not in that dtype's categories is deprecated and will raise in a future version.

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github-actions bot commented Feb 4, 2026

Unit Test Results

See test report for an extended history of previous test failures. This is useful for diagnosing flaky tests.

     21 files  +     18       21 suites  +18   5h 4m 52s ⏱️ + 4h 51m 55s
 18 282 tests +    104   17 009 ✅ +    288   1 272 💤  -    185  1 ❌ +1 
294 733 runs  +259 798  252 413 ✅ +234 567  42 319 💤 +25 230  1 ❌ +1 

For more details on these failures, see this check.

Results for commit 1c44fec. ± Comparison against base commit 1be4042.

@crusaderky crusaderky merged commit f4ba089 into dask:main Feb 4, 2026
29 of 30 checks passed
@crusaderky crusaderky deleted the pandas-cow-warning branch February 4, 2026 18:57
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