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FIX _safe_indexing: improve error message for string indexing on axis=0 #31494

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FIX _safe_indexing: improve error message for string indexing on axis=0 #31494

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This PR makes the ValueError from sklearn.utils._indexing._safe_indexing more informative
when a string index is passed with axis=0. The new message shows the actual value of
indices and suggests using axis=1 for column selection.

  • Modified sklearn/utils/_indexing.py to raise:
    ValueError(f"String indexing (indices={indices}) is not supported with 'axis=0'. "
    "Did you mean to use axis=1 for column selection?")
  • Updated the test in sklearn/utils/tests/test_indexing.py to match this new message
    (using a regex).

All existing indexing tests now pass. The only local test failure is test_openmp_parallelism_enabled,
which can be ignored on macOS; CI will run on Linux with OpenMP support.

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github-actions bot commented Jun 6, 2025 •
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❌ Linting issues

This PR is introducing linting issues. Here's a summary of the issues. Note that you can avoid having linting issues by enabling pre-commit hooks. Instructions to enable them can be found here.

You can see the details of the linting issues under the lint job here

ruff check

ruff detected issues. Please run ruff check --fix --output-format=full locally, fix the remaining issues, and push the changes. Here you can see the detected issues. Note that the installed ruff version is ruff=0.11.7.


sklearn/utils/_indexing.py:294:89: E501 Line too long (131 > 88)
    |
292 |     if axis == 0 and indices_dtype == "str":
293 |           raise ValueError(
294 |             f"String indexing (indices={indices}) is not supported with 'axis=0'. Did you mean to use axis=1 for column selection?"
    |                                                                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ E501
295 |         )
    |

Found 1 error.

ruff format

ruff detected issues. Please run ruff format locally and push the changes. Here you can see the detected issues. Note that the installed ruff version is ruff=0.11.7.


--- sklearn/utils/_indexing.py
+++ sklearn/utils/_indexing.py
@@ -290,7 +290,7 @@
     indices_dtype = _determine_key_type(indices)
 
     if axis == 0 and indices_dtype == "str":
-          raise ValueError(
+        raise ValueError(
             f"String indexing (indices={indices}) is not supported with 'axis=0'. Did you mean to use axis=1 for column selection?"
         )
 

1 file would be reformatted, 923 files already formatted

Generated for commit: cfd4b0e. Link to the linter CI: here

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@Roshangoli