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ENH: say which DataFrame is missing the key in merge KeyError (GH#71819) - #71954
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When a key passed to ``on``, ``left_on`` or ``right_on`` was missing,
merge/join raised a bare ``KeyError: 'key'`` that did not say which
DataFrame lacked it. The error now names the side and lists the
available columns and index levels, e.g.
KeyError: "'customer_id' not found in right DataFrame. Available
columns: ['id', 'name']"
If an ``on`` key is missing from both DataFrames, both are reported.
The exception type is still ``KeyError`` and the original error is
chained as ``__cause__``.
- Add _MergeOperation._get_merge_key_values and use it for all key
lookups in _get_merge_keys
- Update the join(on=...) test in test_join.py for the new message
- Add TestMergeMissingKeyError to test_merge.py
- Add a whatsnew entry to v3.2.0.rst
closes pandas-dev#71819
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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When a key passed to
on,left_onorright_onwas missing, merge/join raised a bareKeyError: 'key'that did not say which DataFrame lacked it. The error now names the side and lists the available columns and index levels, e.g.If an
onkey is missing from both DataFrames, both are reported. The exception type is stillKeyErrorand the original error is chained as__cause__.AI use disclosure: I used Claude Code, model Opus 5.5 on low effort. It was used to explain in detail what the issue entailed, the location within the repository, including the affected files that called on or tested merge.py. It was used to help generate a some rudimentary functions and testing, along with some of the entry in the latest whatsnew snapshot. TLDR is I used a little bit of Claude Opus 5.5 in my workflow.
closes #71819
closes ENH: DataFrame.merge raises bare KeyError without saying which DataFrame is missing the key column #71819 (Replace xxxx with the GitHub issue number)
Tests added and passed if fixing a bug or adding a new feature
All code checks passed.
Added type annotations to new arguments/methods/functions.
Added an entry in the latest
doc/source/whatsnew/vX.X.X.rstfile if fixing a bug or adding a new feature.I have reviewed and followed all the contribution guidelines
Check exactly one of the following, per the automated contributions policy:
I did not use AI to develop this pull request.
I used AI to develop this pull request. I prompted it to follow
AGENTS.md, I have reviewed and understood every change, and I have described above how I used it and exactly which tool, model version, and effort setting — e.g.claude opus 4.8 (xhigh), not justclaude.