BUG: Fix GroupBy aggregate coersion of outputs inconsistency for pyarrow dtypes #61640
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heoh force-pushed the fix/#61636 branch 2 times, most recently from 0dc71b3 to 05470fc Compare 
| This pull request is stale because it has been open for thirty days with no activity. Please update and respond to this comment if you're still interested in working on this. |
| we've introduced a method _cast_pointwise_result that should be used for these cases |
| in fact, trying out your test on main, the string case looks right and the other two are giving better dtypes. so if youd like to update with just the tests, thatd be great |
| The original issue has been addressed by #59601 so closing |
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doc/source/whatsnew/v3.0.0.rstfile if fixing a bug or adding a new feature.Description
Fix a bug in
DataFrameGroupBy.aggwhen applied to columns withArrowDtype, where pandas attempted to cast the result back to the original dtype.Cause
ExtensionArray._from_scalars()should raise onlyValueErrororTypeError, instead ofpa.ArrowNotImplementedError.ArrowExtensionArray._from_sequence()may attempt an invalid cast, and if it succeeds, it can unintentionally alter the actual data type.