Fix alignment in Series subtraction with MultiIndex, Index and NaN values (#60908) #61381
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Fixes an issue where subtracting a Series with a MultiIndex containing NaN values from a regular Index Series led to incorrect results. Updated _align_for_op to properly reindex right-hand Series based on the first level of the MultiIndex and handle empty Series cases. Added test_subtraction_nanindex to ensure correct behavior.
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This pull request fixes #60908 , where subtracting a Series with a MultiIndex containing NaN values from a Series with a regular Index could lead to incorrect results or unexpected behavior.
The issue was caused by the _align_for_op method not properly handling cases where the left-hand Series had a MultiIndex and the right-hand side had a flat Index, especially when NaN values were present. This could lead to misalignment during arithmetic operations.
To fix this, the _align_for_op method was updated to:
Ensure that when the left Series has a MultiIndex and the right Series has a regular Index, the right Series is properly reindexed based on the first level of the left-hand MultiIndex, even when NaN values are involved.
Correctly handle cases where either Series is empty.
Additionally, a new test test_series_subtraction_with_nan_and_levels (added in test_subtraction_nanindex) was introduced to verify that:
Subtracting a Series with a MultiIndex (including NaNs) from a regular Index works correctly.
The result maintains the correct alignment and expected output values.