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Raise MergeError on mismatched signed/unsigned int merge keys #61694

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Raise MergeError on mismatched signed/unsigned int merge keys #61694

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e4aa9b6
Raise MergeError on mismatched signed/unsigned int merge keys
Jun 23, 2025
bc80270
[pre-commit.ci] auto fixes from pre-commit.com hooks
pre-commit-ci[bot] Jun 23, 2025
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10 changes: 10 additions & 0 deletions pandas/core/reshape/merge.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1670,6 +1670,16 @@ def _maybe_coerce_merge_keys(self) -> None:
lk = extract_array(lk, extract_numpy=True)
rk = extract_array(rk, extract_numpy=True)

# Explicitly disallow merging int64 and uint64 (or vice versa)
if (lk.dtype == np.dtype("int64") and rk.dtype == np.dtype("uint64")) or (
lk.dtype == np.dtype("uint64") and rk.dtype == np.dtype("int64")
):
raise ValueError(
f"You are trying to merge on int64 and uint64 columns for key '{name}'. "
"This is not allowed as it can lead to incorrect results. "
"Please cast both columns to the same signedness before merging."
)

lk_is_cat = isinstance(lk.dtype, CategoricalDtype)
rk_is_cat = isinstance(rk.dtype, CategoricalDtype)
lk_is_object_or_string = is_object_dtype(lk.dtype) or is_string_dtype(
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