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ENH: Add validation checks for non-unique merge keys #35373

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ENH: Add validation checks for non-unique merge keys #35373

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8e6f677
Add many-to-many merge validation check
LewisDavies Jul 22, 2020
ebe7be9
Extend checks to 1:m and m:1 merges
LewisDavies Jul 22, 2020
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17 changes: 16 additions & 1 deletion pandas/core/reshape/merge.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1283,16 +1283,31 @@ def _validate(self, validate: str):
raise MergeError(
"Merge keys are not unique in left dataset; not a one-to-many merge"
)
elif right_unique:
raise MergeError(
"Merge keys are unique in right dataset; not a one-to-many merge"
)

elif validate in ["many_to_one", "m:1"]:
if not right_unique:
raise MergeError(
"Merge keys are not unique in right dataset; "
"not a many-to-one merge"
)
elif left_unique:
raise MergeError(
"Merge keys are unique in left dataset; not a many-to-one merge"
)

elif validate in ["many_to_many", "m:m"]:
pass
if left_unique:
raise MergeError(
"Merge keys are unique in left dataset; not a many-to-many merge"
)
elif right_unique:
raise MergeError(
"Merge keys are unique in right dataset; not a many-to-many merge"
)

else:
raise ValueError("Not a valid argument for validate")
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