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BUG: join changes index type when doing join on empty dataframes #53893

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Merged
merged 3 commits into from
Jul 10, 2023
Merged

BUG: join changes index type when doing join on empty dataframes #53893

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1fac76b
BUG: join changes index type when doing join on empty dataframes
topper-123 Jun 27, 2023
5be67d5
Merge branch 'master' into empty_join_bug
topper-123 Jul 7, 2023
8c454ae
move to v2.1.0
topper-123 Jul 7, 2023
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1 change: 1 addition & 0 deletions doc/source/whatsnew/v2.1.0.rst
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Expand Up @@ -512,6 +512,7 @@ Reshaping
- Bug in :meth:`DataFrame.stack` sorting index lexicographically in rare cases (:issue:`53824`)
- Bug in :meth:`DataFrame.transpose` inferring dtype for object column (:issue:`51546`)
- Bug in :meth:`Series.combine_first` converting ``int64`` dtype to ``float64`` and losing precision on very large integers (:issue:`51764`)
- Bug when joining empty :class:`DataFrame` objects, where the joined index would be a :class:`RangeIndex` instead of the joined index type (:issue:`52777`)
-

Sparse
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2 changes: 0 additions & 2 deletions pandas/core/reshape/merge.py
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Expand Up @@ -1157,8 +1157,6 @@ def _get_join_info(
else:
join_index = default_index(len(left_indexer))

if len(join_index) == 0 and not isinstance(join_index, MultiIndex):
join_index = default_index(0).set_names(join_index.name)
return join_index, left_indexer, right_indexer

@final
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9 changes: 9 additions & 0 deletions pandas/tests/reshape/merge/test_merge.py
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Expand Up @@ -438,6 +438,15 @@ def test_left_merge_empty_dataframe(self):
result = merge(right, left, on="key", how="right")
tm.assert_frame_equal(result, left)

@pytest.mark.parametrize("how", ["inner", "left", "right", "outer"])
def test_merge_empty_dataframe(self, index, how):
# GH52777
left = DataFrame([], index=index[:0])
right = left.copy()

result = left.join(right, how=how)
tm.assert_frame_equal(result, left)

@pytest.mark.parametrize(
"kwarg",
[
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