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TST: Add regression test for MultiIndex merge after reset_index #63256

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Merged
mroeschke merged 2 commits into from
Dec 5, 2025
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Merged

TST: Add regression test for MultiIndex merge after reset_index #63256

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TST: Add regression test for MultiIndex merge after reset_index
zachyattack23 Dec 3, 2025
42f2406
Address review feedback: simplify test construction and comments
zachyattack23 Dec 4, 2025
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35 changes: 35 additions & 0 deletions pandas/tests/reshape/merge/test_merge.py
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Original file line number Diff line number Diff line change
Expand Up @@ -2936,6 +2936,41 @@ def test_merge_multiindex_single_level():
tm.assert_frame_equal(result, expected)


def test_merge_multiindex_reset_index_mixed():
# GH#62150
df = DataFrame(
{("column_1", ""): [1, 1], ("column_2", ""): [2, 2]},
index=MultiIndex.from_arrays(
[[1, 1], ["metadata_1", "metadata_2"]], names=["index", "metadata"]
),
)

df2 = DataFrame(
data=[1, 1],
index=Index([1, 1], name="index"),
columns=MultiIndex.from_product([["new_data"], [""]]),
)

with tm.assert_produces_warning(pd.errors.PerformanceWarning):
result = df.reset_index().merge(df2.reset_index(), on="index")

expected = DataFrame(
{
("index", ""): [1, 1, 1, 1],
("metadata", ""): ["metadata_1", "metadata_1", "metadata_2", "metadata_2"],
("column_1", ""): [1, 1, 1, 1],
("column_2", ""): [2, 2, 2, 2],
("new_data", ""): [1, 1, 1, 1],
}
)
expected.columns = MultiIndex.from_tuples(expected.columns)

tm.assert_frame_equal(result, expected)

result2 = df.reset_index().merge(df2.reset_index(), on=[("index", "")])
tm.assert_frame_equal(result2, expected)


@pytest.mark.parametrize("on_index", [True, False])
@pytest.mark.parametrize("left_unique", [True, False])
@pytest.mark.parametrize("left_monotonic", [True, False])
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