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BUG: combine_first ignoring others columns if other is empty #53792

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Merged
merged 2 commits into from
Jun 22, 2023
Merged

BUG: combine_first ignoring others columns if other is empty #53792

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1f7518b
BUG: combine_first ignoring others columns if other is empty
phofl Jun 22, 2023
d176ed4
Fix
phofl Jun 22, 2023
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1 change: 1 addition & 0 deletions doc/source/whatsnew/v2.1.0.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -491,6 +491,7 @@ Reshaping
- Bug in :func:`merge_asof` raising ``KeyError`` for extension dtypes (:issue:`52904`)
- Bug in :func:`merge_asof` raising ``ValueError`` for data backed by read-only ndarrays (:issue:`53513`)
- Bug in :meth:`DataFrame.agg` and :meth:`Series.agg` on non-unique columns would return incorrect type when dist-like argument passed in (:issue:`51099`)
- Bug in :meth:`DataFrame.combine_first` ignoring other's columns if ``other`` is empty (:issue:`53792`)
- Bug in :meth:`DataFrame.idxmin` and :meth:`DataFrame.idxmax`, where the axis dtype would be lost for empty frames (:issue:`53265`)
- Bug in :meth:`DataFrame.merge` not merging correctly when having ``MultiIndex`` with single level (:issue:`52331`)
- Bug in :meth:`DataFrame.stack` losing extension dtypes when columns is a :class:`MultiIndex` and frame contains mixed dtypes (:issue:`45740`)
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8 changes: 7 additions & 1 deletion pandas/core/frame.py
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Expand Up @@ -8344,7 +8344,13 @@ def combiner(x, y):

return expressions.where(mask, y_values, x_values)

combined = self.combine(other, combiner, overwrite=False)
if len(other) == 0:
combined = self.reindex(
self.columns.append(other.columns.difference(self.columns)), axis=1
)
combined = combined.astype(other.dtypes)
else:
combined = self.combine(other, combiner, overwrite=False)

dtypes = {
col: find_common_type([self.dtypes[col], other.dtypes[col]])
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8 changes: 8 additions & 0 deletions pandas/tests/frame/methods/test_combine_first.py
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Expand Up @@ -538,3 +538,11 @@ def test_midx_losing_dtype():
)
expected = DataFrame({"a": [np.nan, 4, 3, 3]}, index=expected_midx)
tm.assert_frame_equal(result, expected)


def test_combine_first_empty_columns():
left = DataFrame(columns=["a", "b"])
right = DataFrame(columns=["a", "c"])
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Could you also test where right has a different dtype than left?

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This is actually covered. The reindex casts to float while right has object dtype

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Oh right. Sounds good

result = left.combine_first(right)
expected = DataFrame(columns=["a", "b", "c"])
tm.assert_frame_equal(result, expected)