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BUG: Fix getitem dtype preservation with multiindexes #51895

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Apr 25, 2023
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2 changes: 1 addition & 1 deletion doc/source/whatsnew/v2.1.0.rst
Original file line number Diff line number Diff line change
Expand Up @@ -347,8 +347,8 @@ Missing

MultiIndex
^^^^^^^^^^
- Bug in :meth:`DataFrame.__getitem__` not preserving dtypes for :class:`MultiIndex` partial keys (:issue:`51895`)
- Bug in :meth:`MultiIndex.set_levels` not preserving dtypes for :class:`Categorical` (:issue:`52125`)
-

I/O
^^^
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14 changes: 2 additions & 12 deletions pandas/core/frame.py
Original file line number Diff line number Diff line change
Expand Up @@ -3831,18 +3831,8 @@ def _getitem_multilevel(self, key):
if isinstance(loc, (slice, np.ndarray)):
new_columns = self.columns[loc]
result_columns = maybe_droplevels(new_columns, key)
if self._is_mixed_type:
result = self.reindex(columns=new_columns)
result.columns = result_columns
else:
new_values = self._values[:, loc]
result = self._constructor(
new_values, index=self.index, columns=result_columns, copy=False
)
if using_copy_on_write() and isinstance(loc, slice):
result._mgr.add_references(self._mgr) # type: ignore[arg-type]

result = result.__finalize__(self)
result = self.iloc[:, loc]
result.columns = result_columns

# If there is only one column being returned, and its name is
# either an empty string, or a tuple with an empty string as its
Expand Down
20 changes: 20 additions & 0 deletions pandas/tests/indexing/multiindex/test_multiindex.py
Original file line number Diff line number Diff line change
Expand Up @@ -6,12 +6,14 @@

import pandas as pd
from pandas import (
CategoricalDtype,
DataFrame,
Index,
MultiIndex,
Series,
)
import pandas._testing as tm
from pandas.core.arrays.boolean import BooleanDtype


class TestMultiIndexBasic:
Expand Down Expand Up @@ -207,6 +209,24 @@ def test_multiindex_with_na_missing_key(self):
with pytest.raises(KeyError, match="missing_key"):
df[[("missing_key",)]]

def test_multiindex_dtype_preservation(self):
# GH51261
columns = MultiIndex.from_tuples([("A", "B")], names=["lvl1", "lvl2"])
df = DataFrame(["value"], columns=columns).astype("category")
df_no_multiindex = df["A"]
assert isinstance(df_no_multiindex["B"].dtype, CategoricalDtype)

# geopandas 1763 analogue
df = DataFrame(
[[1, 0], [0, 1]],
columns=[
["foo", "foo"],
["location", "location"],
["x", "y"],
],
).assign(bools=Series([True, False], dtype="boolean"))
assert isinstance(df["bools"].dtype, BooleanDtype)

def test_multiindex_from_tuples_with_nan(self):
# GH#23578
result = MultiIndex.from_tuples([("a", "b", "c"), np.nan, ("d", "", "")])
Expand Down