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Increase edge case check strictness when cols_droplevel contains empty values #62527

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Merged
merged 5 commits into from
Oct 2, 2025
+26 −0
Merged

Increase edge case check strictness when cols_droplevel contains empty values #62527

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6a2568a
stricter edge case checking
matiaslindgren Oct 1, 2025
c6d65bd
use consistent naming in test
matiaslindgren Oct 1, 2025
968cdeb
fix string dtype check
matiaslindgren Oct 1, 2025
c30cf1d
add separate test for #62518
matiaslindgren Oct 1, 2025
5dd2bb4
assert result after assignment
matiaslindgren Oct 1, 2025
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5 changes: 5 additions & 0 deletions pandas/core/frame.py
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Original file line number Diff line number Diff line change
Expand Up @@ -107,6 +107,7 @@
is_list_like,
is_scalar,
is_sequence,
is_string_dtype,
needs_i8_conversion,
pandas_dtype,
)
Expand Down Expand Up @@ -4454,8 +4455,12 @@ def _set_item_frame_value(self, key, value: DataFrame) -> None:
cols_droplevel = maybe_droplevels(cols, key)
if (
not isinstance(cols_droplevel, MultiIndex)
and is_string_dtype(cols_droplevel.dtype)
and not cols_droplevel.any()
):
# if cols_droplevel contains only empty strings,
# value.reindex(cols_droplevel, axis=1) would be full of NaNs
# see GH#62518 and GH#61841
return
if len(cols_droplevel) and not cols_droplevel.equals(value.columns):
value = value.reindex(cols_droplevel, axis=1)
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21 changes: 21 additions & 0 deletions pandas/tests/indexing/multiindex/test_multiindex.py
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Expand Up @@ -271,3 +271,24 @@ def test_multiindex_assign_aligns_as_implicit_tuple(self):
df1["C"] = s1
tm.assert_frame_equal(df1, df2)
tm.assert_frame_equal(df1, df3)

def test_multiindex_assign_alignment_with_non_string_dtype(self):
# GH 62518
columns = MultiIndex.from_arrays(
[["a", "a", "z", "z"], pd.Categorical([1, 2, 1, 2])]
)

meta = DataFrame(columns=columns, dtype=object)
meta["z"] = meta["z"].astype("int64")
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Sorry, one more thing. Could you use tm.assert_frame.equal to assert that meta is equal to a new DataFrame that's the result of this casting?


result = DataFrame(
data={
("a", 1): Series([], dtype=object),
("a", 2): Series([], dtype=object),
("z", 1): Series([], dtype="int64"),
("z", 2): Series([], dtype="int64"),
},
columns=columns,
)

tm.assert_frame_equal(meta, result)
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