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API: Disallow DataFrame other for Series flex ops #62648

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Oct 16, 2025
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API: Disallow DataFrame other for Series flex ops #62648

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API: Disallow DataFrame other for Series flex ops
jbrockmendel Oct 10, 2025
44d45ef
Merge branch 'main' into bug-46179
jbrockmendel Oct 16, 2025
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1 change: 1 addition & 0 deletions doc/source/whatsnew/v3.0.0.rst
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Expand Up @@ -655,6 +655,7 @@ Other API changes
an empty ``RangeIndex`` or empty ``Index`` with object dtype when determining
the dtype of the resulting Index (:issue:`60797`)
- :class:`IncompatibleFrequency` now subclasses ``TypeError`` instead of ``ValueError``. As a result, joins with mismatched frequencies now cast to object like other non-comparable joins, and arithmetic with indexes with mismatched frequencies align (:issue:`55782`)
- :class:`Series` "flex" methods like :meth:`Series.add` no longer allow passing a :class:`DataFrame` for ``other``; use the DataFrame reversed method instead (:issue:`46179`)
- :meth:`CategoricalIndex.append` no longer attempts to cast different-dtype indexes to the caller's dtype (:issue:`41626`)
- :meth:`ExtensionDtype.construct_array_type` is now a regular method instead of a ``classmethod`` (:issue:`58663`)
- Comparison operations between :class:`Index` and :class:`Series` now consistently return :class:`Series` regardless of which object is on the left or right (:issue:`36759`)
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6 changes: 6 additions & 0 deletions pandas/core/series.py
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Expand Up @@ -6069,6 +6069,12 @@ def _flex_method(self, other, op, *, level=None, fill_value=None, axis: Axis = 0
result = self._binop(other, op, level=level, fill_value=fill_value)
result._name = res_name
return result
elif isinstance(other, ABCDataFrame):
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Actually last minute thought, would be worth still allowing this if the axis are already aligned?

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so just revert and let non-aligned cases raises like before? maybe try to give a better message?

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Or if ser.flex_op(df, axis=0) gives a sensible result when ser.index.equals(df.index), just have a follow up PR to amend and not ser.index.equals(df.index) to this elif. I suppose it would be good to also have a test case for ser.flex_op(df, axis=0) when the indexes are aligned

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i think "DataFrame is not allowed" is significantly simpler to communicate than "DataFrame is not allowed except when aligned"

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Yeah definitely. We can keep this as is and reconsider if anyone seriously complains

# GH#46179
raise TypeError(
f"Series.{op.__name__.strip('_')} does not support a DataFrame "
f"`other`. Use df.{op.__name__.strip('_')}(ser) instead."
)
else:
if fill_value is not None:
if isna(other):
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18 changes: 18 additions & 0 deletions pandas/tests/series/test_arithmetic.py
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Expand Up @@ -156,6 +156,24 @@ def _check_fill(meth, op, a, b, fill_value=0):
# should accept axis=0 or axis='rows'
op(a, b, axis=0)

def test_flex_disallows_dataframe(self):
# GH#46179
df = pd.DataFrame(
{2010: [1], 2020: [3]},
index=pd.MultiIndex.from_product([["a"], ["b"]], names=["scen", "mod"]),
)

ser = Series(
[10.0, 20.0, 30.0],
index=pd.MultiIndex.from_product(
[["a"], ["b"], [0, 1, 2]], names=["scen", "mod", "id"]
),
)

msg = "Series.add does not support a DataFrame `other`"
with pytest.raises(TypeError, match=msg):
ser.add(df, axis=0)


class TestSeriesArithmetic:
# Some of these may end up in tests/arithmetic, but are not yet sorted
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