fix: preserve dtypes when using with_columns and length-1 pandas df (#1201)

* fix: preserve dtypes when using with_columns and length-1 pandas df

* pyarrow versions

Author: MarcoGorelli

narwhals/_pandas_like/series.py

@@ -619,9 +619,7 @@ def quantile(
 
     def zip_with(self: Self, mask: Any, other: Any) -> PandasLikeSeries:
         ser = self._native_series
-        mask = validate_column_comparand(
-            ser.index, mask, treat_length_one_as_scalar=False
-        )
+        mask = validate_column_comparand(ser.index, mask)
         other = validate_column_comparand(ser.index, other)
         res = ser.where(mask, other)
         return self._from_native_series(res)

narwhals/_pandas_like/utils.py

@@ -32,9 +32,7 @@
 }
 
 
-def validate_column_comparand(
-    index: Any, other: Any, *, treat_length_one_as_scalar: bool = True
-) -> Any:
+def validate_column_comparand(index: Any, other: Any) -> Any:
     """Validate RHS of binary operation.
 
     If the comparison isn't supported, return `NotImplemented` so that the
@@ -55,9 +53,10 @@ def validate_column_comparand(
     if isinstance(other, PandasLikeDataFrame):
         return NotImplemented
     if isinstance(other, PandasLikeSeries):
-        if other.len() == 1 and treat_length_one_as_scalar:
+        if other.len() == 1:
             # broadcast
-            return other.item()
+            s = other._native_series
+            return s.__class__(s.iloc[0], index=index, dtype=s.dtype)
         if other._native_series.index is not index:
             return set_axis(
                 other._native_series,
@@ -83,7 +82,8 @@ def validate_dataframe_comparand(index: Any, other: Any) -> Any:
     if isinstance(other, PandasLikeSeries):
         if other.len() == 1:
             # broadcast
-            return other._native_series.iloc[0]
+            s = other._native_series
+            return s.__class__(s.iloc[0], index=index, dtype=s.dtype)
         if other._native_series.index is not index:
             return set_axis(
                 other._native_series,

tests/frame/with_columns_test.py

@@ -1,7 +1,10 @@
 import numpy as np
 import pandas as pd
+import pyarrow as pa
+import pytest
 
 import narwhals.stable.v1 as nw
+from narwhals.utils import parse_version
 from tests.utils import Constructor
 from tests.utils import compare_dicts
 
@@ -40,3 +43,14 @@ def test_with_columns_order_single_row(constructor: Constructor) -> None:
     assert result.collect_schema().names() == ["a", "b", "z", "d"]
     expected = {"a": [2], "b": [4], "z": [7.0], "d": [0]}
     compare_dicts(result, expected)
+
+
+def test_with_columns_dtypes_single_row(
+    constructor: Constructor, request: pytest.FixtureRequest
+) -> None:
+    if "pyarrow_table" in str(constructor) and parse_version(pa.__version__) < (15,):
+        request.applymarker(pytest.mark.xfail)
+    data = {"a": ["foo"]}
+    df = nw.from_native(constructor(data)).with_columns(nw.col("a").cast(nw.Categorical))
+    result = df.with_columns(nw.col("a"))
+    assert result.collect_schema() == {"a": nw.Categorical}