chore: Refactor validate_column_comparand (#1102)

* simplify

* rename argument

narwhals/_pandas_like/series.py

@@ -619,11 +619,9 @@ def quantile(
 
     def zip_with(self: Self, mask: Any, other: Any) -> PandasLikeSeries:
         ser = self._native_series
-        mask = validate_column_comparand(ser.index, mask)
-        if isinstance(mask, str) or not isinstance(
-            mask, (self.__native_namespace__().Series, Sequence)
-        ):
-            mask = [mask]
+        mask = validate_column_comparand(
+            ser.index, mask, treat_length_one_as_scalar=False
+        )
         other = validate_column_comparand(ser.index, other)
         res = ser.where(mask, other)
         return self._from_native_series(res)

narwhals/_pandas_like/utils.py

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