WIP: raise on str and momentaneously xfail on polars expr

narwhals/_arrow/series.py

@@ -263,6 +263,12 @@ def mean(self) -> int:
     def median(self) -> int:
         import pyarrow.compute as pc  # ignore-banned-import()
 
+        from narwhals._exceptions import InvalidOperationError
+
+        if not self.dtype.is_numeric():
+            msg = "`median` operation not supported for non-numeric input type."
+            raise InvalidOperationError(msg)
+
         return pc.approximate_median(self._native_series)  # type: ignore[no-any-return]
 
     def min(self) -> int:

narwhals/_dask/expr.py

@@ -382,11 +382,17 @@ def mean(self) -> Self:
         )
 
     def median(self) -> Self:
-        return self._from_call(
-            lambda _input: _input.median_approximate(),
-            "median",
-            returns_scalar=True,
-        )
+        from dask_expr._shuffle import _is_numeric_cast_type
+
+        from narwhals._exceptions import InvalidOperationError
+
+        def func(_input: dask_expr.Series) -> dask_expr.Series:
+            if not _is_numeric_cast_type(_input.dtype):
+                msg = "`median` operation not supported for non-numeric input type."
+                raise InvalidOperationError(msg)
+            return _input.median_approximate()
+
+        return self._from_call(func, "median", returns_scalar=True)
 
     def min(self) -> Self:
         return self._from_call(

narwhals/_exceptions.py

@@ -2,3 +2,6 @@
 
 
 class ColumnNotFoundError(Exception): ...
+
+
+class InvalidOperationError(Exception): ...

narwhals/_pandas_like/series.py

@@ -422,6 +422,11 @@ def mean(self) -> Any:
         return ser.mean()
 
     def median(self) -> Any:
+        from narwhals._exceptions import InvalidOperationError
+
+        if not self.dtype.is_numeric():
+            msg = "`median` operation not supported for non-numeric input type."
+            raise InvalidOperationError(msg)
         ser = self._native_series
         return ser.median()
 

narwhals/_polars/namespace.py

@@ -120,9 +120,14 @@ def mean_horizontal(self, *exprs: IntoPolarsExpr) -> PolarsExpr:
     def median(self, *column_names: str) -> PolarsExpr:
         import polars as pl  # ignore-banned-import()
 
+        from narwhals._exceptions import InvalidOperationError
         from narwhals._polars.expr import PolarsExpr
 
-        return PolarsExpr(pl.median([*column_names]), dtypes=self._dtypes)  # type: ignore[arg-type]
+        try:
+            return PolarsExpr(pl.median([*column_names]), dtypes=self._dtypes)  # type: ignore[arg-type]
+        except pl.exceptions.InvalidOperationError as e:
+            msg = "`median` operation not supported for non-numeric input type."
+            raise InvalidOperationError(msg) from e
 
     def concat_str(
         self,

narwhals/_polars/series.py

@@ -179,6 +179,15 @@ def __rpow__(self, other: PolarsSeries | Any) -> Self:
     def __invert__(self) -> Self:
         return self._from_native_series(self._native_series.__invert__())
 
+    def median(self) -> Any:
+        from narwhals._exceptions import InvalidOperationError
+
+        if not self.dtype.is_numeric():
+            msg = "`median` operation not supported for non-numeric input type."
+            raise InvalidOperationError(msg)
+
+        return self._native_series.median()
+
     def to_dummies(
         self: Self, *, separator: str = "_", drop_first: bool = False
     ) -> PolarsDataFrame:

tests/expr_and_series/median_test.py

@@ -3,6 +3,7 @@
 import pytest
 
 import narwhals.stable.v1 as nw
+from narwhals._exceptions import InvalidOperationError
 from tests.utils import Constructor
 from tests.utils import ConstructorEager
 from tests.utils import assert_equal_data
@@ -36,3 +37,32 @@ def test_median_series(
     series = nw.from_native(constructor_eager(data), eager_only=True)[col]
     result = series.median()
     assert_equal_data({col: [result]}, {col: [expected]})
+
+
+@pytest.mark.parametrize("expr", [nw.col("s").median(), nw.median("s")])
+def test_median_expr_raises_on_str(
+    constructor: Constructor,
+    expr: nw.Expr,
+    request: pytest.FixtureRequest,
+) -> None:
+    if "polars" in str(constructor):
+        request.applymarker(pytest.mark.xfail)
+
+    df = nw.from_native(constructor(data))
+    with pytest.raises(
+        InvalidOperationError,
+        match="`median` operation not supported for non-numeric input type.",
+    ):
+        df.select(expr)
+
+
+@pytest.mark.parametrize(("col"), [("s")])
+def test_median_series_raises_on_str(
+    constructor_eager: ConstructorEager, col: str
+) -> None:
+    series = nw.from_native(constructor_eager(data), eager_only=True)[col]
+    with pytest.raises(
+        InvalidOperationError,
+        match="`median` operation not supported for non-numeric input type.",
+    ):
+        series.median()