wip

narwhals/dtypes.py

@@ -24,6 +24,18 @@ def _validate_dtype(dtype: DType | type[DType]) -> None:
         raise TypeError(msg)
 
 
+def _validate_datetime(
+    dtype: DType | type[DType], series_dtype: DType | type[DType]
+) -> None:
+    from narwhals.dtypes import Datetime
+
+    if isinstance_or_issubclass(series_dtype, Datetime) and not (
+        isinstance_or_issubclass(dtype, Datetime)
+    ):
+        msg = f"Expected to cast to Narwhals Datetime, got: {dtype}.\n\n"
+        raise TypeError(msg)
+
+
 class DType:
     def __repr__(self) -> str:  # pragma: no cover
         return self.__class__.__qualname__

narwhals/series.py

@@ -12,6 +12,7 @@
 from typing import overload
 
 from narwhals.dependencies import is_numpy_scalar
+from narwhals.dtypes import _validate_datetime
 from narwhals.dtypes import _validate_dtype
 from narwhals.typing import IntoSeriesT
 from narwhals.utils import _validate_rolling_arguments
@@ -630,6 +631,7 @@ def cast(self: Self, dtype: DType | type[DType]) -> Self:
             ]
         """
         _validate_dtype(dtype)
+        _validate_datetime(dtype, self.dtype)
         return self._from_compliant_series(self._compliant_series.cast(dtype))
 
     def to_frame(self) -> DataFrame[Any]:

tests/expr_and_series/cast_test.py

@@ -7,6 +7,7 @@
 
 import pandas as pd
 import polars as pl
+import pyarrow as pa
 import pytest
 
 import narwhals.stable.v1 as nw
@@ -226,3 +227,18 @@ def test_raise_if_polars_dtype(constructor: Constructor, dtype: Any) -> None:
     df = nw.from_native(constructor({"a": [1, 2, 3], "b": [4, 5, 6]}))
     with pytest.raises(TypeError, match="Expected Narwhals dtype, got:"):
         df.select(nw.col("a").cast(dtype))
+
+
+def test_raise_datetime_to_numeric() -> None:
+    data = [datetime(year=2020, month=1, day=1, hour=0, second=0, minute=0)]
+    s_pd = nw.from_native(pd.Series(data), series_only=True)
+    with pytest.raises(TypeError, match="Expected to cast to Narwhals Datetime, got: "):
+        s_pd.cast(nw.Int64)
+
+    s_pa = nw.from_native(pa.chunked_array([data]), series_only=True)
+    with pytest.raises(TypeError, match="Expected to cast to Narwhals Datetime, got: "):
+        s_pa.cast(nw.Int64)
+
+    s_pl = nw.from_native(pl.Series(data), series_only=True)
+    with pytest.raises(TypeError, match="Expected to cast to Narwhals Datetime, got: "):
+        s_pl.cast(nw.Int64)