fix: narwhals_to_native_dtype raise if polars dtype is passed

narwhals/_arrow/utils.py

@@ -4,6 +4,7 @@
 from typing import Any
 from typing import Sequence
 
+from narwhals.dependencies import get_polars
 from narwhals.utils import isinstance_or_issubclass
 
 if TYPE_CHECKING:
@@ -76,6 +77,17 @@ def native_to_narwhals_dtype(dtype: Any, dtypes: DTypes) -> DType:
 
 
 def narwhals_to_native_dtype(dtype: DType | type[DType], dtypes: DTypes) -> Any:
+    if (pl := get_polars()) is not None and isinstance(
+        dtype, (pl.DataType, pl.DataType.__class__)
+    ):
+        msg = (
+            f"Expected Narwhals object, got: {type(dtype)}.\n\n"
+            "Perhaps you:\n"
+            "- Forgot a `nw.from_native` somewhere?\n"
+            "- Used `pl.Int64` instead of `nw.Int64`?"
+        )
+        raise TypeError(msg)
+
     import pyarrow as pa  # ignore-banned-import
 
     if isinstance_or_issubclass(dtype, dtypes.Float64):

narwhals/_dask/utils.py

@@ -4,6 +4,7 @@
 from typing import Any
 
 from narwhals.dependencies import get_pandas
+from narwhals.dependencies import get_polars
 from narwhals.dependencies import get_pyarrow
 from narwhals.utils import isinstance_or_issubclass
 from narwhals.utils import parse_version
@@ -85,6 +86,17 @@ def validate_comparand(lhs: dask_expr.Series, rhs: dask_expr.Series) -> None:
 
 
 def narwhals_to_native_dtype(dtype: DType | type[DType], dtypes: DTypes) -> Any:
+    if (pl := get_polars()) is not None and isinstance(
+        dtype, (pl.DataType, pl.DataType.__class__)
+    ):
+        msg = (
+            f"Expected Narwhals object, got: {type(dtype)}.\n\n"
+            "Perhaps you:\n"
+            "- Forgot a `nw.from_native` somewhere?\n"
+            "- Used `pl.Int64` instead of `nw.Int64`?"
+        )
+        raise TypeError(msg)
+
     if isinstance_or_issubclass(dtype, dtypes.Float64):
         return "float64"
     if isinstance_or_issubclass(dtype, dtypes.Float32):

narwhals/_pandas_like/utils.py

@@ -10,6 +10,7 @@
 from narwhals._arrow.utils import (
     native_to_narwhals_dtype as arrow_native_to_narwhals_dtype,
 )
+from narwhals.dependencies import get_polars
 from narwhals.utils import Implementation
 from narwhals.utils import isinstance_or_issubclass
 
@@ -339,7 +340,9 @@ def narwhals_to_native_dtype(  # noqa: PLR0915
     backend_version: tuple[int, ...],
     dtypes: DTypes,
 ) -> Any:
-    if "polars" in str(type(dtype)):
+    if (pl := get_polars()) is not None and isinstance(
+        dtype, (pl.DataType, pl.DataType.__class__)
+    ):
         msg = (
             f"Expected Narwhals object, got: {type(dtype)}.\n\n"
             "Perhaps you:\n"

narwhals/_polars/utils.py

@@ -8,6 +8,7 @@
     from narwhals.dtypes import DType
     from narwhals.typing import DTypes
 
+from narwhals.dependencies import get_polars
 from narwhals.utils import parse_version
 
 
@@ -94,6 +95,17 @@ def native_to_narwhals_dtype(dtype: Any, dtypes: DTypes) -> DType:
 
 
 def narwhals_to_native_dtype(dtype: DType | type[DType], dtypes: DTypes) -> Any:
+    if (pl := get_polars()) is not None and isinstance(
+        dtype, (pl.DataType, pl.DataType.__class__)
+    ):
+        msg = (
+            f"Expected Narwhals object, got: {type(dtype)}.\n\n"
+            "Perhaps you:\n"
+            "- Forgot a `nw.from_native` somewhere?\n"
+            "- Used `pl.Int64` instead of `nw.Int64`?"
+        )
+        raise TypeError(msg)
+
     import polars as pl  # ignore-banned-import()
 
     if dtype == dtypes.Float64:
@@ -132,7 +144,7 @@ def narwhals_to_native_dtype(dtype: DType | type[DType], dtypes: DTypes) -> Any:
     if dtype == dtypes.Datetime or isinstance(dtype, dtypes.Datetime):
         dt_time_unit = getattr(dtype, "time_unit", "us")
         dt_time_zone = getattr(dtype, "time_zone", None)
-        return pl.Datetime(dt_time_unit, dt_time_zone)  # type: ignore[arg-type]
+        return pl.Datetime(dt_time_unit, dt_time_zone)
     if dtype == dtypes.Duration or isinstance(dtype, dtypes.Duration):
         du_time_unit: Literal["us", "ns", "ms"] = getattr(dtype, "time_unit", "us")
         return pl.Duration(time_unit=du_time_unit)

narwhals/expr.py

@@ -135,10 +135,7 @@ def pipe(self, function: Callable[[Any], Self], *args: Any, **kwargs: Any) -> Se
         """
         return function(self, *args, **kwargs)
 
-    def cast(
-        self,
-        dtype: Any,
-    ) -> Self:
+    def cast(self: Self, dtype: DType | type[DType]) -> Self:
         """
         Redefine an object's data type.
 

narwhals/series.py

@@ -383,10 +383,7 @@ def name(self) -> str:
         """
         return self._compliant_series.name  # type: ignore[no-any-return]
 
-    def cast(
-        self,
-        dtype: Any,
-    ) -> Self:
+    def cast(self: Self, dtype: DType | type[DType]) -> Self:
         """
         Cast between data types.
 

tests/expr_and_series/cast_test.py

@@ -3,8 +3,10 @@
 from datetime import datetime
 from datetime import timedelta
 from datetime import timezone
+from typing import Any
 
 import pandas as pd
+import polars as pl
 import pytest
 
 import narwhals.stable.v1 as nw
@@ -191,7 +193,7 @@ class Banana:
         pass
 
     with pytest.raises(AssertionError, match=r"Unknown dtype"):
-        df.select(nw.col("a").cast(Banana))
+        df.select(nw.col("a").cast(Banana))  # type: ignore[arg-type]
 
 
 def test_cast_datetime_tz_aware(
@@ -222,3 +224,10 @@ def test_cast_datetime_tz_aware(
         .str.slice(offset=0, length=19)
     )
     assert_equal_data(result, expected)
+
+
+@pytest.mark.parametrize("dtype", [pl.String, pl.String()])
+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 object, got:"):
+        df.select(nw.col("a").cast(dtype))

tests/frame/invalid_test.py

@@ -20,7 +20,7 @@ def test_invalid() -> None:
     with pytest.raises(TypeError, match="Perhaps you:"):
         df.select([pl.col("a")])  # type: ignore[list-item]
     with pytest.raises(TypeError, match="Perhaps you:"):
-        df.select([nw.col("a").cast(pl.Int64)])
+        df.select([nw.col("a").cast(pl.Int64)])  # type: ignore[arg-type]
 
 
 def test_native_vs_non_native() -> None: