check for polars specifically

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:
@@ -76,7 +77,9 @@ def native_to_narwhals_dtype(dtype: Any, dtypes: DTypes) -> DType:
 
 
 def narwhals_to_native_dtype(dtype: DType | type[DType], 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/_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
@@ -85,7 +86,9 @@ 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 "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/_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
 
 
@@ -94,7 +95,9 @@ def native_to_narwhals_dtype(dtype: Any, dtypes: DTypes) -> DType:
 
 
 def narwhals_to_native_dtype(dtype: DType | type[DType], 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"
@@ -141,7 +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

@@ -186,19 +186,6 @@ def cast(self: Self, dtype: DType | type[DType]) -> Self:
             foo: [[1,2,3]]
             bar: [[6,7,8]]
         """
-        # from narwhals.dtypes import DType
-
-        # if not (
-        #     isinstance(dtype, DType)
-        #     or (isinstance(dtype, type) and issubclass(dtype, DType))
-        # ):
-        #     msg = (
-        #         f"Expected Narwhals DType, got: {type(dtype)}.\n\n"
-        #         "Hint: Perhaps you used Polars DataType instance `pl.dtype` instead of "
-        #         "Narwhals DType `nw.dtype`?"
-        #     )
-        #     raise TypeError(msg)
-
         return self.__class__(
             lambda plx: self._call(plx).cast(dtype),
         )

narwhals/series.py

@@ -420,16 +420,6 @@ def cast(self: Self, dtype: DType | type[DType]) -> Self:
                1
             ]
         """
-        # from narwhals.dtypes import DType
-
-        # if not (isinstance(dtype, DType) or dtype == DType()):
-        #     msg = (
-        #         f"Expected Narwhals DType, got: {type(dtype)}.\n\n"
-        #         "Hint: Perhaps you used Polars DataType instance `pl.dtype` instead of "
-        #         "Narwhals DType `nw.dtype`?"
-        #     )
-        #     raise TypeError(msg)
-
         return self._from_compliant_series(self._compliant_series.cast(dtype))
 
     def to_frame(self) -> DataFrame[Any]:

tests/expr_and_series/cast_test.py

@@ -7,7 +7,6 @@
 
 import pandas as pd
 import polars as pl
-import pyarrow as pa
 import pytest
 
 import narwhals.stable.v1 as nw
@@ -194,7 +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(
@@ -227,8 +226,8 @@ def test_cast_datetime_tz_aware(
     assert_equal_data(result, expected)
 
 
-@pytest.mark.parametrize("dtype", [pl.String, pl.String(), pa.float64(), str])
-def test_raise_if_not_narwhals_dtype(constructor: Constructor, dtype: Any) -> None:
+@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))