feat: Disallow casting temporal to numeric

narwhals/_arrow/series.py

@@ -28,12 +28,14 @@
 from narwhals._typing_compat import assert_never
 from narwhals._utils import (
     Implementation,
+    Version,
     generate_temporary_column_name,
     is_list_of,
     no_default,
     not_implemented,
 )
 from narwhals.dependencies import is_numpy_array_1d
+from narwhals.dtypes import _validate_cast_temporal_to_numeric
 from narwhals.exceptions import InvalidOperationError, ShapeError
 
 if TYPE_CHECKING:
@@ -61,7 +63,7 @@
     )
     from narwhals._compliant.series import HistData
     from narwhals._typing import NoDefault
-    from narwhals._utils import Version, _LimitedContext
+    from narwhals._utils import _LimitedContext
     from narwhals.dtypes import DType
     from narwhals.typing import (
         ClosedInterval,
@@ -543,6 +545,7 @@ def is_nan(self) -> Self:
         return self._with_native(pc.is_nan(self.native), preserve_broadcast=True)
 
     def cast(self, dtype: IntoDType) -> Self:
+        _validate_cast_temporal_to_numeric(source=self.dtype, target=dtype)
         data_type = narwhals_to_native_dtype(dtype, self._version)
         return self._with_native(pc.cast(self.native, data_type), preserve_broadcast=True)
 

narwhals/_dask/expr.py

@@ -19,10 +19,12 @@
 from narwhals._pandas_like.utils import get_dtype_backend, native_to_narwhals_dtype
 from narwhals._utils import (
     Implementation,
+    Version,
     generate_temporary_column_name,
     no_default,
     not_implemented,
 )
+from narwhals.dtypes import _validate_cast_temporal_to_numeric
 from narwhals.exceptions import InvalidOperationError
 
 if TYPE_CHECKING:
@@ -40,7 +42,7 @@
     from narwhals._dask.dataframe import DaskLazyFrame
     from narwhals._dask.namespace import DaskNamespace
     from narwhals._typing import NoDefault
-    from narwhals._utils import Version, _LimitedContext
+    from narwhals._utils import _LimitedContext
     from narwhals.typing import (
         FillNullStrategy,
         IntoDType,
@@ -613,11 +615,21 @@ def func(df: DaskLazyFrame) -> Sequence[dx.Series]:
         )
 
     def cast(self, dtype: IntoDType) -> Self:
-        def func(expr: dx.Series) -> dx.Series:
+        def func(df: DaskLazyFrame) -> list[dx.Series]:
+            if dtype.is_numeric():
+                schema = df.schema
+                for name in self._evaluate_output_names(df):
+                    _validate_cast_temporal_to_numeric(source=schema[name], target=dtype)
+
             native_dtype = narwhals_to_native_dtype(dtype, self._version)
-            return expr.astype(native_dtype)
+            return [expr.astype(native_dtype) for expr in self._call(df)]
 
-        return self._with_callable(func)
+        return self.__class__(
+            func,
+            evaluate_output_names=self._evaluate_output_names,
+            alias_output_names=self._alias_output_names,
+            version=self._version,
+        )
 
     def is_finite(self) -> Self:
         import dask.array as da

narwhals/_duckdb/expr.py

@@ -22,6 +22,7 @@
 )
 from narwhals._sql.expr import SQLExpr
 from narwhals._utils import Implementation, Version, extend_bool, no_default
+from narwhals.dtypes import _validate_cast_temporal_to_numeric
 
 if TYPE_CHECKING:
     from collections.abc import Sequence
@@ -38,6 +39,7 @@
     )
     from narwhals._duckdb.dataframe import DuckDBLazyFrame
     from narwhals._duckdb.namespace import DuckDBNamespace
+    from narwhals._duckdb.utils import duckdb_dtypes
     from narwhals._typing import NoDefault
     from narwhals._utils import _LimitedContext
     from narwhals.typing import FillNullStrategy, IntoDType, RollingInterpolationMethod
@@ -266,14 +268,23 @@ def _fill_constant(expr: Expression, value: Any) -> Expression:
         return self._with_elementwise(_fill_constant, value=value)
 
     def cast(self, dtype: IntoDType) -> Self:
-        def func(df: DuckDBLazyFrame) -> list[Expression]:
+        def _validated_dtype(
+            dtype: IntoDType, df: DuckDBLazyFrame
+        ) -> duckdb_dtypes.DuckDBPyType:
+            if dtype.is_numeric():
+                schema = df.collect_schema()
+                for name in self._evaluate_output_names(df):
+                    _validate_cast_temporal_to_numeric(source=schema[name], target=dtype)
+
             tz = DeferredTimeZone(df.native)
-            native_dtype = narwhals_to_native_dtype(dtype, self._version, tz)
+            return narwhals_to_native_dtype(dtype, self._version, tz)
+
+        def func(df: DuckDBLazyFrame) -> list[Expression]:
+            native_dtype = _validated_dtype(dtype, df)
             return [expr.cast(native_dtype) for expr in self(df)]
 
         def window_f(df: DuckDBLazyFrame, inputs: DuckDBWindowInputs) -> list[Expression]:
-            tz = DeferredTimeZone(df.native)
-            native_dtype = narwhals_to_native_dtype(dtype, self._version, tz)
+            native_dtype = _validated_dtype(dtype, df)
             return [expr.cast(native_dtype) for expr in self.window_function(df, inputs)]
 
         return self.__class__(

narwhals/_ibis/expr.py

@@ -28,6 +28,7 @@
     not_implemented,
     zip_strict,
 )
+from narwhals.dtypes import _validate_cast_temporal_to_numeric
 
 if TYPE_CHECKING:
     from collections.abc import Iterator, Sequence
@@ -269,12 +270,21 @@ def _fill_null(expr: ir.Value, value: ir.Scalar) -> ir.Value:
         return self._with_callable(_fill_null, value=value)
 
     def cast(self, dtype: IntoDType) -> Self:
-        def _func(expr: ir.Column) -> ir.Value:
+        def func(df: IbisLazyFrame) -> list[ir.Value]:
+            if dtype.is_numeric():
+                schema = df.collect_schema()
+                for name in self._evaluate_output_names(df):
+                    _validate_cast_temporal_to_numeric(source=schema[name], target=dtype)
+
             native_dtype = narwhals_to_native_dtype(dtype, self._version)
-            # ibis `cast` overloads do not include DataType, only literals
-            return expr.cast(native_dtype)  # type: ignore[unused-ignore]
+            return [expr.cast(native_dtype) for expr in self(df)]  # pyright: ignore[reportArgumentType, reportCallIssue]
 
-        return self._with_callable(_func)
+        return self.__class__(
+            func,
+            evaluate_output_names=self._evaluate_output_names,
+            alias_output_names=self._alias_output_names,
+            version=self._version,
+        )
 
     def is_unique(self) -> Self:
         return self._with_callable(

narwhals/_pandas_like/series.py

@@ -24,8 +24,9 @@
     set_index,
 )
 from narwhals._typing_compat import assert_never
-from narwhals._utils import Implementation, is_list_of, no_default
+from narwhals._utils import Implementation, Version, is_list_of, no_default
 from narwhals.dependencies import is_numpy_array_1d, is_pandas_like_series
+from narwhals.dtypes import _validate_cast_temporal_to_numeric
 from narwhals.exceptions import InvalidOperationError
 
 if TYPE_CHECKING:
@@ -43,7 +44,7 @@
     from narwhals._pandas_like.namespace import PandasLikeNamespace
     from narwhals._pandas_like.typing import NativeSeriesT
     from narwhals._typing import NoDefault
-    from narwhals._utils import Version, _LimitedContext
+    from narwhals._utils import _LimitedContext
     from narwhals.dtypes import DType
     from narwhals.typing import (
         ClosedInterval,
@@ -311,6 +312,7 @@ def scatter(
         return None if in_place else self._with_native(series)
 
     def cast(self, dtype: IntoDType) -> Self:
+        _validate_cast_temporal_to_numeric(source=self.dtype, target=dtype)
         if self.dtype == dtype and self.native.dtype != "object":
             # Avoid dealing with pandas' type-system if we can. Note that it's only
             # safe to do this if we're not starting with object dtype, see tests/expr_and_series/cast_test.py::test_cast_object_pandas

narwhals/_polars/series.py

@@ -20,8 +20,9 @@
     narwhals_to_native_dtype,
     native_to_narwhals_dtype,
 )
-from narwhals._utils import Implementation, no_default, requires
+from narwhals._utils import Implementation, Version, no_default, requires
 from narwhals.dependencies import is_numpy_array_1d, is_pandas_index
+from narwhals.dtypes import _validate_cast_temporal_to_numeric
 
 if TYPE_CHECKING:
     from collections.abc import Iterable, Iterator, Mapping, Sequence
@@ -35,7 +36,7 @@
     from narwhals._polars.dataframe import Method, PolarsDataFrame
     from narwhals._polars.namespace import PolarsNamespace
     from narwhals._typing import NoDefault
-    from narwhals._utils import Version, _LimitedContext
+    from narwhals._utils import _LimitedContext
     from narwhals.dtypes import DType
     from narwhals.series import Series
     from narwhals.typing import (
@@ -288,7 +289,8 @@ def __getitem__(self, item: MultiIndexSelector[Self]) -> Any | Self:
         return self._from_native_object(self.native.__getitem__(item))
 
     def cast(self, dtype: IntoDType) -> Self:
-        dtype_pl = narwhals_to_native_dtype(dtype, self._version)
+        _validate_cast_temporal_to_numeric(source=self.dtype, target=dtype)
+        dtype_pl = narwhals_to_native_dtype(dtype, version=self._version)
         return self._with_native(self.native.cast(dtype_pl))
 
     def clip(self, lower_bound: PolarsSeries, upper_bound: PolarsSeries) -> Self:

narwhals/_spark_like/expr.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import operator
+from contextlib import suppress
 from typing import TYPE_CHECKING, Any, Callable, ClassVar, Literal, cast
 
 from narwhals._spark_like.expr_dt import SparkLikeExprDateTimeNamespace
@@ -23,6 +24,7 @@
     not_implemented,
     zip_strict,
 )
+from narwhals.dtypes import DType, _validate_cast_temporal_to_numeric
 
 if TYPE_CHECKING:
     from collections.abc import Iterator, Mapping, Sequence
@@ -40,6 +42,7 @@
     )
     from narwhals._spark_like.dataframe import SparkLikeLazyFrame
     from narwhals._spark_like.namespace import SparkLikeNamespace
+    from narwhals._spark_like.utils import _NativeDType
     from narwhals._typing import NoDefault
     from narwhals._utils import _LimitedContext
     from narwhals.typing import FillNullStrategy, IntoDType, RankMethod
@@ -246,19 +249,31 @@ def __invert__(self) -> Self:
         return self._with_elementwise(invert)
 
     def cast(self, dtype: IntoDType) -> Self:
-        def func(df: SparkLikeLazyFrame) -> Sequence[Column]:
-            spark_dtype = narwhals_to_native_dtype(
+        def _validated_dtype(dtype: IntoDType, df: SparkLikeLazyFrame) -> _NativeDType:
+            if dtype.is_numeric():
+                schema: dict[str, DType] = {}
+                with suppress(Exception):
+                    schema = df.collect_schema()
+
+                if schema:
+                    for name in self._evaluate_output_names(df):
+                        _validate_cast_temporal_to_numeric(
+                            source=schema[name], target=dtype
+                        )
+
+            return narwhals_to_native_dtype(
                 dtype, self._version, self._native_dtypes, df.native.sparkSession
             )
-            return [expr.cast(spark_dtype) for expr in self(df)]
+
+        def func(df: SparkLikeLazyFrame) -> Sequence[Column]:
+            native_dtype = _validated_dtype(dtype, df)
+            return [expr.cast(native_dtype) for expr in self(df)]
 
         def window_f(
             df: SparkLikeLazyFrame, inputs: SparkWindowInputs
         ) -> Sequence[Column]:
-            spark_dtype = narwhals_to_native_dtype(
-                dtype, self._version, self._native_dtypes, df.native.sparkSession
-            )
-            return [expr.cast(spark_dtype) for expr in self.window_function(df, inputs)]
+            native_dtype = _validated_dtype(dtype, df)
+            return [expr.cast(native_dtype) for expr in self.window_function(df, inputs)]
 
         return self.__class__(
             func,

narwhals/dtypes.py

@@ -59,6 +59,29 @@ def _validate_into_dtype(dtype: Any) -> None:
         raise TypeError(msg)
 
 
+def _validate_cast_temporal_to_numeric(
+    source: DType | type[DType], target: IntoDType
+) -> None:
+    """Validate that we're not casting from temporal to numeric types.
+
+    Arguments:
+        source: The source data type.
+        target: The target data type to cast to.
+
+    Raises:
+        InvalidOperationError: If attempting to cast from temporal to integer.
+    """
+    if source.is_temporal() and target.is_numeric():
+        msg = (
+            "Casting from temporal type to numeric is not supported.\n\n"
+            "Hint: Use `.dt` accessor methods instead, such as:\n"
+            "  - `.dt.timestamp()` for Unix timestamp.\n"
+            "  - `.dt.year()`, `.dt.month()`, `.dt.day()`, ..., for date components.\n"
+            "  - `.dt.total_seconds()`, `.dt.total_milliseconds(), ..., for duration total time."
+        )
+        raise InvalidOperationError(msg)
+
+
 class DTypeClass(type):
     """Metaclass for DType classes.
 

narwhals/expr.py

@@ -171,6 +171,15 @@ def cast(self, dtype: IntoDType) -> Self:
         Arguments:
             dtype: Data type that the object will be cast into.
 
+        Note:
+            Unlike polars, we don't allow to cast from a temporal to a numeric data type.
+
+            Use `.dt` accessor methods instead, such as:
+
+            * `.dt.timestamp()` for Unix timestamp.
+            * `.dt.year()`, `.dt.month()`, `.dt.day()`, ..., for date components.
+            * `.dt.total_seconds()`, `.dt.total_milliseconds(), ..., for duration total time.
+
         Examples:
             >>> import pandas as pd
             >>> import narwhals as nw

narwhals/series.py

@@ -628,6 +628,15 @@ def cast(self, dtype: IntoDType) -> Self:
         Arguments:
             dtype: Data type that the object will be cast into.
 
+        Note:
+            Unlike polars, we don't allow to cast from a temporal to a numeric data type.
+
+            Use `.dt` accessor methods instead, such as:
+
+            * `.dt.timestamp()` for Unix timestamp.
+            * `.dt.year()`, `.dt.month()`, `.dt.day()`, ..., for date components.
+            * `.dt.total_seconds()`, `.dt.total_milliseconds(), ..., for duration total time.
+
         Examples:
             >>> import pyarrow as pa
             >>> import narwhals as nw