ENH: EA._cast_pointwise_result

pandas/core/arrays/arrow/array.py

@@ -392,6 +392,73 @@ def _from_sequence_of_strings(
             )
         return cls._from_sequence(scalars, dtype=pa_type, copy=copy)
 
+    def _cast_pointwise_result(self, values) -> ArrayLike:
+        if len(values) == 0:
+            # Retain our dtype
+            return self[:0].copy()
+
+        try:
+            arr = pa.array(values, from_pandas=True)
+        except (ValueError, TypeError):
+            # e.g. test_by_column_values_with_same_starting_value with nested
+            #  values, one entry of which is an ArrowStringArray
+            #  or test_agg_lambda_complex128_dtype_conversion for complex values
+            return super()._cast_pointwise_result(values)
+
+        if pa.types.is_duration(arr.type):
+            # workaround for https://github.com/apache/arrow/issues/40620
+            result = ArrowExtensionArray._from_sequence(values)
+            if pa.types.is_duration(self._pa_array.type):
+                result = result.astype(self.dtype)  # type: ignore[assignment]
+            elif pa.types.is_timestamp(self._pa_array.type):
+                # Try to retain original unit
+                new_dtype = ArrowDtype(pa.duration(self._pa_array.type.unit))
+                try:
+                    result = result.astype(new_dtype)  # type: ignore[assignment]
+                except ValueError:
+                    pass
+            elif pa.types.is_date64(self._pa_array.type):
+                # Try to match unit we get on non-pointwise op
+                dtype = ArrowDtype(pa.duration("ms"))
+                result = result.astype(dtype)  # type: ignore[assignment]
+            elif pa.types.is_date(self._pa_array.type):
+                # Try to match unit we get on non-pointwise op
+                dtype = ArrowDtype(pa.duration("s"))
+                result = result.astype(dtype)  # type: ignore[assignment]
+            return result
+
+        elif pa.types.is_date(arr.type) and pa.types.is_date(self._pa_array.type):
+            arr = arr.cast(self._pa_array.type)
+        elif pa.types.is_time(arr.type) and pa.types.is_time(self._pa_array.type):
+            arr = arr.cast(self._pa_array.type)
+        elif pa.types.is_decimal(arr.type) and pa.types.is_decimal(self._pa_array.type):
+            arr = arr.cast(self._pa_array.type)
+        elif pa.types.is_integer(arr.type) and pa.types.is_integer(self._pa_array.type):
+            try:
+                arr = arr.cast(self._pa_array.type)
+            except pa.lib.ArrowInvalid:
+                # e.g. test_combine_add if we can't cast
+                pass
+        elif pa.types.is_floating(arr.type) and pa.types.is_floating(
+            self._pa_array.type
+        ):
+            try:
+                arr = arr.cast(self._pa_array.type)
+            except pa.lib.ArrowInvalid:
+                # e.g. test_combine_add if we can't cast
+                pass
+
+        if isinstance(self.dtype, StringDtype):
+            if pa.types.is_string(arr.type) or pa.types.is_large_string(arr.type):
+                # ArrowStringArrayNumpySemantics
+                return type(self)(arr).astype(self.dtype)
+            if self.dtype.na_value is np.nan:
+                # ArrowEA has different semantics, so we return numpy-based
+                #  result instead
+                return super()._cast_pointwise_result(values)
+            return ArrowExtensionArray(arr)
+        return type(self)(arr)
+
     @classmethod
     def _box_pa(
         cls, value, pa_type: pa.DataType | None = None

pandas/core/arrays/base.py

@@ -19,7 +19,6 @@
     cast,
     overload,
 )
-import warnings
 
 import numpy as np
 
@@ -35,13 +34,11 @@
     Substitution,
     cache_readonly,
 )
-from pandas.util._exceptions import find_stack_level
 from pandas.util._validators import (
     validate_bool_kwarg,
     validate_insert_loc,
 )
 
-from pandas.core.dtypes.cast import maybe_cast_pointwise_result
 from pandas.core.dtypes.common import (
     is_list_like,
     is_scalar,
@@ -89,7 +86,6 @@
         AstypeArg,
         AxisInt,
         Dtype,
-        DtypeObj,
         FillnaOptions,
         InterpolateOptions,
         NumpySorter,
@@ -311,38 +307,6 @@ def _from_sequence(
         """
         raise AbstractMethodError(cls)
 
-    @classmethod
-    def _from_scalars(cls, scalars, *, dtype: DtypeObj) -> Self:
-        """
-        Strict analogue to _from_sequence, allowing only sequences of scalars
-        that should be specifically inferred to the given dtype.
-
-        Parameters
-        ----------
-        scalars : sequence
-        dtype : ExtensionDtype
-
-        Raises
-        ------
-        TypeError or ValueError
-
-        Notes
-        -----
-        This is called in a try/except block when casting the result of a
-        pointwise operation.
-        """
-        try:
-            return cls._from_sequence(scalars, dtype=dtype, copy=False)
-        except (ValueError, TypeError):
-            raise
-        except Exception:
-            warnings.warn(
-                "_from_scalars should only raise ValueError or TypeError. "
-                "Consider overriding _from_scalars where appropriate.",
-                stacklevel=find_stack_level(),
-            )
-            raise
-
     @classmethod
     def _from_sequence_of_strings(
         cls, strings, *, dtype: ExtensionDtype, copy: bool = False
@@ -371,9 +335,6 @@ def _from_sequence_of_strings(
             from a sequence of scalars.
         api.extensions.ExtensionArray._from_factorized : Reconstruct an ExtensionArray
             after factorization.
-        api.extensions.ExtensionArray._from_scalars : Strict analogue to _from_sequence,
-            allowing only sequences of scalars that should be specifically inferred to
-            the given dtype.
 
         Examples
         --------
@@ -416,6 +377,14 @@ def _from_factorized(cls, values, original):
         """
         raise AbstractMethodError(cls)
 
+    def _cast_pointwise_result(self, values) -> ArrayLike:
+        """
+        Cast the result of a pointwise operation (e.g. Series.map) to an
+        array, preserve dtype_backend if possible.
+        """
+        values = np.asarray(values, dtype=object)
+        return lib.maybe_convert_objects(values, convert_non_numeric=True)
+
     # ------------------------------------------------------------------------
     # Must be a Sequence
     # ------------------------------------------------------------------------
@@ -2842,7 +2811,7 @@ def _maybe_convert(arr):
                     # https://github.com/pandas-dev/pandas/issues/22850
                     # We catch all regular exceptions here, and fall back
                     # to an ndarray.
-                    res = maybe_cast_pointwise_result(arr, self.dtype, same_dtype=False)
+                    res = self._cast_pointwise_result(arr)
                     if not isinstance(res, type(self)):
                         # exception raised in _from_sequence; ensure we have ndarray
                         res = np.asarray(arr)

pandas/core/arrays/categorical.py

@@ -103,7 +103,6 @@
         AstypeArg,
         AxisInt,
         Dtype,
-        DtypeObj,
         NpDtype,
         Ordered,
         Shape,
@@ -529,20 +528,12 @@ def _from_sequence(
     ) -> Self:
         return cls(scalars, dtype=dtype, copy=copy)
 
-    @classmethod
-    def _from_scalars(cls, scalars, *, dtype: DtypeObj) -> Self:
-        if dtype is None:
-            # The _from_scalars strictness doesn't make much sense in this case.
-            raise NotImplementedError
-
-        res = cls._from_sequence(scalars, dtype=dtype)
-
-        # if there are any non-category elements in scalars, these will be
-        #  converted to NAs in res.
-        mask = isna(scalars)
-        if not (mask == res.isna()).all():
-            # Some non-category element in scalars got converted to NA in res.
-            raise ValueError
+    def _cast_pointwise_result(self, values) -> ArrayLike:
+        res = super()._cast_pointwise_result(values)
+        cat = type(self)._from_sequence(res, dtype=self.dtype)
+        if (cat.isna() == isna(res)).all():
+            # i.e. the conversion was non-lossy
+            return cat
         return res
 
     @overload

pandas/core/arrays/datetimes.py

@@ -83,7 +83,6 @@
     from pandas._typing import (
         ArrayLike,
         DateTimeErrorChoices,
-        DtypeObj,
         IntervalClosedType,
         TimeAmbiguous,
         TimeNonexistent,
@@ -293,14 +292,6 @@ def _scalar_type(self) -> type[Timestamp]:
     _dtype: np.dtype[np.datetime64] | DatetimeTZDtype
     _freq: BaseOffset | None = None
 
-    @classmethod
-    def _from_scalars(cls, scalars, *, dtype: DtypeObj) -> Self:
-        if lib.infer_dtype(scalars, skipna=True) not in ["datetime", "datetime64"]:
-            # TODO: require any NAs be valid-for-DTA
-            # TODO: if dtype is passed, check for tzawareness compat?
-            raise ValueError
-        return cls._from_sequence(scalars, dtype=dtype)
-
     @classmethod
     def _validate_dtype(cls, values, dtype):
         # used in TimeLikeOps.__init__

pandas/core/arrays/masked.py

@@ -26,6 +26,7 @@
 from pandas.util._decorators import doc
 
 from pandas.core.dtypes.base import ExtensionDtype
+from pandas.core.dtypes.cast import maybe_downcast_to_dtype
 from pandas.core.dtypes.common import (
     is_bool,
     is_integer_dtype,
@@ -147,6 +148,19 @@ def _from_sequence(cls, scalars, *, dtype=None, copy: bool = False) -> Self:
         values, mask = cls._coerce_to_array(scalars, dtype=dtype, copy=copy)
         return cls(values, mask)
 
+    def _cast_pointwise_result(self, values) -> ArrayLike:
+        values = np.asarray(values, dtype=object)
+        result = lib.maybe_convert_objects(values, convert_to_nullable_dtype=True)
+        lkind = self.dtype.kind
+        rkind = result.dtype.kind
+        if (lkind in "iu" and rkind in "iu") or (lkind == rkind == "f"):
+            result = cast(BaseMaskedArray, result)
+            new_data = maybe_downcast_to_dtype(
+                result._data, dtype=self.dtype.numpy_dtype
+            )
+            result = type(result)(new_data, result._mask)
+        return result
+
     @classmethod
     @doc(ExtensionArray._empty)
     def _empty(cls, shape: Shape, dtype: ExtensionDtype) -> Self:

pandas/core/arrays/numpy_.py

@@ -14,7 +14,10 @@
 from pandas.compat.numpy import function as nv
 
 from pandas.core.dtypes.astype import astype_array
-from pandas.core.dtypes.cast import construct_1d_object_array_from_listlike
+from pandas.core.dtypes.cast import (
+    construct_1d_object_array_from_listlike,
+    maybe_downcast_to_dtype,
+)
 from pandas.core.dtypes.common import pandas_dtype
 from pandas.core.dtypes.dtypes import NumpyEADtype
 from pandas.core.dtypes.missing import isna
@@ -34,6 +37,7 @@
     from collections.abc import Callable
 
     from pandas._typing import (
+        ArrayLike,
         AxisInt,
         Dtype,
         FillnaOptions,
@@ -145,6 +149,24 @@ def _from_sequence(
             result = result.copy()
         return cls(result)
 
+    def _cast_pointwise_result(self, values) -> ArrayLike:
+        result = super()._cast_pointwise_result(values)
+        lkind = self.dtype.kind
+        rkind = result.dtype.kind
+        if (
+            (lkind in "iu" and rkind in "iu")
+            or (lkind == "f" and rkind == "f")
+            or (lkind == rkind == "c")
+        ):
+            result = maybe_downcast_to_dtype(result, self.dtype.numpy_dtype)
+        elif rkind == "M":
+            # Ensure potential subsequent .astype(object) doesn't incorrectly
+            #  convert Timestamps to ints
+            from pandas import array as pd_array
+
+            result = pd_array(result, copy=False)
+        return result
+
     # ------------------------------------------------------------------------
     # Data
 

pandas/core/arrays/sparse/array.py

@@ -607,6 +607,23 @@ def _from_sequence(
     def _from_factorized(cls, values, original) -> Self:
         return cls(values, dtype=original.dtype)
 
+    def _cast_pointwise_result(self, values):
+        result = super()._cast_pointwise_result(values)
+        if result.dtype.kind == self.dtype.kind:
+            try:
+                # e.g. test_groupby_agg_extension
+                res = type(self)._from_sequence(result, dtype=self.dtype)
+                if ((res == result) | (isna(result) & res.isna())).all():
+                    # This does not hold for e.g.
+                    #  test_arith_frame_with_scalar[0-__truediv__]
+                    return res
+                return type(self)._from_sequence(result)
+            except (ValueError, TypeError):
+                return type(self)._from_sequence(result)
+        else:
+            # e.g. test_combine_le avoid casting bools to Sparse[float64, nan]
+            return type(self)._from_sequence(result)
+
     # ------------------------------------------------------------------------
     # Data
     # ------------------------------------------------------------------------

pandas/core/arrays/string_.py

@@ -412,13 +412,6 @@ def tolist(self) -> list:
             return [x.tolist() for x in self]
         return list(self.to_numpy())
 
-    @classmethod
-    def _from_scalars(cls, scalars, dtype: DtypeObj) -> Self:
-        if lib.infer_dtype(scalars, skipna=True) not in ["string", "empty"]:
-            # TODO: require any NAs be valid-for-string
-            raise ValueError
-        return cls._from_sequence(scalars, dtype=dtype)
-
     def _formatter(self, boxed: bool = False):
         formatter = partial(
             printing.pprint_thing,
@@ -732,6 +725,13 @@ def _from_sequence_of_strings(
     ) -> Self:
         return cls._from_sequence(strings, dtype=dtype, copy=copy)
 
+    def _cast_pointwise_result(self, values) -> ArrayLike:
+        result = super()._cast_pointwise_result(values)
+        if isinstance(result.dtype, StringDtype):
+            # Ensure we retain our same na_value/storage
+            result = result.astype(self.dtype)  # type: ignore[call-overload]
+        return result
+
     @classmethod
     def _empty(cls, shape, dtype) -> StringArray:
         values = np.empty(shape, dtype=object)