EA: add back _from_scalar / cast_pointwise_result backwards compat (#63367)

Author: jorisvandenbossche

pandas/core/arrays/_mixins.py

@@ -208,6 +208,10 @@ def _hash_pandas_object(
             values, encoding=encoding, hash_key=hash_key, categorize=categorize
         )
 
+    def _cast_pointwise_result(self, values: ArrayLike) -> ArrayLike:
+        values = np.asarray(values, dtype=object)
+        return lib.maybe_convert_objects(values, convert_non_numeric=True)
+
     # Signature of "argmin" incompatible with supertype "ExtensionArray"
     def argmin(self, axis: AxisInt = 0, skipna: bool = True):  # type: ignore[override]
         # override base class by adding axis keyword

pandas/core/arrays/arrow/array.py

@@ -442,7 +442,8 @@ def _cast_pointwise_result(self, values) -> ArrayLike:
             # 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)
+            values = np.asarray(values, dtype=object)
+            return lib.maybe_convert_objects(values, convert_non_numeric=True)
 
         if pa.types.is_null(arr.type):
             if lib.infer_dtype(values) == "decimal":
@@ -498,7 +499,8 @@ def _cast_pointwise_result(self, values) -> ArrayLike:
             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)
+                values = np.asarray(values, dtype=object)
+                return lib.maybe_convert_objects(values, convert_non_numeric=True)
             return ArrowExtensionArray(arr)
         return self._from_pyarrow_array(arr)
 

pandas/core/arrays/base.py

@@ -19,6 +19,7 @@
     cast,
     overload,
 )
+import warnings
 
 import numpy as np
 
@@ -33,6 +34,7 @@
     cache_readonly,
     set_module,
 )
+from pandas.util._exceptions import find_stack_level
 from pandas.util._validators import (
     validate_bool_kwarg,
     validate_insert_loc,
@@ -86,6 +88,7 @@
         AstypeArg,
         AxisInt,
         Dtype,
+        DtypeObj,
         FillnaOptions,
         InterpolateOptions,
         NumpySorter,
@@ -383,13 +386,67 @@ def _from_factorized(cls, values, original):
         """
         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 in ExtensionArray._cast_pointwise_result.
+        """
+        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
+
     def _cast_pointwise_result(self, values) -> ArrayLike:
         """
+        Construct an ExtensionArray after a pointwise operation.
+
         Cast the result of a pointwise operation (e.g. Series.map) to an
-        array, preserve dtype_backend if possible.
+        array. This is not required to return an ExtensionArray of the same
+        type as self or of the same dtype. It can also return another
+        ExtensionArray of the same "family" if you implement multiple
+        ExtensionArrays/Dtypes that are interoperable (e.g. if you have float
+        array with units, this method can return an int array with units).
+
+        If converting to your own ExtensionArray is not possible, this method
+        falls back to returning an array with the default type inference.
+        If you only need to cast to `self.dtype`, it is recommended to override
+        `_from_scalars` instead of this method.
+
+        Parameters
+        ----------
+        values : sequence
+
+        Returns
+        -------
+        ExtensionArray or ndarray
         """
-        values = np.asarray(values, dtype=object)
-        return lib.maybe_convert_objects(values, convert_non_numeric=True)
+        try:
+            return type(self)._from_scalars(values, dtype=self.dtype)
+        except (ValueError, TypeError):
+            values = np.asarray(values, dtype=object)
+            return lib.maybe_convert_objects(values, convert_non_numeric=True)
 
     # ------------------------------------------------------------------------
     # Must be a Sequence

pandas/core/arrays/sparse/array.py

@@ -622,7 +622,8 @@ 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)
+        values = np.asarray(values, dtype=object)
+        result = lib.maybe_convert_objects(values, convert_non_numeric=True)
         if result.dtype.kind == self.dtype.kind:
             try:
                 # e.g. test_groupby_agg_extension

pandas/tests/extension/decimal/array.py

@@ -111,15 +111,15 @@ def _from_sequence_of_strings(cls, strings, *, dtype: ExtensionDtype, copy=False
     def _from_factorized(cls, values, original):
         return cls(values)
 
-    def _cast_pointwise_result(self, values):
-        result = super()._cast_pointwise_result(values)
-        try:
-            # If this were ever made a non-test EA, special-casing could
-            #  be avoided by handling Decimal in maybe_convert_objects
-            res = type(self)._from_sequence(result, dtype=self.dtype)
-        except (ValueError, TypeError):
-            return result
-        return res
+    # test to ensure that the base class _cast_pointwise_result works as expected
+    # def _cast_pointwise_result(self, values):
+    #     try:
+    #         # If this were ever made a non-test EA, special-casing could
+    #         #  be avoided by handling Decimal in maybe_convert_objects
+    #         res = type(self)._from_sequence(values, dtype=self.dtype)
+    #     except (ValueError, TypeError):
+    #         return values
+    #     return res
 
     _HANDLED_TYPES = (decimal.Decimal, numbers.Number, np.ndarray)
 

pandas/tests/extension/json/array.py

@@ -94,11 +94,14 @@ def _from_factorized(cls, values, original):
         return cls([UserDict(x) for x in values if x != ()])
 
     def _cast_pointwise_result(self, values):
-        result = super()._cast_pointwise_result(values)
         try:
-            return type(self)._from_sequence(result, dtype=self.dtype)
+            return type(self)._from_sequence(values, dtype=self.dtype)
         except (ValueError, TypeError):
-            return result
+            # TODO replace with public function
+            from pandas._libs import lib
+
+            values = np.asarray(values, dtype=object)
+            return lib.maybe_convert_objects(values, convert_non_numeric=True)
 
     def __getitem__(self, item):
         if isinstance(item, tuple):