REF: simplify factorize, fix factorize_array return type

pandas/core/algorithms.py

@@ -11,7 +11,6 @@
     Hashable,
     Literal,
     Sequence,
-    Union,
     cast,
     final,
 )
@@ -30,7 +29,6 @@
     ArrayLike,
     DtypeObj,
     IndexLabel,
-    Scalar,
     TakeIndexer,
     npt,
 )
@@ -105,9 +103,7 @@
     )
     from pandas.core.arrays import (
         BaseMaskedArray,
-        DatetimeArray,
         ExtensionArray,
-        TimedeltaArray,
     )
 
 
@@ -539,13 +535,24 @@ def factorize_array(
     codes : ndarray[np.intp]
     uniques : ndarray
     """
+    original = values
+    if values.dtype.kind in ["m", "M"]:
+        # _get_hashtable_algo will cast dt64/td64 to i8 via _ensure_data, so we
+        #  need to do the same to na_value. We are assuming here that the passed
+        #  na_value is an appropriately-typed NaT.
+        # e.g. test_where_datetimelike_categorical
+        na_value = iNaT
+
     hash_klass, values = _get_hashtable_algo(values)
 
     table = hash_klass(size_hint or len(values))
     uniques, codes = table.factorize(
         values, na_sentinel=na_sentinel, na_value=na_value, mask=mask
     )
 
+    # re-cast e.g. i8->dt64/td64, uint8->bool
+    uniques = _reconstruct_data(uniques, original.dtype, original)
+
     codes = ensure_platform_int(codes)
     return codes, uniques
 
@@ -720,33 +727,18 @@ def factorize(
         isinstance(values, (ABCDatetimeArray, ABCTimedeltaArray))
         and values.freq is not None
     ):
+        # The presence of 'freq' means we can fast-path sorting and know there
+        #  aren't NAs
         codes, uniques = values.factorize(sort=sort)
-        if isinstance(original, ABCIndex):
-            uniques = original._shallow_copy(uniques, name=None)
-        elif isinstance(original, ABCSeries):
-            from pandas import Index
-
-            uniques = Index(uniques)
-        return codes, uniques
+        return _re_wrap_factorize(original, uniques, codes)
 
     if not isinstance(values.dtype, np.dtype):
         # i.e. ExtensionDtype
         codes, uniques = values.factorize(na_sentinel=na_sentinel)
-        dtype = original.dtype
     else:
-        dtype = values.dtype
-        values = _ensure_data(values)
-        na_value: Scalar | None
-
-        if original.dtype.kind in ["m", "M"]:
-            # Note: factorize_array will cast NaT bc it has a __int__
-            #  method, but will not cast the more-correct dtype.type("nat")
-            na_value = iNaT
-        else:
-            na_value = None
-
+        values = np.asarray(values)  # convert DTA/TDA/MultiIndex
         codes, uniques = factorize_array(
-            values, na_sentinel=na_sentinel, size_hint=size_hint, na_value=na_value
+            values, na_sentinel=na_sentinel, size_hint=size_hint
         )
 
     if sort and len(uniques) > 0:
@@ -759,23 +751,20 @@ def factorize(
         # na_value is set based on the dtype of uniques, and compat set to False is
         # because we do not want na_value to be 0 for integers
         na_value = na_value_for_dtype(uniques.dtype, compat=False)
-        # Argument 2 to "append" has incompatible type "List[Union[str, float, Period,
-        # Timestamp, Timedelta, Any]]"; expected "Union[_SupportsArray[dtype[Any]],
-        # _NestedSequence[_SupportsArray[dtype[Any]]]
-        # , bool, int, float, complex, str, bytes, _NestedSequence[Union[bool, int,
-        # float, complex, str, bytes]]]"  [arg-type]
-        uniques = np.append(uniques, [na_value])  # type: ignore[arg-type]
+        uniques = np.append(uniques, [na_value])
         codes = np.where(code_is_na, len(uniques) - 1, codes)
 
-    uniques = _reconstruct_data(uniques, dtype, original)
+    uniques = _reconstruct_data(uniques, original.dtype, original)
+
+    return _re_wrap_factorize(original, uniques, codes)
+
 
-    # return original tenor
+def _re_wrap_factorize(original, uniques, codes: np.ndarray):
+    """
+    Wrap factorize results in Series or Index depending on original type.
+    """
     if isinstance(original, ABCIndex):
-        if original.dtype.kind in ["m", "M"] and isinstance(uniques, np.ndarray):
-            original._data = cast(
-                "Union[DatetimeArray, TimedeltaArray]", original._data
-            )
-            uniques = type(original._data)._simple_new(uniques, dtype=original.dtype)
+        uniques = ensure_wrapped_if_datetimelike(uniques)
         uniques = original._shallow_copy(uniques, name=None)
     elif isinstance(original, ABCSeries):
         from pandas import Index

pandas/core/arrays/_mixins.py

@@ -182,6 +182,10 @@ def equals(self, other) -> bool:
             return False
         return bool(array_equivalent(self._ndarray, other._ndarray))
 
+    def _from_factorized(cls, values, original):
+        assert values.dtype == original._ndarray.dtype
+        return original._from_backing_data(values)
+
     def _values_for_argsort(self) -> np.ndarray:
         return self._ndarray
 

pandas/core/arrays/categorical.py

@@ -2305,12 +2305,6 @@ def unique(self):
     def _values_for_factorize(self):
         return self._ndarray, -1
 
-    @classmethod
-    def _from_factorized(cls, uniques, original):
-        # ensure we have the same itemsize for codes
-        codes = coerce_indexer_dtype(uniques, original.dtype.categories)
-        return original._from_backing_data(codes)
-
     def _cast_quantile_result(self, res_values: np.ndarray) -> np.ndarray:
         # make sure we have correct itemsize for resulting codes
         res_values = coerce_indexer_dtype(res_values, self.dtype.categories)

pandas/core/arrays/datetimelike.py

@@ -550,14 +550,7 @@ def copy(self: DatetimeLikeArrayT, order="C") -> DatetimeLikeArrayT:
         return new_obj
 
     def _values_for_factorize(self):
-        # int64 instead of int ensures we have a "view" method
-        return self._ndarray, np.int64(iNaT)
-
-    @classmethod
-    def _from_factorized(
-        cls: type[DatetimeLikeArrayT], values, original: DatetimeLikeArrayT
-    ) -> DatetimeLikeArrayT:
-        return cls(values, dtype=original.dtype)
+        return self._ndarray, self._internal_fill_value
 
     # ------------------------------------------------------------------
     # Validation Methods

pandas/core/arrays/masked.py

@@ -874,8 +874,9 @@ def factorize(self, na_sentinel: int = -1) -> tuple[np.ndarray, ExtensionArray]:
 
         codes, uniques = factorize_array(arr, na_sentinel=na_sentinel, mask=mask)
 
-        # the hashtables don't handle all different types of bits
-        uniques = uniques.astype(self.dtype.numpy_dtype, copy=False)
+        # check that factorize_array correctly preserves dtype.
+        assert uniques.dtype == self.dtype.numpy_dtype, (uniques.dtype, self.dtype)
+
         uniques_ea = type(self)(uniques, np.zeros(len(uniques), dtype=bool))
         return codes, uniques_ea
 

pandas/core/arrays/numpy_.py

@@ -109,10 +109,6 @@ def _from_sequence(
             result = result.copy()
         return cls(result)
 
-    @classmethod
-    def _from_factorized(cls, values, original) -> PandasArray:
-        return original._from_backing_data(values)
-
     def _from_backing_data(self, arr: np.ndarray) -> PandasArray:
         return type(self)(arr)