DEPR: Categorical fastpath (#58157)

Author: jbrockmendel

doc/source/whatsnew/v3.0.0.rst

@@ -212,6 +212,7 @@ Removal of prior version deprecations/changes
 - Disallow passing a pandas type to :meth:`Index.view` (:issue:`55709`)
 - Disallow units other than "s", "ms", "us", "ns" for datetime64 and timedelta64 dtypes in :func:`array` (:issue:`53817`)
 - Removed "freq" keyword from :class:`PeriodArray` constructor, use "dtype" instead (:issue:`52462`)
+- Removed 'fastpath' keyword in :class:`Categorical` constructor (:issue:`20110`)
 - Removed alias :class:`arrays.PandasArray` for :class:`arrays.NumpyExtensionArray` (:issue:`53694`)
 - Removed deprecated "method" and "limit" keywords from :meth:`Series.replace` and :meth:`DataFrame.replace` (:issue:`53492`)
 - Removed extension test classes ``BaseNoReduceTests``, ``BaseNumericReduceTests``, ``BaseBooleanReduceTests`` (:issue:`54663`)

pandas/core/arrays/categorical.py

@@ -276,9 +276,6 @@ class Categorical(NDArrayBackedExtensionArray, PandasObject, ObjectStringArrayMi
         provided).
     dtype : CategoricalDtype
         An instance of ``CategoricalDtype`` to use for this categorical.
-    fastpath : bool
-        The 'fastpath' keyword in Categorical is deprecated and will be
-        removed in a future version. Use Categorical.from_codes instead.
     copy : bool, default True
         Whether to copy if the codes are unchanged.
 
@@ -391,33 +388,15 @@ def __init__(
         categories=None,
         ordered=None,
         dtype: Dtype | None = None,
-        fastpath: bool | lib.NoDefault = lib.no_default,
         copy: bool = True,
     ) -> None:
-        if fastpath is not lib.no_default:
-            # GH#20110
-            warnings.warn(
-                "The 'fastpath' keyword in Categorical is deprecated and will "
-                "be removed in a future version. Use Categorical.from_codes instead",
-                DeprecationWarning,
-                stacklevel=find_stack_level(),
-            )
-        else:
-            fastpath = False
-
         dtype = CategoricalDtype._from_values_or_dtype(
             values, categories, ordered, dtype
         )
         # At this point, dtype is always a CategoricalDtype, but
         # we may have dtype.categories be None, and we need to
         # infer categories in a factorization step further below
 
-        if fastpath:
-            codes = coerce_indexer_dtype(values, dtype.categories)
-            dtype = CategoricalDtype(ordered=False).update_dtype(dtype)
-            super().__init__(codes, dtype)
-            return
-
         if not is_list_like(values):
             # GH#38433
             raise TypeError("Categorical input must be list-like")

pandas/tests/arrays/categorical/test_constructors.py

@@ -35,13 +35,6 @@
 
 
 class TestCategoricalConstructors:
-    def test_fastpath_deprecated(self):
-        codes = np.array([1, 2, 3])
-        dtype = CategoricalDtype(categories=["a", "b", "c", "d"], ordered=False)
-        msg = "The 'fastpath' keyword in Categorical is deprecated"
-        with tm.assert_produces_warning(DeprecationWarning, match=msg):
-            Categorical(codes, dtype=dtype, fastpath=True)
-
     def test_categorical_from_cat_and_dtype_str_preserve_ordered(self):
         # GH#49309 we should preserve orderedness in `res`
         cat = Categorical([3, 1], categories=[3, 2, 1], ordered=True)