REGR: Fix bug when replacing categorical value with self (#33292)

Author: dsaxton

doc/source/whatsnew/v1.1.0.rst

@@ -294,6 +294,7 @@ Categorical
 - Bug when passing categorical data to :class:`Index` constructor along with ``dtype=object`` incorrectly returning a :class:`CategoricalIndex` instead of object-dtype :class:`Index` (:issue:`32167`)
 - Bug where :class:`Categorical` comparison operator ``__ne__`` would incorrectly evaluate to ``False`` when either element was missing (:issue:`32276`)
 - :meth:`Categorical.fillna` now accepts :class:`Categorical` ``other`` argument (:issue:`32420`)
+- Bug where :meth:`Categorical.replace` would replace with ``NaN`` whenever the new value and replacement value were equal (:issue:`33288`)
 
 Datetimelike
 ^^^^^^^^^^^^

pandas/core/arrays/categorical.py

@@ -2447,6 +2447,8 @@ def replace(self, to_replace, value, inplace: bool = False):
         # other cases, like if both to_replace and value are list-like or if
         # to_replace is a dict, are handled separately in NDFrame
         for replace_value, new_value in replace_dict.items():
+            if new_value == replace_value:
+                continue
             if replace_value in cat.categories:
                 if isna(new_value):
                     cat.remove_categories(replace_value, inplace=True)

pandas/tests/arrays/categorical/test_algos.py

@@ -64,6 +64,8 @@ def test_isin_cats():
     [
         ("b", "c", ["a", "c"], "Categorical.categories are different"),
         ("c", "d", ["a", "b"], None),
+        # https://github.com/pandas-dev/pandas/issues/33288
+        ("a", "a", ["a", "b"], None),
         ("b", None, ["a", None], "Categorical.categories length are different"),
     ],
 )