BUG: Categorical.unique should keep dtype unchanged (pandas-dev#38140)

Author: topper-123

doc/source/whatsnew/v1.3.0.rst

@@ -230,6 +230,38 @@ Notable bug fixes
 
 These are bug fixes that might have notable behavior changes.
 
+``Categorical.unique`` now always maintains same dtype as original
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Previously, when calling :meth:`~Categorical.unique` with categorical data, unused categories in the new array
+would be removed, meaning that the dtype of the new array would be different than the
+original, if some categories are not present in the unique array (:issue:`18291`)
+
+As an example of this, given:
+
+.. ipython:: python
+
+        dtype = pd.CategoricalDtype(['bad', 'neutral', 'good'], ordered=True)
+        cat = pd.Categorical(['good', 'good', 'bad', 'bad'], dtype=dtype)
+        original = pd.Series(cat)
+        unique = original.unique()
+
+*pandas < 1.3.0*:
+
+.. code-block:: ipython
+
+    In [1]: unique
+    ['good', 'bad']
+    Categories (2, object): ['bad' < 'good']
+    In [2]: original.dtype == unique.dtype
+    False
+
+*pandas >= 1.3.0*
+
+.. ipython:: python
+
+        unique
+        original.dtype == unique.dtype
 
 Preserve dtypes in  :meth:`~pandas.DataFrame.combine_first`
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

pandas/core/series.py

@@ -1993,15 +1993,12 @@ def unique(self) -> ArrayLike:
         ['2016-01-01 00:00:00-05:00']
         Length: 1, dtype: datetime64[ns, US/Eastern]
 
-        An unordered Categorical will return categories in the order of
-        appearance.
+        An Categorical will return categories in the order of
+        appearance and with the same dtype.
 
         >>> pd.Series(pd.Categorical(list('baabc'))).unique()
         ['b', 'a', 'c']
-        Categories (3, object): ['b', 'a', 'c']
-
-        An ordered Categorical preserves the category ordering.
-
+        Categories (3, object): ['a', 'b', 'c']
         >>> pd.Series(pd.Categorical(list('baabc'), categories=list('abc'),
         ...                          ordered=True)).unique()
         ['b', 'a', 'c']