TST: Asserts all types are caterogical in function fillna with correct error message (#27933)

* Fixes issue #13628

* Fixes issue #13628 fix

* isort Fixes #13628

* move to 1.0

Author: LiuSeeker

doc/source/whatsnew/v0.25.1.rst

@@ -27,7 +27,6 @@ Categorical
 
 - Bug in :meth:`Categorical.fillna` would replace all values, not just those that are ``NaN`` (:issue:`26215`)
 -
--
 
 Datetimelike
 ^^^^^^^^^^^^

doc/source/whatsnew/v1.0.0.rst

@@ -87,6 +87,7 @@ Bug fixes
 Categorical
 ^^^^^^^^^^^
 
+- Added test to assert the :func:`fillna` raises the correct ValueError message when the value isn't a value from categories (:issue:`13628`)
 -
 -
 

pandas/tests/arrays/categorical/test_missing.py

@@ -5,7 +5,7 @@
 
 from pandas.core.dtypes.dtypes import CategoricalDtype
 
-from pandas import Categorical, Index, isna
+from pandas import Categorical, Index, Series, isna
 import pandas.util.testing as tm
 
 
@@ -59,11 +59,13 @@ def test_set_item_nan(self):
             ),
             (dict(), "Must specify a fill 'value' or 'method'."),
             (dict(method="bad"), "Invalid fill method. Expecting .* bad"),
+            (dict(value=Series([1, 2, 3, 4, "a"])), "fill value must be in categories"),
         ],
     )
     def test_fillna_raises(self, fillna_kwargs, msg):
         # https://github.com/pandas-dev/pandas/issues/19682
-        cat = Categorical([1, 2, 3])
+        # https://github.com/pandas-dev/pandas/issues/13628
+        cat = Categorical([1, 2, 3, None, None])
 
         with pytest.raises(ValueError, match=msg):
             cat.fillna(**fillna_kwargs)