add PR reference in comments

Author: saldanhad

pandas/core/apply.py

@@ -1464,8 +1464,8 @@ def curried(x):
             curried = func
 
         #  remove the `na_action="ignore"` as default has been changed in
-        #  Categorical (GH51645).
-        # Reference for below fix (GH )
+        #  Categorical (GH 51645).
+        # Reference for below fix (GH 59966)
         mapped = obj._map_values(mapper=curried)
 
         if len(mapped) and isinstance(mapped[0], ABCSeries):

pandas/tests/apply/test_frame_apply.py

@@ -742,7 +742,7 @@ def test_apply_category_equalness(val):
     result = df.a.apply(lambda x: x == val)
     expected = Series(
         [False if pd.isnull(x) else x == val for x in df_values], name="a"
-    )
+    ) # False since behavior of NaN for categorical dtype has been changed (GH 59966)
     tm.assert_series_equal(result, expected)