add unit test for writeable flag after unique

Author: Fariba Aalamifar

pandas/tests/test_categorical.py

@@ -1227,6 +1227,10 @@ def test_set_categories(self):
         exp_categories = Index(["a", "b", "c", "d"])
         tm.assert_index_equal(cat.categories, exp_categories)
 
+        # all-nan categories GH 18051
+        cat_nan = Categorical([np.nan])
+        assert cat_nan.unique()._codes.flags.writeable
+
         # internals...
         c = Categorical([1, 2, 3, 4, 1], categories=[1, 2, 3, 4], ordered=True)
         tm.assert_numpy_array_equal(c._codes, np.array([0, 1, 2, 3, 0],