BUG: Copy categorical codes if empty (fixes pandas-dev#18051) (pandas-dev#18436)

Author: topper-123

doc/source/whatsnew/v0.21.1.txt

@@ -136,6 +136,7 @@ Categorical
 - Bug in :meth:`DataFrame.astype` where casting to 'category' on an empty ``DataFrame`` causes a segmentation fault (:issue:`18004`)
 - Error messages in the testing module have been improved when items have different ``CategoricalDtype`` (:issue:`18069`)
 - ``CategoricalIndex`` can now correctly take a ``pd.api.types.CategoricalDtype`` as its dtype (:issue:`18116`)
+- Bug in ``Categorical.unique()`` returning read-only ``codes`` array when all categories were ``NaN`` (:issue:`18051`)
 
 Other
 ^^^^^

pandas/core/categorical.py

@@ -2276,7 +2276,7 @@ def _recode_for_categories(codes, old_categories, new_categories):
 
  if len(old_categories) == 0:
  # All null anyway, so just retain the nulls
- return codes
+ return codes.copy()
  indexer = coerce_indexer_dtype(new_categories.get_indexer(old_categories),
  new_categories)
  new_codes = take_1d(indexer, codes.copy(), fill_value=-1)

pandas/tests/series/test_analytics.py

@@ -848,6 +848,12 @@ def test_value_counts_nunique(self):
  result = series.nunique()
  assert result == 11
 
+ # GH 18051
+ s = pd.Series(pd.Categorical([]))
+ assert s.nunique() == 0
+ s = pd.Series(pd.Categorical([np.nan]))
+ assert s.nunique() == 0
+
  def test_unique(self):
 
  # 714 also, dtype=float
@@ -873,6 +879,14 @@ def test_unique(self):
  expected = np.array([1, 2, 3, None], dtype=object)
  tm.assert_numpy_array_equal(result, expected)
 
+ # GH 18051
+ s = pd.Series(pd.Categorical([]))
+ tm.assert_categorical_equal(s.unique(), pd.Categorical([]),
+ check_dtype=False)
+ s = pd.Series(pd.Categorical([np.nan]))
+ tm.assert_categorical_equal(s.unique(), pd.Categorical([np.nan]),
+ check_dtype=False)
+
  @pytest.mark.parametrize(
  "tc1, tc2",
  [