BUG: fixed .str.contains(..., na=False) for categorical series

pandas/core/strings.py

@@ -1844,7 +1844,7 @@ def __iter__(self):
             g = self.get(i)
 
     def _wrap_result(self, result, use_codes=True,
-                     name=None, expand=None):
+                     name=None, expand=None, na=np.nan):
 
         from pandas.core.index import Index, MultiIndex
 
@@ -1855,6 +1855,12 @@ def _wrap_result(self, result, use_codes=True,
         # before the transformation...
         if use_codes and self._is_categorical:
             result = take_1d(result, self._orig.cat.codes)
+            missing = isna(result)
+
+            if missing.any():
+                result_type = np.result_type(result, na)
+                result = result.astype(result_type, copy=False)
+                result[isna(result)] = na
 
         if not hasattr(result, 'ndim') or not hasattr(result, 'dtype'):
             return result
@@ -2466,13 +2472,13 @@ def join(self, sep):
     def contains(self, pat, case=True, flags=0, na=np.nan, regex=True):
         result = str_contains(self._parent, pat, case=case, flags=flags, na=na,
                               regex=regex)
-        return self._wrap_result(result)
+        return self._wrap_result(result, na=na)
 
     @copy(str_match)
     def match(self, pat, case=True, flags=0, na=np.nan, as_indexer=None):
         result = str_match(self._parent, pat, case=case, flags=flags, na=na,
                            as_indexer=as_indexer)
-        return self._wrap_result(result)
+        return self._wrap_result(result, na=na)
 
     @copy(str_replace)
     def replace(self, pat, repl, n=-1, case=None, flags=0, regex=True):

pandas/tests/test_strings.py

@@ -572,6 +572,13 @@ def test_contains(self):
         res = values.str.contains('foo', na="foo")
         assert res.loc[2] == "foo"
 
+        # category
+        values = Series(["a", "b", "c", "a", np.nan], dtype="category")
+        result = values.str.contains('a', na=True)
+        expected = Series([True, False, False, True, True], dtype=np.object_)
+        assert isinstance(result, Series)
+        tm.assert_series_equal(result, expected)
+
     def test_startswith(self):
         values = Series(['om', NA, 'foo_nom', 'nom', 'bar_foo', NA, 'foo'])