Fix indexing.setitem when type is pd.Categorical

Author: keechongtan

pandas/core/internals/blocks.py

@@ -883,6 +883,14 @@ def setitem(self, indexer, value):
         ):
             values[indexer] = value
             try:
+                # GH25495 may need to convert to categorical block
+                if self.is_categorical_astype(
+                    arr_value.dtype
+                ) and not is_categorical_dtype(values):
+                    return self.make_block(
+                        Categorical(self.values, dtype=arr_value.dtype)
+                    )
+
                 values = values.astype(arr_value.dtype)
             except ValueError:
                 pass

pandas/tests/frame/test_indexing.py

@@ -204,6 +204,15 @@ def test_setitem_list_of_tuples(self, float_frame):
         expected = Series(tuples, index=float_frame.index, name="tuples")
         tm.assert_series_equal(result, expected)
 
+    def test_setitem_single_row_categorical(self):
+        df = DataFrame({"Alpha": ["a"], "Numeric": [0]})
+        categories = pd.Categorical(df["Alpha"], categories=["a", "b", "c"])
+        df.loc[:, "Alpha"] = categories
+
+        result = df["Alpha"]
+        expected = Series(categories, index=df.index, name="Alpha")
+        tm.assert_series_equal(result, expected)
+
     def test_setitem_mulit_index(self):
         # GH7655, test that assigning to a sub-frame of a frame
         # with multi-index columns aligns both rows and columns