FIX: REGR: setting numeric value in Categorical Series with enlargement raise internal error

doc/source/whatsnew/v1.5.0.rst

@@ -962,6 +962,7 @@ Indexing
 - Bug in :meth:`DataFrame.sum` min_count changes dtype if input contains NaNs (:issue:`46947`)
 - Bug in :class:`IntervalTree` that lead to an infinite recursion. (:issue:`46658`)
 - Bug in :class:`PeriodIndex` raising ``AttributeError`` when indexing on ``NA``, rather than putting ``NaT`` in its place. (:issue:`46673`)
+- Bug in :meth:`DataFrame.loc` when enlarging a :class:`Series` with dtype :class:`CategoricalDtype` with a scalar (:issue:`47677`)
 -
 
 Missing

pandas/core/dtypes/cast.py

@@ -591,7 +591,9 @@ def _maybe_promote(dtype: np.dtype, fill_value=np.nan):
         return dtype, fv
 
     elif isna(fill_value):
-        dtype = _dtype_obj
+        # preserve dtype in case of categoricaldtype
+        if not isinstance(dtype, CategoricalDtype):
+            dtype = _dtype_obj
         if fill_value is None:
             # but we retain e.g. pd.NA
             fill_value = np.nan
@@ -646,6 +648,12 @@ def _maybe_promote(dtype: np.dtype, fill_value=np.nan):
 
         return np.dtype("object"), fill_value
 
+    elif isinstance(dtype, CategoricalDtype):
+        if fill_value in dtype.categories:
+            return dtype, fill_value
+        else:
+            return object, ensure_object(fill_value)
+
     elif is_float(fill_value):
         if issubclass(dtype.type, np.bool_):
             dtype = np.dtype(np.object_)

pandas/tests/indexing/test_loc.py

@@ -18,6 +18,7 @@
 import pandas as pd
 from pandas import (
     Categorical,
+    CategoricalDtype,
     CategoricalIndex,
     DataFrame,
     DatetimeIndex,
@@ -1820,6 +1821,46 @@ def test_loc_getitem_sorted_index_level_with_duplicates(self):
         result = df.loc[("foo", "bar")]
         tm.assert_frame_equal(result, expected)
 
+    def test_additional_element_to_categorical_series_loc(self):
+        # GH#47677
+        result = Series(["a", "b", "c"], dtype="category")
+        result.loc[3] = 0
+        expected = Series(["a", "b", "c", 0], dtype="object")
+        tm.assert_series_equal(result, expected)
+
+    def test_additional_categorical_element_loc(self):
+        # GH#47677
+        result = Series(["a", "b", "c"], dtype="category")
+        result.loc[3] = "a"
+        expected = Series(["a", "b", "c", "a"], dtype="category")
+        tm.assert_series_equal(result, expected)
+
+    def test_loc_set_nan_in_categorical_series(self, any_numeric_ea_dtype):
+        # GH#47677
+        srs = Series(
+            [1, 2, 3],
+            dtype=CategoricalDtype(Index([1, 2, 3], dtype=any_numeric_ea_dtype)),
+        )
+        # enlarge
+        srs.loc[3] = np.nan
+        assert srs.values.dtype._categories.dtype == any_numeric_ea_dtype
+        # set into
+        srs.loc[1] = np.nan
+        assert srs.values.dtype._categories.dtype == any_numeric_ea_dtype
+
+    @pytest.mark.parametrize("na", (np.nan, pd.NA, None))
+    def test_loc_consistency_series_enlarge_set_into(self, na):
+        # GH#47677
+        srs_enlarge = Series(["a", "b", "c"], dtype="category")
+        srs_enlarge.loc[3] = na
+
+        srs_setinto = Series(["a", "b", "c", "a"], dtype="category")
+        srs_setinto.loc[3] = na
+
+        tm.assert_series_equal(srs_enlarge, srs_setinto)
+        expected = Series(["a", "b", "c", na], dtype="category")
+        tm.assert_series_equal(srs_enlarge, expected)
+
     def test_loc_getitem_preserves_index_level_category_dtype(self):
         # GH#15166
         df = DataFrame(