API: CategoricalIndex.append fallback to concat_compat (#38098)

doc/source/whatsnew/v1.2.0.rst

@@ -454,6 +454,7 @@ Other API changes
 - Passing an invalid ``fill_value`` to :meth:`Series.shift` with a ``CategoricalDtype`` now raises a ``TypeError`` instead of a ``ValueError`` (:issue:`37733`)
 - Passing an invalid value to :meth:`IntervalIndex.insert` or :meth:`CategoricalIndex.insert` now raises a ``TypeError`` instead of a ``ValueError`` (:issue:`37733`)
 - Attempting to reindex a Series with a :class:`CategoricalIndex` with an invalid ``fill_value`` now raises a ``TypeError`` instead of a ``ValueError`` (:issue:`37733`)
+- :meth:`CategoricalIndex.append` with an index that contains non-category values will now cast instead of raising ``TypeError`` (:issue:`38098`)
 
 .. ---------------------------------------------------------------------------
 
@@ -635,6 +636,7 @@ Indexing
 - Bug in :meth:`DataFrame.loc` returning and assigning elements in wrong order when indexer is differently ordered than the :class:`MultiIndex` to filter (:issue:`31330`, :issue:`34603`)
 - Bug in :meth:`DataFrame.loc` and :meth:`DataFrame.__getitem__`  raising ``KeyError`` when columns were :class:`MultiIndex` with only one level (:issue:`29749`)
 - Bug in :meth:`Series.__getitem__` and :meth:`DataFrame.__getitem__` raising blank ``KeyError`` without missing keys for :class:`IntervalIndex` (:issue:`27365`)
+- Bug in setting a new label on a :class:`DataFrame` or :class:`Series` with a :class:`CategoricalIndex` incorrectly raising ``TypeError`` when the new label is not among the index's categories (:issue:`38098`)
 
 Missing
 ^^^^^^^

pandas/core/indexes/base.py

@@ -4180,12 +4180,6 @@ def _coerce_scalar_to_index(self, item):
 
         return Index([item], dtype=dtype, **self._get_attributes_dict())
 
-    def _to_safe_for_reshape(self):
-        """
-        Convert to object if we are a categorical.
-        """
-        return self
-
     def _validate_fill_value(self, value):
         """
         Check if the value can be inserted into our array, and convert

pandas/core/indexes/category.py

@@ -399,10 +399,6 @@ def unique(self, level=None):
         #  of result, not self.
         return type(self)._simple_new(result, name=self.name)
 
-    def _to_safe_for_reshape(self):
-        """ convert to object if we are a categorical """
-        return self.astype("object")
-
     def reindex(self, target, method=None, level=None, limit=None, tolerance=None):
         """
         Create index with target's values (move/add/delete values as necessary)
@@ -637,11 +633,19 @@ def map(self, mapper):
         mapped = self._values.map(mapper)
         return Index(mapped, name=self.name)
 
-    def _concat(self, to_concat: List["Index"], name: Label) -> "CategoricalIndex":
+    def _concat(self, to_concat: List["Index"], name: Label) -> Index:
         # if calling index is category, don't check dtype of others
-        codes = np.concatenate([self._is_dtype_compat(c).codes for c in to_concat])
-        cat = self._data._from_backing_data(codes)
-        return type(self)._simple_new(cat, name=name)
+        try:
+            codes = np.concatenate([self._is_dtype_compat(c).codes for c in to_concat])
+        except TypeError:
+            # not all to_concat elements are among our categories (or NA)
+            from pandas.core.dtypes.concat import concat_compat
+
+            res = concat_compat(to_concat)
+            return Index(res, name=name)
+        else:
+            cat = self._data._from_backing_data(codes)
+            return type(self)._simple_new(cat, name=name)
 
     def _delegate_method(self, name: str, *args, **kwargs):
         """ method delegation to the ._values """

pandas/core/indexes/multi.py

@@ -1684,10 +1684,6 @@ def unique(self, level=None):
             level = self._get_level_number(level)
             return self._get_level_values(level=level, unique=True)
 
-    def _to_safe_for_reshape(self):
-        """ convert to object if we are a categorical """
-        return self.set_levels([i._to_safe_for_reshape() for i in self.levels])
-
     def to_frame(self, index=True, name=None):
         """
         Create a DataFrame with the levels of the MultiIndex as columns.

pandas/core/reshape/pivot.py

@@ -268,19 +268,13 @@ def _add_margins(
     margin_dummy = DataFrame(row_margin, columns=[key]).T
 
     row_names = result.index.names
-    try:
-        # check the result column and leave floats
-        for dtype in set(result.dtypes):
-            cols = result.select_dtypes([dtype]).columns
-            margin_dummy[cols] = margin_dummy[cols].apply(
-                maybe_downcast_to_dtype, args=(dtype,)
-            )
-        result = result.append(margin_dummy)
-    except TypeError:
-
-        # we cannot reshape, so coerce the axis
-        result.index = result.index._to_safe_for_reshape()
-        result = result.append(margin_dummy)
+    # check the result column and leave floats
+    for dtype in set(result.dtypes):
+        cols = result.select_dtypes([dtype]).columns
+        margin_dummy[cols] = margin_dummy[cols].apply(
+            maybe_downcast_to_dtype, args=(dtype,)
+        )
+    result = result.append(margin_dummy)
     result.index.names = row_names
 
     return result
@@ -328,16 +322,7 @@ def _all_key(key):
 
                 # we are going to mutate this, so need to copy!
                 piece = piece.copy()
-                try:
-                    piece[all_key] = margin[key]
-                except ValueError:
-                    # we cannot reshape, so coerce the axis
-                    piece.set_axis(
-                        piece._get_axis(cat_axis)._to_safe_for_reshape(),
-                        axis=cat_axis,
-                        inplace=True,
-                    )
-                    piece[all_key] = margin[key]
+                piece[all_key] = margin[key]
 
                 table_pieces.append(piece)
                 margin_keys.append(all_key)

pandas/tests/indexes/categorical/test_category.py

@@ -57,10 +57,10 @@ def test_append(self):
         expected = CategoricalIndex(list("aabbcaca"), categories=categories)
         tm.assert_index_equal(result, expected, exact=True)
 
-        # invalid objects
-        msg = "cannot append a non-category item to a CategoricalIndex"
-        with pytest.raises(TypeError, match=msg):
-            ci.append(Index(["a", "d"]))
+        # invalid objects -> cast to object via concat_compat
+        result = ci.append(Index(["a", "d"]))
+        expected = Index(["a", "a", "b", "b", "c", "a", "a", "d"])
+        tm.assert_index_equal(result, expected, exact=True)
 
         # GH14298 - if base object is not categorical -> coerce to object
         result = Index(["c", "a"]).append(ci)

pandas/tests/indexing/test_categorical.py

@@ -57,9 +57,12 @@ def test_loc_scalar(self):
         with pytest.raises(KeyError, match=r"^'d'$"):
             df.loc["d"]
 
-        msg = "cannot append a non-category item to a CategoricalIndex"
-        with pytest.raises(TypeError, match=msg):
-            df.loc["d"] = 10
+        df2 = df.copy()
+        expected = df2.copy()
+        expected.index = expected.index.astype(object)
+        expected.loc["d"] = 10
+        df2.loc["d"] = 10
+        tm.assert_frame_equal(df2, expected)
 
         msg = "'fill_value=d' is not present in this Categorical's categories"
         with pytest.raises(TypeError, match=msg):

pandas/tests/reshape/concat/test_categorical.py

@@ -1,5 +1,4 @@
 import numpy as np
-import pytest
 
 from pandas.core.dtypes.dtypes import CategoricalDtype
 
@@ -137,13 +136,18 @@ def test_categorical_index_preserver(self):
         ).set_index("B")
         tm.assert_frame_equal(result, expected)
 
-        # wrong categories
+        # wrong categories -> uses concat_compat, which casts to object
         df3 = DataFrame(
             {"A": a, "B": Categorical(b, categories=list("abe"))}
         ).set_index("B")
-        msg = "categories must match existing categories when appending"
-        with pytest.raises(TypeError, match=msg):
-            pd.concat([df2, df3])
+        result = pd.concat([df2, df3])
+        expected = pd.concat(
+            [
+                df2.set_axis(df2.index.astype(object), 0),
+                df3.set_axis(df3.index.astype(object), 0),
+            ]
+        )
+        tm.assert_frame_equal(result, expected)
 
     def test_concat_categorical_tz(self):
         # GH-23816