DataFrame.append preserves columns dtype if possible

Author: tp

doc/source/whatsnew/v0.23.0.txt

@@ -202,6 +202,7 @@ Other Enhancements
 - ``Resampler`` objects now have a functioning :attr:`~pandas.core.resample.Resampler.pipe` method.
   Previously, calls to ``pipe`` were diverted to  the ``mean`` method (:issue:`17905`).
 - :func:`~pandas.api.types.is_scalar` now returns ``True`` for ``DateOffset`` objects (:issue:`18943`).
+- :meth:`DataFrame.append` now preserves the type of the calling dataframe's columns, when possible (:issue:`18359`)
 - Added :func:`pandas.api.extensions.register_dataframe_accessor`,
   :func:`pandas.api.extensions.register_series_accessor`, and
   :func:`pandas.api.extensions.register_index_accessor`, accessor for libraries downstream of pandas

pandas/tests/reshape/test_concat.py

@@ -834,7 +834,6 @@ def test_append_preserve_index_name(self):
     @pytest.mark.parametrize("df_columns", [
         pd.RangeIndex(3),
         pd.CategoricalIndex('A B C'.split()),
-        pd.CategoricalIndex('A B C'.split(), ordered=True),
         pd.MultiIndex.from_arrays(['A B C'.split(), 'D E F'.split()]),
         pd.IntervalIndex.from_breaks([0, 1, 2, 3]),
         pd.DatetimeIndex([dt.datetime(2013, 1, 3, 0, 0),
@@ -913,10 +912,8 @@ def test_append_multi_index_raises(self, other_type):
 
         df = pd.DataFrame([[1, 2, 3], [4, 5, 6]], columns=mi)
         ser = pd.Series([7, 8, 9], index=other_type, name=2)
-        if isinstance(other_type, pd.IntervalIndex):
-            pytest.raises(ValueError, df.append, ser)
-        else:
-            pytest.raises(TypeError, df.append, ser)
+        with pytest.raises(TypeError):
+            df.append(ser)
 
         df = pd.DataFrame([[1, 2, 3], [4, 5, 6]], columns=other_type)
         ser = pd.Series([7, 8, 9], index=mi, name=2)
@@ -947,7 +944,7 @@ def test_append_interval_index_raises(self, other_type):
 
         df = pd.DataFrame([[1, 2, 3], [4, 5, 6]], columns=other_type)
         ser = pd.Series([7, 8, 9], index=ii, name=2)
-        with pytest.raises(ValueError):
+        with pytest.raises(TypeError):
             df.append(ser)
 
     def test_append_dtype_coerce(self):