BUG: raise when sort_index with ascending=None

pandas/core/generic.py

@@ -4533,6 +4533,8 @@ def sort_index(
 
         inplace = validate_bool_kwarg(inplace, "inplace")
         axis = self._get_axis_number(axis)
+        ascending = validate_ascending(ascending)
+
         target = self._get_axis(axis)
 
         indexer = get_indexer_indexer(
@@ -11772,3 +11774,12 @@ def _doc_params(cls):
     The required number of valid values to perform the operation. If fewer than
     ``min_count`` non-NA values are present the result will be NA.
 """
+
+
+def validate_ascending(ascending):
+    """Validate ``ascending`` kwargs for ``sort_index`` method."""
+    kwargs = {"none_allowed": False, "int_allowed": True}
+    if not isinstance(ascending, (list, tuple)):
+        return validate_bool_kwarg(ascending, "ascending", **kwargs)
+
+    return [validate_bool_kwarg(item, "ascending", **kwargs) for item in ascending]

pandas/tests/frame/methods/test_sort_index.py

@@ -761,6 +761,22 @@ def test_sort_index_with_categories(self, categories):
         )
         tm.assert_frame_equal(result, expected)
 
+    @pytest.mark.parametrize(
+        "ascending",
+        [
+            None,
+            (True, None),
+            (False, "True"),
+        ],
+    )
+    def test_sort_index_ascending_bad_value_raises(self, ascending):
+        df = DataFrame(np.arange(64))
+        length = len(df.index)
+        df.index = [(i - length / 2) % length for i in range(length)]
+        match = 'For argument "ascending" expected type bool'
+        with pytest.raises(ValueError, match=match):
+            df.sort_index(axis=0, ascending=ascending, na_position="first")
+
 
 class TestDataFrameSortIndexKey:
     def test_sort_multi_index_key(self):

pandas/tests/series/methods/test_sort_index.py

@@ -198,6 +198,20 @@ def test_sort_index_ascending_list(self):
         expected = ser.iloc[[0, 4, 1, 5, 2, 6, 3, 7]]
         tm.assert_series_equal(result, expected)
 
+    @pytest.mark.parametrize(
+        "ascending",
+        [
+            None,
+            (True, None),
+            (False, "True"),
+        ],
+    )
+    def test_sort_index_ascending_bad_value_raises(self, ascending):
+        ser = Series(range(10), index=[0, 3, 2, 1, 4, 5, 7, 6, 8, 9])
+        match = 'For argument "ascending" expected type bool'
+        with pytest.raises(ValueError, match=match):
+            ser.sort_index(ascending=ascending)
+
 
 class TestSeriesSortIndexKey:
     def test_sort_index_multiindex_key(self):

pandas/util/_validators.py

@@ -205,9 +205,39 @@ def validate_args_and_kwargs(fname, args, kwargs, max_fname_arg_count, compat_ar
     validate_kwargs(fname, kwargs, compat_args)
 
 
-def validate_bool_kwarg(value, arg_name):
-    """ Ensures that argument passed in arg_name is of type bool. """
-    if not (is_bool(value) or value is None):
+def validate_bool_kwarg(value, arg_name, none_allowed=True, int_allowed=False):
+    """
+    Ensure that argument passed in arg_name can be interpreted as boolean.
+
+    Parameters
+    ----------
+    value : bool
+        Value to be validated.
+    arg_name : str
+        Name of the argument. To be reflected in the error message.
+    none_allowed : bool, optional
+        Whether to consider None to be a valid boolean.
+    int_allowed : bool, optional
+        Whether to consider integer value to be a valid boolean.
+
+    Returns
+    -------
+    value
+        The same value as input.
+
+    Raises
+    ------
+    ValueError
+        If the value is not a valid boolean.
+    """
+    good_value = is_bool(value)
+    if none_allowed:
+        good_value = good_value or value is None
+
+    if int_allowed:
+        good_value = good_value or isinstance(value, int)
+
+    if not good_value:
         raise ValueError(
             f'For argument "{arg_name}" expected type bool, received '
             f"type {type(value).__name__}."