BUG: raise when sort_index with ascending=None

pandas/core/generic.py

@@ -11776,20 +11776,10 @@ def _doc_params(cls):
 """
 
 
-def validate_ascending(
-    ascending: Union[bool, Sequence[bool]],
-) -> Union[bool, Sequence[bool]]:
+def validate_ascending(ascending):
     """Validate ``ascending`` kwargs for ``sort_index`` method."""
+    kwargs = {"none_allowed": False, "int_allowed": True}
     if not isinstance(ascending, (list, tuple)):
-        _check_ascending_element(ascending)
-        return ascending
+        return validate_bool_kwarg(ascending, "ascending", **kwargs)
 
-    for item in ascending:
-        _check_ascending_element(item)
-    return ascending
-
-
-def _check_ascending_element(value):
-    """Ensure that each item in ``ascending`` kwarg is either bool or int."""
-    if value is None or not isinstance(value, (bool, int)):
-        raise ValueError("ascending must be either a bool or a sequence of bools")
+    return [validate_bool_kwarg(item, "ascending", **kwargs) for item in ascending]

pandas/tests/frame/methods/test_sort_index.py

@@ -773,7 +773,7 @@ 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 = "ascending must be either a bool or a sequence of bools"
+        match = 'For argument "ascending" expected type bool'
         with pytest.raises(ValueError, match=match):
             df.sort_index(axis=0, ascending=ascending, na_position="first")
 

pandas/tests/series/methods/test_sort_index.py

@@ -208,7 +208,7 @@ def test_sort_index_ascending_list(self):
     )
     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 = "ascending must be either a bool or a sequence of bools"
+        match = 'For argument "ascending" expected type bool'
         with pytest.raises(ValueError, match=match):
             ser.sort_index(ascending=ascending)
 

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__}."