BUG Series.sort_index does not accept parameters kind and na_position

Author: brandonmburroughs

pandas/core/frame.py

@@ -3323,16 +3323,6 @@ def sort_index(self, axis=0, level=None, ascending=True, inplace=False,
         else:
             from pandas.core.groupby import _nargsort
 
-            # GH11080 - Check monotonic-ness before sort an index
-            # if monotonic (already sorted), return None or copy() according
-            # to 'inplace'
-            if ((ascending and labels.is_monotonic_increasing) or
-                    (not ascending and labels.is_monotonic_decreasing)):
-                if inplace:
-                    return
-                else:
-                    return self.copy()
-
             indexer = _nargsort(labels, kind=kind, ascending=ascending,
                                 na_position=na_position)
 

pandas/core/groupby.py

@@ -4280,6 +4280,11 @@ def _nargsort(items, kind='quicksort', ascending=True, na_position='last'):
     if is_categorical_dtype(items):
         return items.argsort(ascending=ascending)
 
+    # # GH11080 - Check monotonic-ness before sort an index
+    if ((ascending and all(x <= y for x, y in zip(items, items[1:]))) or
+            (not ascending and all(x >= y for x, y in zip(items, items[1:])))):
+        return np.arange(len(items))
+
     items = np.asanyarray(items)
     idx = np.arange(len(items))
     mask = isnull(items)

pandas/core/series.py

@@ -1788,13 +1788,6 @@ def sort_index(self, axis=0, level=None, ascending=True, inplace=False,
         else:
             from pandas.core.groupby import _nargsort
 
-            if ((ascending and index.is_monotonic_increasing) or
-                    (not ascending and index.is_monotonic_decreasing)):
-                if inplace:
-                    return
-                else:
-                    return self.copy()
-
             indexer = _nargsort(index, kind=kind, ascending=ascending,
                                 na_position=na_position)
             new_index = index.take(indexer)