Revert "Remove _values_for_argsort"

This reverts commit 44b6d72.

Author: TomAugspurger

pandas/core/arrays/base.py

@@ -217,8 +217,20 @@ def isna(self):
         """
         raise AbstractMethodError(self)
 
+    def _values_for_argsort(self):
+        # type: () -> ndarray
+        """Get the ndarray to be passed to np.argsort.
+
+        This is called from within 'ExtensionArray.argsort'.
+
+        Returns
+        -------
+        values : ndarray
+        """
+        return np.array(self)
+
     def argsort(self, ascending=True, kind='quicksort', *args, **kwargs):
-        """Return the indices that would sort this array.
+        """Returns the indices that would sort this array.
 
         Parameters
         ----------
@@ -239,8 +251,13 @@ def argsort(self, ascending=True, kind='quicksort', *args, **kwargs):
         --------
         numpy.argsort
         """
+        # Implementor note: You have two places to override the behavior of
+        # argsort.
+        # 1. _values_for_argsort : construct the values passed to np.argsort
+        # 2. argsort : total control over sorting.
+
         ascending = nv.validate_argsort_with_ascending(ascending, args, kwargs)
-        values = self.astype(object)
+        values = self._values_for_argsort()
         result = np.argsort(values, kind=kind, **kwargs)
         if not ascending:
             result = result[::-1]

pandas/core/arrays/categorical.py

@@ -1386,6 +1386,9 @@ def check_for_ordered(self, op):
                             "you can use .as_ordered() to change the "
                             "Categorical to an ordered one\n".format(op=op))
 
+    def _values_for_argsort(self):
+        return self._codes.copy()
+
     def argsort(self, ascending=True, kind='quicksort', *args, **kwargs):
         """
         Returns the indices that would sort the Categorical instance if
@@ -1406,11 +1409,9 @@ def argsort(self, ascending=True, kind='quicksort', *args, **kwargs):
         --------
         numpy.ndarray.argsort
         """
-        ascending = nv.validate_argsort_with_ascending(ascending, args, kwargs)
-        result = np.argsort(self._codes.copy(), kind=kind, **kwargs)
-        if not ascending:
-            result = result[::-1]
-        return result
+        # Keep the implementation here just for the docstring.
+        return super(Categorical, self).argsort(ascending=ascending, kind=kind,
+                                                *args, **kwargs)
 
     def sort_values(self, inplace=False, ascending=True, na_position='last'):
         """ Sorts the Categorical by category value returning a new