ask Categorical for ranking function

Author: jeet63

pandas/core/categorical.py

@@ -1404,6 +1404,25 @@ def sort_values(self, inplace=False, ascending=True, na_position='last'):
             return self._constructor(values=codes, categories=self.categories,
                                      ordered=self.ordered, fastpath=True)
 
+    def _funcs_for_rank(self):
+        """
+        For correctly ranking ordered categorical data. See GH#15420
+
+        Ordered categorical data should be ranked on the basis of
+        codes with -1 translated to NaN, as floats, and unordered
+        as objects
+
+        Returns
+        -------
+        numpy array
+
+        """
+        if self._ordered:
+            f = _algos.rank_1d_float64
+        else:
+            f = _algos.rank_1d_object
+        return f
+
     def _values_for_rank(self):
         """
         For correctly ranking ordered categorical data. See GH#15420