PERF: do not expand categoricals when re-coding

Author: toobaz

pandas/core/categorical.py

@@ -2219,6 +2219,12 @@ def _get_codes_for_values(values, categories):
     """
 
     from pandas.core.algorithms import _get_data_algo, _hashtables
+    if is_categorical_dtype(values.dtype):
+        codes = (values.cat.codes if isinstance(values, ABCSeries)
+                 else values.codes)
+        return _recode_for_categories(codes, values.dtype.categories,
+                                      categories)
+
     if not is_dtype_equal(values.dtype, categories.dtype):
         values = _ensure_object(values)
         categories = _ensure_object(categories)