PERF: do not expand categoricals when re-coding

Author: toobaz

pandas/core/categorical.py

@@ -293,13 +293,10 @@ def __init__(self, values, categories=None, ordered=None, dtype=None,
 
         # sanitize input
         if is_categorical_dtype(values):
-            values = values.get_values()
-
-        elif isinstance(values, (ABCIndexClass, ABCSeries)):
-            # we'll do inference later
-            pass
+            if dtype.categories is None:
+                dtype = CategoricalDtype(values.categories, dtype.ordered)
 
-        else:
+        elif not isinstance(values, (ABCIndexClass, ABCSeries)):
             if not isinstance(values, np.ndarray):
                 values = _convert_to_list_like(values)
                 from pandas.core.series import _sanitize_array
@@ -330,9 +327,14 @@ def __init__(self, values, categories=None, ordered=None, dtype=None,
                 raise NotImplementedError("> 1 ndim Categorical are not "
                                           "supported at this time")
 
-            if dtype.categories is None:
-                # we're inferring from values
-                dtype = CategoricalDtype(categories, dtype.ordered)
+            # we're inferring from values
+            dtype = CategoricalDtype(categories, dtype.ordered)
+
+        elif is_categorical_dtype(values):
+            old_codes = (values.cat.codes if isinstance(values, ABCSeries)
+                         else values.codes)
+            codes = _recode_for_categories(old_codes, values.dtype.categories,
+                                           dtype.categories)
 
         else:
             codes = _get_codes_for_values(values, dtype.categories)