PERF: avoid casting categorical to array

Author: toobaz

pandas/core/categorical.py

@@ -293,7 +293,8 @@ def __init__(self, values, categories=None, ordered=None, dtype=None,
 
         # sanitize input
         if is_categorical_dtype(values):
-            values = values.get_values()
+            if dtype.categories is None:
+                dtype = CategoricalDtype(values.categories, dtype.ordered)
 
         elif isinstance(values, (ABCIndexClass, ABCSeries)):
             # we'll do inference later
@@ -337,9 +338,8 @@ 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)
 
         else:
             # there were two ways if categories are present