accept dtype

Author: TomAugspurger

pandas/core/categorical.py

@@ -202,6 +202,7 @@ class Categorical(PandasObject):
         categorical, read only.
     ordered : boolean
         Whether or not this Categorical is ordered.
+    dtype : CategoricalDtype
 
     Raises
     ------
@@ -248,11 +249,24 @@ class Categorical(PandasObject):
     __array_priority__ = 1000
     _typ = 'categorical'
 
-    def __init__(self, values, categories=None, ordered=False, fastpath=False):
+    def __init__(self, values, categories=None, ordered=None, dtype=None,
+                 fastpath=False):
+
+        if dtype is not None:
+            if categories is not None or ordered is not None:
+                raise ValueError("Cannot specify both `dtype` and `categories`"
+                                 " or `ordered`.")
+            categories = dtype.categories
+            ordered = dtype.ordered
+
+        if ordered is None:
+            ordered = False
 
         if fastpath:
-            self._dtype = CategoricalDtype(categories, ordered)
+            if dtype is None:
+                dtype = CategoricalDtype(categories, ordered)
             self._codes = coerce_indexer_dtype(values, categories)
+            self._dtype = dtype
             return
 
         # sanitize input
@@ -308,7 +322,8 @@ def __init__(self, values, categories=None, ordered=False, fastpath=False):
                 raise NotImplementedError("> 1 ndim Categorical are not "
                                           "supported at this time")
 
-            dtype = CategoricalDtype(categories, ordered)
+            if dtype is None or isinstance(dtype, str):
+                dtype = CategoricalDtype(categories, ordered)
 
         else:
             # there were two ways if categories are present
@@ -320,7 +335,9 @@ def __init__(self, values, categories=None, ordered=False, fastpath=False):
 
             # make sure that we always have the same type here, no matter what
             # we get passed in
-            dtype = CategoricalDtype(categories, ordered)
+            if dtype is None or isinstance(dtype, str):
+                dtype = CategoricalDtype(categories, ordered)
+
             codes = _get_codes_for_values(values, dtype.categories)
 
             # TODO: check for old style usage. These warnings should be removes
@@ -598,6 +615,9 @@ def _codes_for_groupby(self, sort):
 
         return self.reorder_categories(cat.categories)
 
+    def _set_dtype(self, dtype):
+        return type(self)(self, dtype=dtype, fastpath=True)
+
     def set_ordered(self, value, inplace=False):
         """
         Sets the ordered attribute to the boolean value