Make CI.astype('category') not change anything

Author: jschendel

pandas/core/indexes/category.py

@@ -341,21 +341,21 @@ def __array__(self, dtype=None):
 
     @Appender(_index_shared_docs['astype'])
     def astype(self, dtype, copy=True):
+        if isinstance(dtype, compat.string_types) and dtype == 'category':
+            # GH 18630: CI.astype('category') should not change anything
+            return self.copy() if copy else self
+
         dtype = pandas_dtype(dtype)
         if is_interval_dtype(dtype):
             from pandas import IntervalIndex
             return IntervalIndex.from_intervals(np.array(self))
         elif is_categorical_dtype(dtype):
-            # want to maintain existing categories/ordered if they are None
+            # GH 18630: keep current categories if None (ordered can't be None)
             if dtype.categories is None:
                 new_categories = self.categories
             else:
                 new_categories = dtype.categories
-            if dtype.ordered is None:
-                new_ordered = self.ordered
-            else:
-                new_ordered = dtype.ordered
-            dtype = CategoricalDtype(new_categories, new_ordered)
+            dtype = CategoricalDtype(new_categories, dtype.ordered)
 
             # fastpath if dtypes are equal
             if dtype == self.dtype:

pandas/tests/indexes/test_category.py

@@ -437,10 +437,9 @@ def test_astype_category(self, copy, name, dtype_ordered, index_ordered):
         tm.assert_index_equal(result, expected)
 
         if dtype_ordered is False:
-            # dtype='category' defaults to ordered=False, so only test once
+            # dtype='category' can't specify ordered, so only test once
             result = index.astype('category', copy=copy)
-            expected = CategoricalIndex(
-                index.tolist(), categories=index.categories, name=name)
+            expected = index
             tm.assert_index_equal(result, expected)
 
     def test_reindex_base(self):