cover additional case and review edits

Author: jschendel

pandas/core/dtypes/dtypes.py

@@ -546,7 +546,7 @@ def update_dtype(self, dtype):
                 msg = ("ordered=None is deprecated and will default to False "
                        "in a future version; ordered=True must be explicitly "
                        "passed in order to be retained")
-                warnings.warn(msg, FutureWarning, stacklevel=2)
+                warnings.warn(msg, FutureWarning, stacklevel=3)
 
         return CategoricalDtype(new_categories, new_ordered)
 

pandas/core/generic.py

@@ -175,7 +175,10 @@ def is_copy(self, msg):
     def _validate_dtype(self, dtype):
         """ validate the passed dtype """
 
-        if dtype is not None:
+        # GH 26336: don't convert 'category' to CategoricalDtype
+        if isinstance(dtype, str) and dtype == 'category':
+            pass
+        elif dtype is not None:
             dtype = pandas_dtype(dtype)
 
             # a compound dtype

pandas/core/internals/construction.py

@@ -538,7 +538,10 @@ def sanitize_array(data, index, dtype=None, copy=False,
     Sanitize input data to an ndarray, copy if specified, coerce to the
     dtype if specified.
     """
-    if dtype is not None:
+    # GH 26336: don't convert 'category' to CategoricalDtype
+    if isinstance(dtype, str) and dtype == 'category':
+        pass
+    elif dtype is not None:
         dtype = pandas_dtype(dtype)
 
     if isinstance(data, ma.MaskedArray):

pandas/io/packers.py

@@ -623,7 +623,12 @@ def decode(obj):
         return Interval(obj['left'], obj['right'], obj['closed'])
     elif typ == 'series':
         dtype = dtype_for(obj['dtype'])
-        pd_dtype = pandas_dtype(dtype)
+
+        # GH 26336: don't convert 'category' to CategoricalDtype
+        if isinstance(dtype, str) and dtype == 'category':
+            pd_dtype = dtype
+        else:
+            pd_dtype = pandas_dtype(dtype)
 
         index = obj['index']
         result = Series(unconvert(obj['data'], dtype, obj['compress']),

pandas/tests/arrays/categorical/test_dtypes.py

@@ -165,7 +165,7 @@ def test_astype_category_ordered_none_deprecated(self):
         cdt1 = CategoricalDtype(categories=list('cdab'), ordered=True)
         cdt2 = CategoricalDtype(categories=list('cedafb'))
         cat = Categorical(list('abcdaba'), dtype=cdt1)
-        with tm.assert_produces_warning(FutureWarning, check_stacklevel=False):
+        with tm.assert_produces_warning(FutureWarning):
             cat.astype(cdt2)
 
     def test_iter_python_types(self):

pandas/tests/dtypes/test_dtypes.py

@@ -819,7 +819,8 @@ def test_update_dtype(self, ordered_fixture, new_categories, new_ordered):
 
         # GH 26336
         if new_ordered is None and ordered_fixture is True:
-            with tm.assert_produces_warning(FutureWarning):
+            with tm.assert_produces_warning(FutureWarning,
+                                            check_stacklevel=False):
                 result = dtype.update_dtype(new_dtype)
         else:
             result = dtype.update_dtype(new_dtype)

pandas/tests/indexes/test_category.py

@@ -495,7 +495,7 @@ def test_astype_category_ordered_none_deprecated(self):
         cdt1 = CategoricalDtype(categories=list('cdab'), ordered=True)
         cdt2 = CategoricalDtype(categories=list('cedafb'))
         idx = CategoricalIndex(list('abcdaba'), dtype=cdt1)
-        with tm.assert_produces_warning(FutureWarning, check_stacklevel=False):
+        with tm.assert_produces_warning(FutureWarning):
             idx.astype(cdt2)
 
     def test_reindex_base(self):

pandas/tests/series/test_constructors.py

@@ -365,6 +365,33 @@ def test_constructor_categorical_dtype(self):
                           dtype=CategoricalDtype(['a', 'b'], ordered=True))
         tm.assert_series_equal(result, expected, check_categorical=True)
 
+    def test_constructor_categorical_string(self):
+        # GH 26336: the string 'category' maintains existing CategoricalDtype
+        cdt = CategoricalDtype(categories=list('dabc'), ordered=True)
+        expected = Series(list('abcabc'), dtype=cdt)
+
+        # Series(Categorical, dtype='category') keeps existing dtype
+        cat = Categorical(list('abcabc'), dtype=cdt)
+        result = Series(cat, dtype='category')
+        tm.assert_series_equal(result, expected)
+
+        # Series(Series[Categorical], dtype='category') keeps existing dtype
+        result = Series(result, dtype='category')
+        tm.assert_series_equal(result, expected)
+
+    def test_categorical_ordered_none_deprecated(self):
+        # GH 26336
+        cdt1 = CategoricalDtype(categories=list('cdab'), ordered=True)
+        cdt2 = CategoricalDtype(categories=list('cedafb'))
+
+        cat = Categorical(list('abcdaba'), dtype=cdt1)
+        with tm.assert_produces_warning(FutureWarning, check_stacklevel=False):
+            Series(cat, dtype=cdt2)
+
+        s = Series(cat)
+        with tm.assert_produces_warning(FutureWarning, check_stacklevel=False):
+            Series(s, dtype=cdt2)
+
     def test_categorical_sideeffects_free(self):
         # Passing a categorical to a Series and then changing values in either
         # the series or the categorical should not change the values in the