consolidate special casing

pandas/core/generic.py

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

pandas/core/internals/construction.py

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

pandas/core/series.py

@@ -18,7 +18,7 @@
 from pandas.util._validators import validate_bool_kwarg
 
 from pandas.core.dtypes.common import (
-    _is_unorderable_exception, ensure_platform_int, is_bool,
+    _is_unorderable_exception, ensure_platform_int, is_bool, is_categorical,
     is_categorical_dtype, is_datetime64_dtype, is_datetimelike, is_dict_like,
     is_extension_array_dtype, is_extension_type, is_hashable, is_integer,
     is_iterator, is_list_like, is_scalar, is_string_like, is_timedelta64_dtype)
@@ -168,6 +168,12 @@ def __init__(self, data=None, index=None, dtype=None, name=None,
             if data is None:
                 data = {}
             if dtype is not None:
+                # GH 26336: explicitly handle 'category' to avoid warning
+                # TODO: Remove after CategoricalDtype defaults to ordered=False
+                if (isinstance(dtype, str) and dtype == 'category' and
+                        is_categorical(data)):
+                    dtype = data.dtype
+
                 dtype = self._validate_dtype(dtype)
 
             if isinstance(data, MultiIndex):

pandas/io/packers.py

@@ -623,19 +623,9 @@ def decode(obj):
         return Interval(obj['left'], obj['right'], obj['closed'])
     elif typ == 'series':
         dtype = dtype_for(obj['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']),
-                        index=index,
-                        dtype=pd_dtype,
-                        name=obj['name'])
-        return result
+        data = unconvert(obj['data'], dtype, obj['compress'])
+        return Series(data, index=index, dtype=dtype, name=obj['name'])
 
     elif typ == 'block_manager':
         axes = obj['axes']