From 8137437e3e9219f0ad0ee436b6a121dc26c11bc7 Mon Sep 17 00:00:00 2001
From: jbrockmendel <jbrockmendel@gmail.com>
Date: Tue, 7 Apr 2020 12:15:57 -0700
Subject: [PATCH] BUG: is_categorical shouldnt recognize Dtype objects

---
 pandas/core/dtypes/common.py       | 11 ++++++-----
 pandas/core/indexes/base.py        |  2 +-
 pandas/tests/dtypes/test_common.py |  1 +
 pandas/tests/dtypes/test_dtypes.py |  2 +-
 4 files changed, 9 insertions(+), 7 deletions(-)

diff --git a/pandas/core/dtypes/common.py b/pandas/core/dtypes/common.py
index 16373bd697c1f..efb27db26345e 100644
--- a/pandas/core/dtypes/common.py
+++ b/pandas/core/dtypes/common.py
@@ -19,7 +19,7 @@
     PeriodDtype,
     registry,
 )
-from pandas.core.dtypes.generic import ABCCategorical, ABCIndexClass
+from pandas.core.dtypes.generic import ABCCategorical, ABCIndexClass, ABCSeries
 from pandas.core.dtypes.inference import (  # noqa:F401
     is_array_like,
     is_bool,
@@ -68,9 +68,6 @@
 ensure_float64 = algos.ensure_float64
 ensure_float32 = algos.ensure_float32
 
-_ensure_datetime64ns = conversion.ensure_datetime64ns
-_ensure_timedelta64ns = conversion.ensure_timedelta64ns
-
 
 def ensure_float(arr):
     """
@@ -359,8 +356,12 @@ def is_categorical(arr) -> bool:
     True
     >>> is_categorical(pd.CategoricalIndex([1, 2, 3]))
     True
+    >>> is_categorical(cat.dtype)
+    False
     """
-    return isinstance(arr, ABCCategorical) or is_categorical_dtype(arr)
+    return isinstance(arr, ABCCategorical) or (
+        isinstance(arr, (ABCIndexClass, ABCSeries)) and is_categorical_dtype(arr.dtype)
+    )
 
 
 def is_datetime64_dtype(arr_or_dtype) -> bool:
diff --git a/pandas/core/indexes/base.py b/pandas/core/indexes/base.py
index d0319e9181bad..9f147a6e6c8fd 100644
--- a/pandas/core/indexes/base.py
+++ b/pandas/core/indexes/base.py
@@ -4716,7 +4716,7 @@ def groupby(self, values) -> PrettyDict[Hashable, np.ndarray]:
         # TODO: if we are a MultiIndex, we can do better
         # that converting to tuples
         if isinstance(values, ABCMultiIndex):
-            values = values.values
+            values = values._values
         values = ensure_categorical(values)
         result = values._reverse_indexer()
 
diff --git a/pandas/tests/dtypes/test_common.py b/pandas/tests/dtypes/test_common.py
index 66bf696cbe912..7dc8984f6d9b7 100644
--- a/pandas/tests/dtypes/test_common.py
+++ b/pandas/tests/dtypes/test_common.py
@@ -208,6 +208,7 @@ def test_is_categorical():
     assert com.is_categorical(pd.CategoricalIndex([1, 2, 3]))
 
     assert not com.is_categorical([1, 2, 3])
+    assert not com.is_categorical(cat.dtype)  # Categorical obj, not dtype
 
 
 def test_is_datetime64_dtype():
diff --git a/pandas/tests/dtypes/test_dtypes.py b/pandas/tests/dtypes/test_dtypes.py
index d0831ea514a64..d9b0473d6c3a3 100644
--- a/pandas/tests/dtypes/test_dtypes.py
+++ b/pandas/tests/dtypes/test_dtypes.py
@@ -159,7 +159,7 @@ def test_basic(self, dtype):
         assert is_categorical_dtype(s)
         assert not is_categorical_dtype(np.dtype("float64"))
 
-        assert is_categorical(s.dtype)
+        assert not is_categorical(s.dtype)
         assert is_categorical(s)
         assert not is_categorical(np.dtype("float64"))
         assert not is_categorical(1.0)