From 5db7e01cc50632306b43efe4346d23642d365922 Mon Sep 17 00:00:00 2001
From: Oliver Hofkens <oliver@novemberfive.co>
Date: Thu, 12 Dec 2019 11:40:02 +0100
Subject: [PATCH 01/16] TST: Add failing test for min/max on empty categorical.

---
 pandas/tests/reductions/test_reductions.py | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/pandas/tests/reductions/test_reductions.py b/pandas/tests/reductions/test_reductions.py
index d66472b1c2054..2846d51b22d56 100644
--- a/pandas/tests/reductions/test_reductions.py
+++ b/pandas/tests/reductions/test_reductions.py
@@ -1060,6 +1060,14 @@ def test_min_max_skipna(self, skipna):
             assert np.isnan(_min)
             assert np.isnan(_max)
 
+    def test_min_max_empty(self):
+        cat = Series(
+            Categorical([], categories=list("ABC"), ordered=True)
+        )
+
+        assert isna(cat.min())
+        assert isna(cat.max())
+
 
 class TestSeriesMode:
     # Note: the name TestSeriesMode indicates these tests

From 9c36841bf81abbca683b210b63d70235e7ddd376 Mon Sep 17 00:00:00 2001
From: Oliver Hofkens <oliver@novemberfive.co>
Date: Thu, 12 Dec 2019 11:42:58 +0100
Subject: [PATCH 02/16] BUG: min/max on empty categorical crashed

---
 pandas/core/arrays/categorical.py | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/pandas/core/arrays/categorical.py b/pandas/core/arrays/categorical.py
index 7170aec3820a8..41eb19d155004 100644
--- a/pandas/core/arrays/categorical.py
+++ b/pandas/core/arrays/categorical.py
@@ -2126,6 +2126,10 @@ def min(self, skipna=True):
         min : the minimum of this `Categorical`
         """
         self.check_for_ordered("min")
+
+        if len(self._codes) == 0:
+            return np.nan
+
         good = self._codes != -1
         if not good.all():
             if skipna:
@@ -2153,6 +2157,10 @@ def max(self, skipna=True):
         max : the maximum of this `Categorical`
         """
         self.check_for_ordered("max")
+
+        if len(self._codes) == 0:
+            return np.nan
+
         good = self._codes != -1
         if not good.all():
             if skipna:

From 4d5df3a94b4b17e6b7aa1189d081ee051aaaf42c Mon Sep 17 00:00:00 2001
From: Oliver Hofkens <oliver@novemberfive.co>
Date: Thu, 12 Dec 2019 12:25:25 +0100
Subject: [PATCH 03/16] DOC: Added whatsnew.

---
 doc/source/whatsnew/v1.0.0.rst | 1 +
 1 file changed, 1 insertion(+)

diff --git a/doc/source/whatsnew/v1.0.0.rst b/doc/source/whatsnew/v1.0.0.rst
index 29060a93923eb..6609a3c1914e2 100644
--- a/doc/source/whatsnew/v1.0.0.rst
+++ b/doc/source/whatsnew/v1.0.0.rst
@@ -667,6 +667,7 @@ Categorical
   same type as if one used the :meth:`.str.` / :meth:`.dt.` on a :class:`Series` of that type. E.g. when accessing :meth:`Series.dt.tz_localize` on a
   :class:`Categorical` with duplicate entries, the accessor was skipping duplicates (:issue:`27952`)
 - Bug in :meth:`DataFrame.replace` and :meth:`Series.replace` that would give incorrect results on categorical data (:issue:`26988`)
+- Bug where calling :meth:`Categorical.min` or :meth:`Categorical.max` on an empty Categorical would raise a numpy exception.
 
 
 Datetimelike

From d788332e0953991ec88cdc6a09b1152824221982 Mon Sep 17 00:00:00 2001
From: Oliver Hofkens <oliver@novemberfive.co>
Date: Thu, 12 Dec 2019 12:27:46 +0100
Subject: [PATCH 04/16] CLN: Black test_reductions.

---
 pandas/tests/reductions/test_reductions.py | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/pandas/tests/reductions/test_reductions.py b/pandas/tests/reductions/test_reductions.py
index 2846d51b22d56..a73ad1a09d103 100644
--- a/pandas/tests/reductions/test_reductions.py
+++ b/pandas/tests/reductions/test_reductions.py
@@ -1061,9 +1061,7 @@ def test_min_max_skipna(self, skipna):
             assert np.isnan(_max)
 
     def test_min_max_empty(self):
-        cat = Series(
-            Categorical([], categories=list("ABC"), ordered=True)
-        )
+        cat = Series(Categorical([], categories=list("ABC"), ordered=True))
 
         assert isna(cat.min())
         assert isna(cat.max())

From 3ec816c0f53c7c0ba257569421606d73eeeab9e0 Mon Sep 17 00:00:00 2001
From: Oliver Hofkens <oliver@novemberfive.co>
Date: Thu, 12 Dec 2019 14:29:28 +0100
Subject: [PATCH 05/16] Added .. versionchanged and correct NA value for dtype.

---
 pandas/core/arrays/categorical.py | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/pandas/core/arrays/categorical.py b/pandas/core/arrays/categorical.py
index 41eb19d155004..c84159fd1cdee 100644
--- a/pandas/core/arrays/categorical.py
+++ b/pandas/core/arrays/categorical.py
@@ -39,7 +39,7 @@
 from pandas.core.dtypes.dtypes import CategoricalDtype
 from pandas.core.dtypes.generic import ABCIndexClass, ABCSeries
 from pandas.core.dtypes.inference import is_hashable
-from pandas.core.dtypes.missing import isna, notna
+from pandas.core.dtypes.missing import isna, na_value_for_dtype, notna
 
 from pandas._typing import ArrayLike, Dtype, Ordered
 from pandas.core import ops
@@ -2116,6 +2116,10 @@ def min(self, skipna=True):
 
         Only ordered `Categoricals` have a minimum!
 
+        .. versionchanged:: 1.0.0
+
+           Returns an NA value on empty arrays
+
         Raises
         ------
         TypeError
@@ -2128,7 +2132,7 @@ def min(self, skipna=True):
         self.check_for_ordered("min")
 
         if len(self._codes) == 0:
-            return np.nan
+            return na_value_for_dtype(self.dtype)
 
         good = self._codes != -1
         if not good.all():
@@ -2147,6 +2151,10 @@ def max(self, skipna=True):
 
         Only ordered `Categoricals` have a maximum!
 
+        .. versionchanged:: 1.0.0
+
+           Returns an NA value on empty arrays
+
         Raises
         ------
         TypeError
@@ -2159,7 +2167,7 @@ def max(self, skipna=True):
         self.check_for_ordered("max")
 
         if len(self._codes) == 0:
-            return np.nan
+            return na_value_for_dtype(self.dtype)
 
         good = self._codes != -1
         if not good.all():

From 51f21d56a2a32afb06de78339e251bab11e61ad3 Mon Sep 17 00:00:00 2001
From: Oliver Hofkens <oliver@novemberfive.co>
Date: Thu, 12 Dec 2019 14:56:24 +0100
Subject: [PATCH 06/16] DOC: Add issue number to whatsnew.

---
 doc/source/whatsnew/v1.0.0.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/doc/source/whatsnew/v1.0.0.rst b/doc/source/whatsnew/v1.0.0.rst
index 6609a3c1914e2..d4f58ec3086b0 100644
--- a/doc/source/whatsnew/v1.0.0.rst
+++ b/doc/source/whatsnew/v1.0.0.rst
@@ -667,7 +667,7 @@ Categorical
   same type as if one used the :meth:`.str.` / :meth:`.dt.` on a :class:`Series` of that type. E.g. when accessing :meth:`Series.dt.tz_localize` on a
   :class:`Categorical` with duplicate entries, the accessor was skipping duplicates (:issue:`27952`)
 - Bug in :meth:`DataFrame.replace` and :meth:`Series.replace` that would give incorrect results on categorical data (:issue:`26988`)
-- Bug where calling :meth:`Categorical.min` or :meth:`Categorical.max` on an empty Categorical would raise a numpy exception.
+- Bug where calling :meth:`Categorical.min` or :meth:`Categorical.max` on an empty Categorical would raise a numpy exception (:issue:`30227`)
 
 
 Datetimelike

From 8dee71e2aa9a83a7e8f75a95f67009ee5c16ad2a Mon Sep 17 00:00:00 2001
From: Oliver Hofkens <oliver@novemberfive.co>
Date: Thu, 12 Dec 2019 15:00:01 +0100
Subject: [PATCH 07/16] CLN: Cleaner empty checks.

---
 pandas/core/arrays/categorical.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/pandas/core/arrays/categorical.py b/pandas/core/arrays/categorical.py
index c84159fd1cdee..ba0cfad20c748 100644
--- a/pandas/core/arrays/categorical.py
+++ b/pandas/core/arrays/categorical.py
@@ -2131,7 +2131,7 @@ def min(self, skipna=True):
         """
         self.check_for_ordered("min")
 
-        if len(self._codes) == 0:
+        if not len(self._codes):
             return na_value_for_dtype(self.dtype)
 
         good = self._codes != -1
@@ -2166,7 +2166,7 @@ def max(self, skipna=True):
         """
         self.check_for_ordered("max")
 
-        if len(self._codes) == 0:
+        if not len(self._codes):
             return na_value_for_dtype(self.dtype)
 
         good = self._codes != -1

From c3f0d77b3474de6fad2a16fda24f1bdb839c8996 Mon Sep 17 00:00:00 2001
From: Oliver Hofkens <oliver@novemberfive.co>
Date: Thu, 12 Dec 2019 15:09:34 +0100
Subject: [PATCH 08/16] CLN: Move and clean up test_min_max_empty.

---
 pandas/tests/arrays/categorical/test_analytics.py | 9 +++++++++
 pandas/tests/reductions/test_reductions.py        | 6 ------
 2 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/pandas/tests/arrays/categorical/test_analytics.py b/pandas/tests/arrays/categorical/test_analytics.py
index 637a47eba0597..226033e9aebb1 100644
--- a/pandas/tests/arrays/categorical/test_analytics.py
+++ b/pandas/tests/arrays/categorical/test_analytics.py
@@ -35,6 +35,15 @@ def test_min_max(self):
         assert _min == "d"
         assert _max == "a"
 
+    def test_min_max_empty(self):
+        cat = Categorical([], categories=list("ABC"), ordered=True)
+
+        result = cat.min()
+        assert isinstance(result, float) and np.isnan(result)
+
+        result = cat.max()
+        assert isinstance(result, float) and np.isnan(result)
+
     @pytest.mark.parametrize("skipna", [True, False])
     def test_min_max_with_nan(self, skipna):
         # GH 25303
diff --git a/pandas/tests/reductions/test_reductions.py b/pandas/tests/reductions/test_reductions.py
index a73ad1a09d103..d66472b1c2054 100644
--- a/pandas/tests/reductions/test_reductions.py
+++ b/pandas/tests/reductions/test_reductions.py
@@ -1060,12 +1060,6 @@ def test_min_max_skipna(self, skipna):
             assert np.isnan(_min)
             assert np.isnan(_max)
 
-    def test_min_max_empty(self):
-        cat = Series(Categorical([], categories=list("ABC"), ordered=True))
-
-        assert isna(cat.min())
-        assert isna(cat.max())
-
 
 class TestSeriesMode:
     # Note: the name TestSeriesMode indicates these tests

From f0efeeb05b362f1a27a9943c525bca8779b0ffaa Mon Sep 17 00:00:00 2001
From: Oliver Hofkens <oliver@novemberfive.co>
Date: Thu, 12 Dec 2019 15:13:36 +0100
Subject: [PATCH 09/16] DOC: Add issue number comment to test.

---
 pandas/tests/arrays/categorical/test_analytics.py | 1 +
 1 file changed, 1 insertion(+)

diff --git a/pandas/tests/arrays/categorical/test_analytics.py b/pandas/tests/arrays/categorical/test_analytics.py
index 226033e9aebb1..3a7da69bc3edc 100644
--- a/pandas/tests/arrays/categorical/test_analytics.py
+++ b/pandas/tests/arrays/categorical/test_analytics.py
@@ -36,6 +36,7 @@ def test_min_max(self):
         assert _max == "a"
 
     def test_min_max_empty(self):
+        # GH 30227
         cat = Categorical([], categories=list("ABC"), ordered=True)
 
         result = cat.min()

From 1888238169933dc41ba2ff4fd9f9fd8cc283e1a0 Mon Sep 17 00:00:00 2001
From: Oliver Hofkens <oliver@novemberfive.co>
Date: Thu, 12 Dec 2019 15:37:40 +0100
Subject: [PATCH 10/16] TST: Parametrize test case on different categorical
 dtypes.

---
 .../tests/arrays/categorical/test_analytics.py   | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/pandas/tests/arrays/categorical/test_analytics.py b/pandas/tests/arrays/categorical/test_analytics.py
index 3a7da69bc3edc..febf37a0d0b48 100644
--- a/pandas/tests/arrays/categorical/test_analytics.py
+++ b/pandas/tests/arrays/categorical/test_analytics.py
@@ -5,7 +5,7 @@
 
 from pandas.compat import PYPY
 
-from pandas import Categorical, Index, Series
+from pandas import Categorical, Index, Series, date_range, NaT
 from pandas.api.types import is_scalar
 import pandas.util.testing as tm
 
@@ -35,15 +35,23 @@ def test_min_max(self):
         assert _min == "d"
         assert _max == "a"
 
-    def test_min_max_empty(self):
+    @pytest.mark.parametrize(
+        "categories,expected",
+        [
+            (list("ABC"), np.NaN),
+            ([1, 2, 3], np.NaN),
+            (Series(date_range("2020-01-01", periods=3), dtype="category"), NaT),
+        ],
+    )
+    def test_min_max_empty(self, categories, expected):
         # GH 30227
         cat = Categorical([], categories=list("ABC"), ordered=True)
 
         result = cat.min()
-        assert isinstance(result, float) and np.isnan(result)
+        assert result is expected
 
         result = cat.max()
-        assert isinstance(result, float) and np.isnan(result)
+        assert result is expected
 
     @pytest.mark.parametrize("skipna", [True, False])
     def test_min_max_with_nan(self, skipna):

From c74b8d095d64992b0ce808c95a1956a593159ad2 Mon Sep 17 00:00:00 2001
From: Oliver Hofkens <oliver@novemberfive.co>
Date: Thu, 12 Dec 2019 15:54:25 +0100
Subject: [PATCH 11/16] TST: Split and rename tests.

---
 pandas/tests/arrays/categorical/test_analytics.py | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/pandas/tests/arrays/categorical/test_analytics.py b/pandas/tests/arrays/categorical/test_analytics.py
index febf37a0d0b48..bcce9e3df5932 100644
--- a/pandas/tests/arrays/categorical/test_analytics.py
+++ b/pandas/tests/arrays/categorical/test_analytics.py
@@ -11,8 +11,7 @@
 
 
 class TestCategoricalAnalytics:
-    def test_min_max(self):
-
+    def test_min_max_not_ordered(self):
         # unordered cats have no min/max
         cat = Categorical(["a", "b", "c", "d"], ordered=False)
         msg = "Categorical is not ordered for operation {}"
@@ -21,6 +20,7 @@ def test_min_max(self):
         with pytest.raises(TypeError, match=msg.format("max")):
             cat.max()
 
+    def test_min_max_ordered(self):
         cat = Categorical(["a", "b", "c", "d"], ordered=True)
         _min = cat.min()
         _max = cat.max()
@@ -43,7 +43,7 @@ def test_min_max(self):
             (Series(date_range("2020-01-01", periods=3), dtype="category"), NaT),
         ],
     )
-    def test_min_max_empty(self, categories, expected):
+    def test_min_max_ordered_empty(self, categories, expected):
         # GH 30227
         cat = Categorical([], categories=list("ABC"), ordered=True)
 

From 34ad68d1bd47b83724f8304869331c11ff5760c8 Mon Sep 17 00:00:00 2001
From: Oliver Hofkens <oliver@novemberfive.co>
Date: Mon, 16 Dec 2019 12:57:25 +0100
Subject: [PATCH 12/16] TST; Fix test on datetime categorical.

---
 pandas/core/arrays/categorical.py                 | 4 ++--
 pandas/tests/arrays/categorical/test_analytics.py | 2 +-
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/pandas/core/arrays/categorical.py b/pandas/core/arrays/categorical.py
index ba0cfad20c748..5f98e66691331 100644
--- a/pandas/core/arrays/categorical.py
+++ b/pandas/core/arrays/categorical.py
@@ -2132,7 +2132,7 @@ def min(self, skipna=True):
         self.check_for_ordered("min")
 
         if not len(self._codes):
-            return na_value_for_dtype(self.dtype)
+            return self.dtype.na_value
 
         good = self._codes != -1
         if not good.all():
@@ -2167,7 +2167,7 @@ def max(self, skipna=True):
         self.check_for_ordered("max")
 
         if not len(self._codes):
-            return na_value_for_dtype(self.dtype)
+            return self.dtype.na_value
 
         good = self._codes != -1
         if not good.all():
diff --git a/pandas/tests/arrays/categorical/test_analytics.py b/pandas/tests/arrays/categorical/test_analytics.py
index bcce9e3df5932..c0e823d75fdb6 100644
--- a/pandas/tests/arrays/categorical/test_analytics.py
+++ b/pandas/tests/arrays/categorical/test_analytics.py
@@ -40,7 +40,7 @@ def test_min_max_ordered(self):
         [
             (list("ABC"), np.NaN),
             ([1, 2, 3], np.NaN),
-            (Series(date_range("2020-01-01", periods=3), dtype="category"), NaT),
+            (Series(date_range("2020-01-01", periods=3), dtype="category"), np.NaN),
         ],
     )
     def test_min_max_ordered_empty(self, categories, expected):

From d8346ee1db49ac3da86696e7f15f86ac8053a66d Mon Sep 17 00:00:00 2001
From: Oliver Hofkens <oliver@novemberfive.co>
Date: Mon, 16 Dec 2019 13:24:27 +0100
Subject: [PATCH 13/16] CLN: Fixed unused import.

---
 pandas/tests/arrays/categorical/test_analytics.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pandas/tests/arrays/categorical/test_analytics.py b/pandas/tests/arrays/categorical/test_analytics.py
index c0e823d75fdb6..91c5121f55430 100644
--- a/pandas/tests/arrays/categorical/test_analytics.py
+++ b/pandas/tests/arrays/categorical/test_analytics.py
@@ -5,7 +5,7 @@
 
 from pandas.compat import PYPY
 
-from pandas import Categorical, Index, Series, date_range, NaT
+from pandas import Categorical, Index, Series, date_range
 from pandas.api.types import is_scalar
 import pandas.util.testing as tm
 

From 4dca2d94e5c49f67998933e1d57821656aedccef Mon Sep 17 00:00:00 2001
From: Oliver Hofkens <oliver@novemberfive.co>
Date: Mon, 16 Dec 2019 15:09:21 +0100
Subject: [PATCH 14/16] CLN: Parametrize tests over min,max

---
 .../arrays/categorical/test_analytics.py      | 20 +++++++++----------
 1 file changed, 10 insertions(+), 10 deletions(-)

diff --git a/pandas/tests/arrays/categorical/test_analytics.py b/pandas/tests/arrays/categorical/test_analytics.py
index 91c5121f55430..6b066b385fe0a 100644
--- a/pandas/tests/arrays/categorical/test_analytics.py
+++ b/pandas/tests/arrays/categorical/test_analytics.py
@@ -11,14 +11,15 @@
 
 
 class TestCategoricalAnalytics:
-    def test_min_max_not_ordered(self):
+    @pytest.mark.parametrize("aggregation", ["min", "max"])
+    def test_min_max_not_ordered_raises(self, aggregation):
         # unordered cats have no min/max
         cat = Categorical(["a", "b", "c", "d"], ordered=False)
         msg = "Categorical is not ordered for operation {}"
-        with pytest.raises(TypeError, match=msg.format("min")):
-            cat.min()
-        with pytest.raises(TypeError, match=msg.format("max")):
-            cat.max()
+        agg_func = getattr(cat, aggregation)
+
+        with pytest.raises(TypeError, match=msg.format(aggregation)):
+            agg_func()
 
     def test_min_max_ordered(self):
         cat = Categorical(["a", "b", "c", "d"], ordered=True)
@@ -43,14 +44,13 @@ def test_min_max_ordered(self):
             (Series(date_range("2020-01-01", periods=3), dtype="category"), np.NaN),
         ],
     )
-    def test_min_max_ordered_empty(self, categories, expected):
+    @pytest.mark.parametrize("aggregation", ["min", "max"])
+    def test_min_max_ordered_empty(self, categories, expected, aggregation):
         # GH 30227
         cat = Categorical([], categories=list("ABC"), ordered=True)
 
-        result = cat.min()
-        assert result is expected
-
-        result = cat.max()
+        agg_func = getattr(cat, aggregation)
+        result = agg_func()
         assert result is expected
 
     @pytest.mark.parametrize("skipna", [True, False])

From 38bab518f628cbf6adac0e7e2f868e96a6b482e8 Mon Sep 17 00:00:00 2001
From: Oliver Hofkens <oliver@novemberfive.co>
Date: Mon, 16 Dec 2019 16:42:09 +0100
Subject: [PATCH 15/16] CLN: Remove unused import.

---
 pandas/core/arrays/categorical.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pandas/core/arrays/categorical.py b/pandas/core/arrays/categorical.py
index 5f98e66691331..761d9907609c3 100644
--- a/pandas/core/arrays/categorical.py
+++ b/pandas/core/arrays/categorical.py
@@ -39,7 +39,7 @@
 from pandas.core.dtypes.dtypes import CategoricalDtype
 from pandas.core.dtypes.generic import ABCIndexClass, ABCSeries
 from pandas.core.dtypes.inference import is_hashable
-from pandas.core.dtypes.missing import isna, na_value_for_dtype, notna
+from pandas.core.dtypes.missing import isna, notna
 
 from pandas._typing import ArrayLike, Dtype, Ordered
 from pandas.core import ops

From 890b27869585ad45c27dd235a43c5c58b01356ff Mon Sep 17 00:00:00 2001
From: Oliver Hofkens <oliver@novemberfive.co>
Date: Tue, 17 Dec 2019 15:32:01 +0100
Subject: [PATCH 16/16] TST: xfail datetime categorical tests.

---
 pandas/tests/arrays/categorical/test_analytics.py | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/pandas/tests/arrays/categorical/test_analytics.py b/pandas/tests/arrays/categorical/test_analytics.py
index 6b066b385fe0a..3c3d9d7a44aec 100644
--- a/pandas/tests/arrays/categorical/test_analytics.py
+++ b/pandas/tests/arrays/categorical/test_analytics.py
@@ -5,7 +5,7 @@
 
 from pandas.compat import PYPY
 
-from pandas import Categorical, Index, Series, date_range
+from pandas import Categorical, Index, NaT, Series, date_range
 from pandas.api.types import is_scalar
 import pandas.util.testing as tm
 
@@ -41,7 +41,13 @@ def test_min_max_ordered(self):
         [
             (list("ABC"), np.NaN),
             ([1, 2, 3], np.NaN),
-            (Series(date_range("2020-01-01", periods=3), dtype="category"), np.NaN),
+            pytest.param(
+                Series(date_range("2020-01-01", periods=3), dtype="category"),
+                NaT,
+                marks=pytest.mark.xfail(
+                    reason="https://github.com/pandas-dev/pandas/issues/29962"
+                ),
+            ),
         ],
     )
     @pytest.mark.parametrize("aggregation", ["min", "max"])