From 44aca56effaceb7a424ffb9cd8d9ddd590c01267 Mon Sep 17 00:00:00 2001
From: HHest <3169669+HHest@users.noreply.github.com>
Date: Sat, 27 Apr 2019 18:16:10 +0200
Subject: [PATCH 1/9] Fix .transform crash when SeriesGroupBy is empty
 (pandas-dev#26208)

---
 pandas/core/groupby/generic.py         |  7 +++++--
 pandas/tests/groupby/test_transform.py | 10 ++++++++++
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/pandas/core/groupby/generic.py b/pandas/core/groupby/generic.py
index f8b9ddce6000e..eeaa3f908893c 100644
--- a/pandas/core/groupby/generic.py
+++ b/pandas/core/groupby/generic.py
@@ -917,8 +917,11 @@ def transform(self, func, *args, **kwargs):
             s = klass(res, indexer)
             results.append(s)
 
-        from pandas.core.reshape.concat import concat
-        result = concat(results).sort_index()
+        if results:
+            from pandas.core.reshape.concat import concat
+            result = concat(results).sort_index()
+        else:
+            result = Series([])
 
         # we will only try to coerce the result type if
         # we have a numeric dtype, as these are *always* udfs
diff --git a/pandas/tests/groupby/test_transform.py b/pandas/tests/groupby/test_transform.py
index e330329644269..fb6fa0aecc7bb 100644
--- a/pandas/tests/groupby/test_transform.py
+++ b/pandas/tests/groupby/test_transform.py
@@ -880,3 +880,13 @@ def test_transform_absent_categories(func):
     result = getattr(df.y.groupby(df.x), func)()
     expected = df.y
     assert_series_equal(result, expected)
+
+
+def test_transform_empty_df():
+    # 26208
+    # handle empty SeriesGroupBy
+    d = pd.DataFrame({1: [], 2: []})
+    g = d.groupby(1)
+    result = g[2].transform(lambda x: x)
+    expected = pd.Series([], name=2)
+    assert_series_equal(result, expected)

From cbffeb68c65c83ebb8e5221c0cefcccc6f555541 Mon Sep 17 00:00:00 2001
From: HHest <3169669+HHest@users.noreply.github.com>
Date: Sat, 27 Apr 2019 18:16:10 +0200
Subject: [PATCH 2/9] Fix .transform crash when SeriesGroupBy is empty
 (pandas-dev#26208)

---
 pandas/core/groupby/generic.py         |  7 +++++--
 pandas/tests/groupby/test_transform.py | 10 ++++++++++
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/pandas/core/groupby/generic.py b/pandas/core/groupby/generic.py
index f8b9ddce6000e..eeaa3f908893c 100644
--- a/pandas/core/groupby/generic.py
+++ b/pandas/core/groupby/generic.py
@@ -917,8 +917,11 @@ def transform(self, func, *args, **kwargs):
             s = klass(res, indexer)
             results.append(s)
 
-        from pandas.core.reshape.concat import concat
-        result = concat(results).sort_index()
+        if results:
+            from pandas.core.reshape.concat import concat
+            result = concat(results).sort_index()
+        else:
+            result = Series([])
 
         # we will only try to coerce the result type if
         # we have a numeric dtype, as these are *always* udfs
diff --git a/pandas/tests/groupby/test_transform.py b/pandas/tests/groupby/test_transform.py
index e330329644269..fb6fa0aecc7bb 100644
--- a/pandas/tests/groupby/test_transform.py
+++ b/pandas/tests/groupby/test_transform.py
@@ -880,3 +880,13 @@ def test_transform_absent_categories(func):
     result = getattr(df.y.groupby(df.x), func)()
     expected = df.y
     assert_series_equal(result, expected)
+
+
+def test_transform_empty_df():
+    # 26208
+    # handle empty SeriesGroupBy
+    d = pd.DataFrame({1: [], 2: []})
+    g = d.groupby(1)
+    result = g[2].transform(lambda x: x)
+    expected = pd.Series([], name=2)
+    assert_series_equal(result, expected)

From d86d07bd8dcc3531eb81eddb9cd03e00742f39ec Mon Sep 17 00:00:00 2001
From: HHest <3169669+HHest@users.noreply.github.com>
Date: Sat, 27 Apr 2019 19:55:10 +0200
Subject: [PATCH 3/9] whats new for issue #26208

---
 doc/source/whatsnew/v0.25.0.rst | 1 +
 1 file changed, 1 insertion(+)

diff --git a/doc/source/whatsnew/v0.25.0.rst b/doc/source/whatsnew/v0.25.0.rst
index 485a0a06fc247..56777f23698c7 100644
--- a/doc/source/whatsnew/v0.25.0.rst
+++ b/doc/source/whatsnew/v0.25.0.rst
@@ -391,6 +391,7 @@ Groupby/Resample/Rolling
 - Bug in :meth:`pandas.core.window.Rolling.min` and :meth:`pandas.core.window.Rolling.max` that caused a memory leak (:issue:`25893`)
 - Bug in :meth:`pandas.core.groupby.GroupBy.idxmax` and :meth:`pandas.core.groupby.GroupBy.idxmin` with datetime column would return incorrect dtype (:issue:`25444`, :issue:`15306`)
 - Bug in :meth:`pandas.core.groupby.GroupBy.cumsum`, :meth:`pandas.core.groupby.GroupBy.cumprod`, :meth:`pandas.core.groupby.GroupBy.cummin` and :meth:`pandas.core.groupby.GroupBy.cummax` with categorical column having absent categories, would return incorrect result or segfault (:issue:`16771`)
+- Bug in :meth:`pandas.core.groupby.SeriesGroupBy.transform` where transforming an empty SeriesGroupBy would raise error. Now returns empty Series, just like .apply and .agg. (:issue:`26208`)
 
 
 Reshaping

From c85491db5c0c53f147f6b6329ce6dfbd63e169fe Mon Sep 17 00:00:00 2001
From: HHest <3169669+HHest@users.noreply.github.com>
Date: Fri, 3 May 2019 23:11:11 +0200
Subject: [PATCH 4/9] Simplify what's new comment

---
 doc/source/whatsnew/v0.25.0.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/doc/source/whatsnew/v0.25.0.rst b/doc/source/whatsnew/v0.25.0.rst
index 56777f23698c7..e4bf729d44138 100644
--- a/doc/source/whatsnew/v0.25.0.rst
+++ b/doc/source/whatsnew/v0.25.0.rst
@@ -391,7 +391,7 @@ Groupby/Resample/Rolling
 - Bug in :meth:`pandas.core.window.Rolling.min` and :meth:`pandas.core.window.Rolling.max` that caused a memory leak (:issue:`25893`)
 - Bug in :meth:`pandas.core.groupby.GroupBy.idxmax` and :meth:`pandas.core.groupby.GroupBy.idxmin` with datetime column would return incorrect dtype (:issue:`25444`, :issue:`15306`)
 - Bug in :meth:`pandas.core.groupby.GroupBy.cumsum`, :meth:`pandas.core.groupby.GroupBy.cumprod`, :meth:`pandas.core.groupby.GroupBy.cummin` and :meth:`pandas.core.groupby.GroupBy.cummax` with categorical column having absent categories, would return incorrect result or segfault (:issue:`16771`)
-- Bug in :meth:`pandas.core.groupby.SeriesGroupBy.transform` where transforming an empty SeriesGroupBy would raise error. Now returns empty Series, just like .apply and .agg. (:issue:`26208`)
+- Bug in :meth:`pandas.core.groupby.SeriesGroupBy.transform` where transforming an empty group would raise error.
 
 
 Reshaping

From 9b5c16a08ab5331bb84378c2eb97355c6d7d08ea Mon Sep 17 00:00:00 2001
From: HHest <3169669+HHest@users.noreply.github.com>
Date: Fri, 3 May 2019 23:22:26 +0200
Subject: [PATCH 5/9] Add comment to explain change

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

diff --git a/pandas/core/groupby/generic.py b/pandas/core/groupby/generic.py
index eeaa3f908893c..0a0b672f3360f 100644
--- a/pandas/core/groupby/generic.py
+++ b/pandas/core/groupby/generic.py
@@ -917,7 +917,7 @@ def transform(self, func, *args, **kwargs):
             s = klass(res, indexer)
             results.append(s)
 
-        if results:
+        if results:  # otherwise concat raises on empty results
             from pandas.core.reshape.concat import concat
             result = concat(results).sort_index()
         else:

From cf92a15d21f1d539b18d022050b20aea400c65cd Mon Sep 17 00:00:00 2001
From: HHest <3169669+HHest@users.noreply.github.com>
Date: Sun, 12 May 2019 22:10:07 +0200
Subject: [PATCH 6/9] Update fixes according to requests by reviewer.

---
 doc/source/whatsnew/v0.25.0.rst        |  2 +-
 pandas/core/groupby/generic.py         |  3 ++-
 pandas/tests/groupby/test_grouping.py  | 11 +++++++++++
 pandas/tests/groupby/test_transform.py | 10 ----------
 4 files changed, 14 insertions(+), 12 deletions(-)

diff --git a/doc/source/whatsnew/v0.25.0.rst b/doc/source/whatsnew/v0.25.0.rst
index e4bf729d44138..a6127c91a7a25 100644
--- a/doc/source/whatsnew/v0.25.0.rst
+++ b/doc/source/whatsnew/v0.25.0.rst
@@ -391,7 +391,7 @@ Groupby/Resample/Rolling
 - Bug in :meth:`pandas.core.window.Rolling.min` and :meth:`pandas.core.window.Rolling.max` that caused a memory leak (:issue:`25893`)
 - Bug in :meth:`pandas.core.groupby.GroupBy.idxmax` and :meth:`pandas.core.groupby.GroupBy.idxmin` with datetime column would return incorrect dtype (:issue:`25444`, :issue:`15306`)
 - Bug in :meth:`pandas.core.groupby.GroupBy.cumsum`, :meth:`pandas.core.groupby.GroupBy.cumprod`, :meth:`pandas.core.groupby.GroupBy.cummin` and :meth:`pandas.core.groupby.GroupBy.cummax` with categorical column having absent categories, would return incorrect result or segfault (:issue:`16771`)
-- Bug in :meth:`pandas.core.groupby.SeriesGroupBy.transform` where transforming an empty group would raise error.
+- Bug in :meth:`pandas.core.groupby.SeriesGroupBy.transform` where transforming an empty group would raise error (:issue:`26208`)
 
 
 Reshaping
diff --git a/pandas/core/groupby/generic.py b/pandas/core/groupby/generic.py
index 0a0b672f3360f..f1681062a5630 100644
--- a/pandas/core/groupby/generic.py
+++ b/pandas/core/groupby/generic.py
@@ -917,7 +917,8 @@ def transform(self, func, *args, **kwargs):
             s = klass(res, indexer)
             results.append(s)
 
-        if results:  # otherwise concat raises on empty results
+        # check for empty "results" to avoid concat ValueError
+        if results:
             from pandas.core.reshape.concat import concat
             result = concat(results).sort_index()
         else:
diff --git a/pandas/tests/groupby/test_grouping.py b/pandas/tests/groupby/test_grouping.py
index 013c370899de1..13c1ceb5eee20 100644
--- a/pandas/tests/groupby/test_grouping.py
+++ b/pandas/tests/groupby/test_grouping.py
@@ -556,6 +556,17 @@ def test_list_grouper_with_nat(self):
         expected = {pd.Timestamp('2011-01-01'): 365}
         tm.assert_dict_equal(result.groups, expected)
 
+    @pytest.mark.parametrize('func', ['transform'])
+    def test_evaluate_with_empty_groups(self, func):
+        # 26208
+        # test transform'ing empty groups
+        # (not testing other agg fns, because they return
+        # different index objects.
+        d = pd.DataFrame({1: [], 2: []})
+        g = d.groupby(1)
+        result = getattr(g[2], func)(lambda x: x)
+        expected = pd.Series([], name=2)
+        assert_series_equal(result, expected)
 
 # get_group
 # --------------------------------
diff --git a/pandas/tests/groupby/test_transform.py b/pandas/tests/groupby/test_transform.py
index fb6fa0aecc7bb..e330329644269 100644
--- a/pandas/tests/groupby/test_transform.py
+++ b/pandas/tests/groupby/test_transform.py
@@ -880,13 +880,3 @@ def test_transform_absent_categories(func):
     result = getattr(df.y.groupby(df.x), func)()
     expected = df.y
     assert_series_equal(result, expected)
-
-
-def test_transform_empty_df():
-    # 26208
-    # handle empty SeriesGroupBy
-    d = pd.DataFrame({1: [], 2: []})
-    g = d.groupby(1)
-    result = g[2].transform(lambda x: x)
-    expected = pd.Series([], name=2)
-    assert_series_equal(result, expected)

From 61b652c61deb4f7cf1d39844be01e8a2436a1048 Mon Sep 17 00:00:00 2001
From: HHest <3169669+HHest@users.noreply.github.com>
Date: Tue, 14 May 2019 18:48:23 +0200
Subject: [PATCH 7/9] Add other test cases, per reviewer req.

---
 pandas/tests/groupby/test_grouping.py | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/pandas/tests/groupby/test_grouping.py b/pandas/tests/groupby/test_grouping.py
index 6924eede41c12..b3a4181c79456 100644
--- a/pandas/tests/groupby/test_grouping.py
+++ b/pandas/tests/groupby/test_grouping.py
@@ -552,18 +552,22 @@ def test_list_grouper_with_nat(self):
         expected = {pd.Timestamp('2011-01-01'): 365}
         tm.assert_dict_equal(result.groups, expected)
 
-    @pytest.mark.parametrize('func', ['transform'])
-    def test_evaluate_with_empty_groups(self, func):
+    @pytest.mark.parametrize('func,expected', [
+            ('transform', pd.Series([], name=2, index=pd.RangeIndex(0, 0, 1))),
+            ('agg', pd.Series([], name=2, index=pd.Float64Index([], name=1))),
+            ('apply', pd.Series([], name=2, index=pd.Float64Index([], name=1))),
+        ])
+    def test_evaluate_with_empty_groups(self, func, expected):
         # 26208
         # test transform'ing empty groups
         # (not testing other agg fns, because they return
         # different index objects.
-        d = pd.DataFrame({1: [], 2: []})
-        g = d.groupby(1)
+        df = pd.DataFrame({1: [], 2: []})
+        g = df.groupby(1)
         result = getattr(g[2], func)(lambda x: x)
-        expected = pd.Series([], name=2)
         assert_series_equal(result, expected)
 
+
 # get_group
 # --------------------------------
 

From bbbc7f4915edb6474bcc14ee5308428cb84f4f64 Mon Sep 17 00:00:00 2001
From: HHest <3169669+HHest@users.noreply.github.com>
Date: Tue, 14 May 2019 18:53:26 +0200
Subject: [PATCH 8/9] fix pep8

---
 pandas/tests/groupby/test_grouping.py | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/pandas/tests/groupby/test_grouping.py b/pandas/tests/groupby/test_grouping.py
index b3a4181c79456..72b795f2c249d 100644
--- a/pandas/tests/groupby/test_grouping.py
+++ b/pandas/tests/groupby/test_grouping.py
@@ -552,7 +552,9 @@ def test_list_grouper_with_nat(self):
         expected = {pd.Timestamp('2011-01-01'): 365}
         tm.assert_dict_equal(result.groups, expected)
 
-    @pytest.mark.parametrize('func,expected', [
+    @pytest.mark.parametrize(
+        'func,expected',
+        [
             ('transform', pd.Series([], name=2, index=pd.RangeIndex(0, 0, 1))),
             ('agg', pd.Series([], name=2, index=pd.Float64Index([], name=1))),
             ('apply', pd.Series([], name=2, index=pd.Float64Index([], name=1))),

From 807b7ccf664e7adfca07eed8db0fbac775d09f2b Mon Sep 17 00:00:00 2001
From: HHest <3169669+HHest@users.noreply.github.com>
Date: Tue, 14 May 2019 19:10:27 +0200
Subject: [PATCH 9/9] Remove [] when creating empty Series, per reviewer.

---
 pandas/core/groupby/generic.py        | 2 +-
 pandas/tests/groupby/test_grouping.py | 6 +++---
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/pandas/core/groupby/generic.py b/pandas/core/groupby/generic.py
index f72d27f6b9b51..f4c53151bddc1 100644
--- a/pandas/core/groupby/generic.py
+++ b/pandas/core/groupby/generic.py
@@ -921,7 +921,7 @@ def transform(self, func, *args, **kwargs):
             from pandas.core.reshape.concat import concat
             result = concat(results).sort_index()
         else:
-            result = Series([])
+            result = Series()
 
         # we will only try to coerce the result type if
         # we have a numeric dtype, as these are *always* udfs
diff --git a/pandas/tests/groupby/test_grouping.py b/pandas/tests/groupby/test_grouping.py
index 72b795f2c249d..4c84c29ff98cb 100644
--- a/pandas/tests/groupby/test_grouping.py
+++ b/pandas/tests/groupby/test_grouping.py
@@ -555,9 +555,9 @@ def test_list_grouper_with_nat(self):
     @pytest.mark.parametrize(
         'func,expected',
         [
-            ('transform', pd.Series([], name=2, index=pd.RangeIndex(0, 0, 1))),
-            ('agg', pd.Series([], name=2, index=pd.Float64Index([], name=1))),
-            ('apply', pd.Series([], name=2, index=pd.Float64Index([], name=1))),
+            ('transform', pd.Series(name=2, index=pd.RangeIndex(0, 0, 1))),
+            ('agg', pd.Series(name=2, index=pd.Float64Index([], name=1))),
+            ('apply', pd.Series(name=2, index=pd.Float64Index([], name=1))),
         ])
     def test_evaluate_with_empty_groups(self, func, expected):
         # 26208