From f0ce12cf553a49565b7d2879472a8eae88706cfa Mon Sep 17 00:00:00 2001
From: Adam Hooper <adam@adamhooper.com>
Date: Tue, 16 Apr 2019 17:48:13 -0400
Subject: [PATCH 1/2] BUG: GroupBy.size with all-null data raises ValueError

---
 doc/source/whatsnew/v0.25.0.rst       | 1 +
 pandas/core/groupby/ops.py            | 2 +-
 pandas/tests/groupby/test_function.py | 6 ++++++
 3 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/doc/source/whatsnew/v0.25.0.rst b/doc/source/whatsnew/v0.25.0.rst
index da712f84eb1b5..5f6483ede5544 100644
--- a/doc/source/whatsnew/v0.25.0.rst
+++ b/doc/source/whatsnew/v0.25.0.rst
@@ -381,6 +381,7 @@ Groupby/Resample/Rolling
 - Bug in :meth:`pandas.core.groupby.DataFrameGroupBy.nunique` in which the names of column levels were lost (:issue:`23222`)
 - Bug in :func:`pandas.core.groupby.GroupBy.agg` when applying a aggregation function to timezone aware data (:issue:`23683`)
 - Bug in :func:`pandas.core.groupby.GroupBy.first` and :func:`pandas.core.groupby.GroupBy.last` where timezone information would be dropped (:issue:`21603`)
+- Bug in :func:`pandas.core.groupby.GroupBy.size` when grouping only NA values (:issue:`23050`)
 - Bug in :func:`Series.groupby` where using ``groupby`` with a :class:`MultiIndex` Series with a list of labels equal to the length of the series caused incorrect grouping (:issue:`25704`)
 - Ensured that ordering of outputs in ``groupby`` aggregation functions is consistent across all versions of Python (:issue:`25692`)
 - Ensured that result group order is correct when grouping on an ordered ``Categorical`` and specifying ``observed=True`` (:issue:`25871`, :issue:`25167`)
diff --git a/pandas/core/groupby/ops.py b/pandas/core/groupby/ops.py
index ec22548de6da3..8a6ec285cc79e 100644
--- a/pandas/core/groupby/ops.py
+++ b/pandas/core/groupby/ops.py
@@ -246,7 +246,7 @@ def size(self):
         if ngroup:
             out = np.bincount(ids[ids != -1], minlength=ngroup)
         else:
-            out = ids
+            out = []
         return Series(out,
                       index=self.result_index,
                       dtype='int64')
diff --git a/pandas/tests/groupby/test_function.py b/pandas/tests/groupby/test_function.py
index 187fea5403aea..2909f8e840029 100644
--- a/pandas/tests/groupby/test_function.py
+++ b/pandas/tests/groupby/test_function.py
@@ -1089,6 +1089,12 @@ def test_size(df):
     out = Series(dtype='int64', index=Index([], name='A'))
     tm.assert_series_equal(df.groupby('A').size(), out)
 
+    # GH23050
+    # Assert no 'Value Error : Length of passed values is 2, index implies 0'
+    df = DataFrame({'A': [None, None]})  # all-null groups
+    out = Series(dtype='int64', index=Index([], name='A'))
+    tm.assert_series_equal(df.groupby('A').size(), out)
+
 
 # quantile
 # --------------------------------

From 3f9f66138c858e3ad6bf2d49624384039dcdbbec Mon Sep 17 00:00:00 2001
From: Adam Hooper <adam@adamhooper.com>
Date: Tue, 16 Apr 2019 22:21:58 -0400
Subject: [PATCH 2/2] Move bug-23050 test to separate function

---
 pandas/tests/groupby/test_function.py | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/pandas/tests/groupby/test_function.py b/pandas/tests/groupby/test_function.py
index 2909f8e840029..7ca483e12b332 100644
--- a/pandas/tests/groupby/test_function.py
+++ b/pandas/tests/groupby/test_function.py
@@ -1089,11 +1089,14 @@ def test_size(df):
     out = Series(dtype='int64', index=Index([], name='A'))
     tm.assert_series_equal(df.groupby('A').size(), out)
 
+
+def test_size_groupby_all_null():
     # GH23050
     # Assert no 'Value Error : Length of passed values is 2, index implies 0'
     df = DataFrame({'A': [None, None]})  # all-null groups
-    out = Series(dtype='int64', index=Index([], name='A'))
-    tm.assert_series_equal(df.groupby('A').size(), out)
+    result = df.groupby('A').size()
+    expected = Series(dtype='int64', index=Index([], name='A'))
+    tm.assert_series_equal(result, expected)
 
 
 # quantile