From b8b10b02ac2123f9d5d6f1cb1b1c1d7c30855e22 Mon Sep 17 00:00:00 2001
From: Nicholas Ver Halen <verhalenn@yahoo.com>
Date: Mon, 6 Feb 2017 18:30:34 -0600
Subject: [PATCH 1/3] Added the on arg to resample on a grouped dataframe.

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

diff --git a/pandas/core/groupby.py b/pandas/core/groupby.py
index 99220232114ce..9cdbcf5d07402 100644
--- a/pandas/core/groupby.py
+++ b/pandas/core/groupby.py
@@ -1140,13 +1140,13 @@ def ohlc(self):
 
     @Substitution(name='groupby')
     @Appender(_doc_template)
-    def resample(self, rule, *args, **kwargs):
+    def resample(self, rule, on=None, *args, **kwargs):
         """
         Provide resampling when using a TimeGrouper
         Return a new grouper with our resampler appended
         """
         from pandas.tseries.resample import get_resampler_for_grouping
-        return get_resampler_for_grouping(self, rule, *args, **kwargs)
+        return get_resampler_for_grouping(self, rule, key=on, *args, **kwargs)
 
     @Substitution(name='groupby')
     @Appender(_doc_template)

From ec1f316302a4ae2094200aedeb53e386fe3dc1a6 Mon Sep 17 00:00:00 2001
From: Nicholas Ver Halen <verhalenn@yahoo.com>
Date: Mon, 6 Feb 2017 19:56:43 -0600
Subject: [PATCH 2/3] Created a test for GH 15021

---
 pandas/tseries/tests/test_resample.py | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/pandas/tseries/tests/test_resample.py b/pandas/tseries/tests/test_resample.py
index 56953541265a6..193dec44c0d7e 100755
--- a/pandas/tseries/tests/test_resample.py
+++ b/pandas/tseries/tests/test_resample.py
@@ -217,6 +217,20 @@ def test_groupby_resample_api(self):
             lambda x: x.resample('1D').ffill())[['val']]
         assert_frame_equal(result, expected)
 
+    def test_grouby_resample_on_api(self):
+
+        # GH 15021
+        # .groupby(...).resample(on=...) results in an unexpected
+        # keyword warning.
+        df = pd.DataFrame({'key': ['A', 'B'] * 5,
+                           'dates': pd.date_range('2016-01-01', periods=10),
+                           'values': np.random.randn(10)})
+
+        expected = df.set_index('dates').groupby('key').resample('D').mean()
+
+        result = df.groupby('key').resample('D', on='dates').mean()
+        assert_frame_equal(result, expected)
+
     def test_plot_api(self):
         tm._skip_if_no_mpl()
 

From 9fc3b4fa45112c1eb69197bb4d9c553546ea5a78 Mon Sep 17 00:00:00 2001
From: Nicholas Ver Halen <verhalenn@yahoo.com>
Date: Mon, 6 Feb 2017 20:48:42 -0600
Subject: [PATCH 3/3] Updated the whatsnew for issue 15021

---
 doc/source/whatsnew/v0.20.0.txt | 1 +
 1 file changed, 1 insertion(+)

diff --git a/doc/source/whatsnew/v0.20.0.txt b/doc/source/whatsnew/v0.20.0.txt
index 1a32498d53c23..1c00c18a24717 100644
--- a/doc/source/whatsnew/v0.20.0.txt
+++ b/doc/source/whatsnew/v0.20.0.txt
@@ -149,6 +149,7 @@ Other enhancements
 - ``Series/DataFrame.asfreq()`` have gained a ``fill_value`` parameter, to fill missing values (:issue:`3715`).
 - ``Series/DataFrame.resample.asfreq`` have gained a ``fill_value`` parameter, to fill missing values during resampling (:issue:`3715`).
 - ``pandas.tools.hashing`` has gained a ``hash_tuples`` routine, and ``hash_pandas_object`` has gained the ability to hash a ``MultiIndex`` (:issue:`15224`)
+- ``groupby(..).resample()`` is now able to use the ``on`` argument. (:issue:`15021`)
 
 .. _ISO 8601 duration: https://en.wikipedia.org/wiki/ISO_8601#Durations