Fixed bug in df.resample using 'on' parameter

Author: discort

doc/source/whatsnew/v0.23.0.txt

@@ -515,7 +515,7 @@ Groupby/Resample/Rolling
 - Bug in :func:`DataFrame.resample` which silently ignored unsupported (or mistyped) options for ``label``, ``closed`` and ``convention`` (:issue:`19303`)
 - Bug in :func:`DataFrame.groupby` where tuples were interpreted as lists of keys rather than as keys (:issue:`17979`, :issue:`18249`)
 - Bug in ``transform`` where particular aggregation functions were being incorrectly cast to match the dtype(s) of the grouped data (:issue:`19200`)
--
+- Bug in `DataFrame.groupby` with resample using `on` parameter when selecting column to apply function (:issue:`17813`)
 
 Sparse
 ^^^^^^

pandas/core/groupby.py

@@ -466,7 +466,7 @@ def _set_grouper(self, obj, sort=False):
                 "The Grouper cannot specify both a key and a level!")
 
         # the key must be a valid info item
-        if self.key is not None:
+        if self.key is not None and not isinstance(obj, Series):
             key = self.key
             if key not in obj._info_axis:
                 raise KeyError("The grouper name {0} is not found".format(key))

pandas/core/resample.py

@@ -12,6 +12,7 @@
                                  _pipe_template)
 
 from pandas.tseries.frequencies import to_offset, is_subperiod, is_superperiod
+from pandas.core.index import Index
 from pandas.core.indexes.datetimes import DatetimeIndex, date_range
 from pandas.core.indexes.timedeltas import TimedeltaIndex
 from pandas.tseries.offsets import DateOffset, Tick, Day, delta_to_nanoseconds
@@ -742,6 +743,11 @@ def func(x):
 
             return x.apply(f, **kwargs)
 
+        # Patch the index in case we're applying after __getitem__. GH 17813
+        if self.groupby.key:
+            key = self.groupby.key
+            self._groupby.obj.index = Index(self.groupby.obj[key], key=key)
+
         result = self._groupby.apply(func)
         return self._wrap_result(result)
 

pandas/tests/test_resample.py

@@ -252,6 +252,15 @@ def test_pipe(self):
         result = r.pipe(lambda x: x.max() - x.mean())
         tm.assert_frame_equal(result, expected)
 
+    def test_groupby_resample_on_api_with_getitem(self):
+        # GH 17813
+        df = pd.DataFrame({'id': list('aabbb'),
+                           'date': pd.date_range('1-1-2016', periods=5),
+                           'data': 1})
+        exp = df.set_index('date').groupby('id').resample('2D')['data'].sum()
+        result = df.groupby('id').resample('2D', on='date')['data'].sum()
+        assert_series_equal(result, exp)
+
     @td.skip_if_no_mpl
     def test_plot_api(self):
         # .resample(....).plot(...)