From e4d67354aa0cc0404da22d6d4c79b2c7d0b354c7 Mon Sep 17 00:00:00 2001
From: phofl <patrick_hoefler@gmx.net>
Date: Mon, 7 Sep 2020 21:23:52 +0200
Subject: [PATCH 1/9] Bug in groupby and resample miscalculatet aggregation or
 raised error if no RangeIndex was used

---
 doc/source/whatsnew/v1.2.0.rst                |  2 +-
 pandas/core/groupby/grouper.py                | 14 +++++
 .../tests/resample/test_resampler_grouper.py  | 62 +++++++++++++++++++
 3 files changed, 77 insertions(+), 1 deletion(-)

diff --git a/doc/source/whatsnew/v1.2.0.rst b/doc/source/whatsnew/v1.2.0.rst
index b1229a5d5823d..767609eec80ac 100644
--- a/doc/source/whatsnew/v1.2.0.rst
+++ b/doc/source/whatsnew/v1.2.0.rst
@@ -314,7 +314,7 @@ Groupby/resample/rolling
 - Bug when subsetting columns on a :class:`~pandas.core.groupby.DataFrameGroupBy` (e.g. ``df.groupby('a')[['b']])``) would reset the attributes ``axis``, ``dropna``, ``group_keys``, ``level``, ``mutated``, ``sort``, and ``squeeze`` to their default values. (:issue:`9959`)
 - Bug in :meth:`DataFrameGroupby.tshift` failing to raise ``ValueError`` when a frequency cannot be inferred for the index of a group (:issue:`35937`)
 - Bug in :meth:`DataFrame.groupby` does not always maintain column index name for ``any``, ``all``, ``bfill``, ``ffill``, ``shift`` (:issue:`29764`)
--
+- Bug when combining methods :meth:`DataFrame.groupby` with :meth:`DataFrame.resample` and restricting to `Series` or using `agg` did miscalculate the aggregation (:issue:`27343`, :issue:`33548`, :issue:`35275`).
 
 Reshaping
 ^^^^^^^^^
diff --git a/pandas/core/groupby/grouper.py b/pandas/core/groupby/grouper.py
index 59ea7781025c4..0d2a744afa7da 100644
--- a/pandas/core/groupby/grouper.py
+++ b/pandas/core/groupby/grouper.py
@@ -338,6 +338,20 @@ def _set_grouper(self, obj: FrameOrSeries, sort: bool = False):
             if getattr(self.grouper, "name", None) == key and isinstance(
                 obj, ABCSeries
             ):
+                # In some cases the self._grouper may be sorted differently than obj.
+                # self.obj has the right order with the old index in the first go around.
+                # We align the index from obj with the self.obj index to select the correct
+                # values. Additionally we have to sort obj.index correctly to aggregate
+                # correctly.
+                if not hasattr(self, "_index_mapping"):
+                    self._index_mapping = DataFrame(
+                        self.obj.index, columns=["index"]
+                    ).sort_values(by="index")
+                obj.sort_index(inplace=True)
+                obj.index = self._index_mapping.loc[
+                    self._index_mapping["index"].isin(obj.index)
+                ].index
+                obj.sort_index(inplace=True)
                 ax = self._grouper.take(obj.index)
             else:
                 if key not in obj._info_axis:
diff --git a/pandas/tests/resample/test_resampler_grouper.py b/pandas/tests/resample/test_resampler_grouper.py
index 73bf7dafac254..d95deaad2b36e 100644
--- a/pandas/tests/resample/test_resampler_grouper.py
+++ b/pandas/tests/resample/test_resampler_grouper.py
@@ -347,3 +347,65 @@ def test_median_duplicate_columns():
     result = df.resample("5s").median()
     expected.columns = result.columns
     tm.assert_frame_equal(result, expected)
+
+
+def test_resample_different_result_with_agg():
+    data = pd.DataFrame(
+        {
+            "cat": ["cat1", "cat1", "cat2", "cat1", "cat2", "cat1", "cat2", "cat1"],
+            "num": [5, 20, 22, 3, 4, 30, 10, 50],
+            "date": [
+                "2019-2-1",
+                "2018-02-03",
+                "2020-3-11",
+                "2019-2-2",
+                "2019-2-2",
+                "2018-12-4",
+                "2020-3-11",
+                "2020-12-12",
+            ],
+        }
+    )
+    data["date"] = pd.to_datetime(data["date"])
+
+    resampled = data.groupby("cat").resample("Y", on="date")
+
+    index = pd.MultiIndex.from_tuples(
+        [
+            ("cat1", "2018-12-31"),
+            ("cat1", "2019-12-31"),
+            ("cat1", "2020-12-31"),
+            ("cat2", "2019-12-31"),
+            ("cat2", "2020-12-31"),
+        ],
+        names=["cat", "date"],
+    )
+    index = index.set_levels([index.levels[0], pd.to_datetime(index.levels[1])])
+    expected = DataFrame([25, 4, 50, 4, 16], columns=pd.Index(["num"]), index=index)
+    result = resampled.agg({"num": "mean"})
+    tm.assert_frame_equal(result, expected)
+    result = resampled["num"].mean()
+    tm.assert_series_equal(result, expected["num"])
+    result = resampled.mean()
+    tm.assert_frame_equal(result, expected)
+
+
+def test_resample_agg_different_results_on_keyword():
+    df = pd.DataFrame.from_records(
+        {
+            "ref": ["a", "a", "a", "b", "b"],
+            "time": [
+                "2014-12-31",
+                "2015-12-31",
+                "2016-12-31",
+                "2012-12-31",
+                "2014-12-31",
+            ],
+            "value": 5 * [1],
+        }
+    )
+    df["time"] = pd.to_datetime(df["time"])
+
+    expected = df.set_index("time").groupby("ref").resample(rule="M")["value"].sum()
+    result = df.groupby("ref").resample(rule="M", on="time")["value"].sum()
+    tm.assert_series_equal(result, expected)

From bc127f233212c3d06335da277b435b2eca3de594 Mon Sep 17 00:00:00 2001
From: phofl <patrick_hoefler@gmx.net>
Date: Mon, 7 Sep 2020 21:32:30 +0200
Subject: [PATCH 2/9] Fix Pep8 issues

---
 pandas/core/groupby/grouper.py | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/pandas/core/groupby/grouper.py b/pandas/core/groupby/grouper.py
index 0d2a744afa7da..0f050a4c10893 100644
--- a/pandas/core/groupby/grouper.py
+++ b/pandas/core/groupby/grouper.py
@@ -339,10 +339,10 @@ def _set_grouper(self, obj: FrameOrSeries, sort: bool = False):
                 obj, ABCSeries
             ):
                 # In some cases the self._grouper may be sorted differently than obj.
-                # self.obj has the right order with the old index in the first go around.
-                # We align the index from obj with the self.obj index to select the correct
-                # values. Additionally we have to sort obj.index correctly to aggregate
-                # correctly.
+                # self.obj has the right order with the old index in the first go
+                # around. We align the index from obj with the self.obj index to
+                # select the correct values. Additionally we have to sort
+                # obj.index correctly to aggregate correctly.
                 if not hasattr(self, "_index_mapping"):
                     self._index_mapping = DataFrame(
                         self.obj.index, columns=["index"]

From fcc11e7adb1a5e63d7a04bf23b9f55e629617018 Mon Sep 17 00:00:00 2001
From: phofl <patrick_hoefler@gmx.net>
Date: Mon, 7 Sep 2020 21:41:40 +0200
Subject: [PATCH 3/9] Add gh numbers to tests

---
 pandas/tests/resample/test_resampler_grouper.py | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/pandas/tests/resample/test_resampler_grouper.py b/pandas/tests/resample/test_resampler_grouper.py
index d95deaad2b36e..e90052eb941c1 100644
--- a/pandas/tests/resample/test_resampler_grouper.py
+++ b/pandas/tests/resample/test_resampler_grouper.py
@@ -350,6 +350,7 @@ def test_median_duplicate_columns():
 
 
 def test_resample_different_result_with_agg():
+    # GH: 35275 and 33548
     data = pd.DataFrame(
         {
             "cat": ["cat1", "cat1", "cat2", "cat1", "cat2", "cat1", "cat2", "cat1"],
@@ -391,6 +392,7 @@ def test_resample_different_result_with_agg():
 
 
 def test_resample_agg_different_results_on_keyword():
+    # GH: 27343
     df = pd.DataFrame.from_records(
         {
             "ref": ["a", "a", "a", "b", "b"],

From d76f1c517f5e5c59e59432080358c020853ad967 Mon Sep 17 00:00:00 2001
From: phofl <patrick_hoefler@gmx.net>
Date: Tue, 8 Sep 2020 16:18:28 +0200
Subject: [PATCH 4/9] Use take on indexer based on group indices

---
 pandas/core/groupby/grouper.py | 26 +++++++++++++++-----------
 pandas/core/resample.py        |  4 ++--
 2 files changed, 17 insertions(+), 13 deletions(-)

diff --git a/pandas/core/groupby/grouper.py b/pandas/core/groupby/grouper.py
index 0f050a4c10893..959432bed7716 100644
--- a/pandas/core/groupby/grouper.py
+++ b/pandas/core/groupby/grouper.py
@@ -311,7 +311,7 @@ def _get_grouper(self, obj, validate: bool = True):
         )
         return self.binner, self.grouper, self.obj
 
-    def _set_grouper(self, obj: FrameOrSeries, sort: bool = False):
+    def _set_grouper(self, obj: FrameOrSeries, sort: bool = False, _group_indices: Dict = None):
         """
         given an object and the specifications, setup the internal grouper
         for this particular specification
@@ -330,6 +330,7 @@ def _set_grouper(self, obj: FrameOrSeries, sort: bool = False):
         # Keep self.grouper value before overriding
         if self._grouper is None:
             self._grouper = self.grouper
+            self._indexer = self.indexer
 
         # the key must be a valid info item
         if self.key is not None:
@@ -343,16 +344,19 @@ def _set_grouper(self, obj: FrameOrSeries, sort: bool = False):
                 # around. We align the index from obj with the self.obj index to
                 # select the correct values. Additionally we have to sort
                 # obj.index correctly to aggregate correctly.
-                if not hasattr(self, "_index_mapping"):
-                    self._index_mapping = DataFrame(
-                        self.obj.index, columns=["index"]
-                    ).sort_values(by="index")
-                obj.sort_index(inplace=True)
-                obj.index = self._index_mapping.loc[
-                    self._index_mapping["index"].isin(obj.index)
-                ].index
-                obj.sort_index(inplace=True)
-                ax = self._grouper.take(obj.index)
+                # if not hasattr(self, "_index_mapping"):
+                #     self._index_mapping = DataFrame(
+                #         self.obj.index, columns=["index"]
+                #     ).sort_values(by="index")
+                # from pandas.core.sorting import get_indexer_dict
+                # obj.sort_index(inplace=True)
+                indices = _group_indices.get(obj.name)
+                ax = self._grouper.take(self._indexer.argsort()).take(indices)
+                # obj.index = self._index_mapping.loc[
+                #     self._index_mapping["index"].isin(obj.index)
+                # ].index
+                # obj.sort_index(inplace=True)
+                # ax = self._grouper.take(obj.index)
             else:
                 if key not in obj._info_axis:
                     raise KeyError(f"The grouper name {key} is not found")
diff --git a/pandas/core/resample.py b/pandas/core/resample.py
index 7b5154756e613..45e5c18bfcac2 100644
--- a/pandas/core/resample.py
+++ b/pandas/core/resample.py
@@ -91,7 +91,7 @@ def __init__(self, obj, groupby=None, axis=0, kind=None, **kwargs):
         self.grouper = None
 
         if self.groupby is not None:
-            self.groupby._set_grouper(self._convert_obj(obj), sort=True)
+            self.groupby._set_grouper(self._convert_obj(obj), sort=True, _group_indices=kwargs.get("_group_indices"))
 
     def __str__(self) -> str:
         """
@@ -980,7 +980,7 @@ def _apply(self, f, grouper=None, *args, **kwargs):
         """
 
         def func(x):
-            x = self._shallow_copy(x, groupby=self.groupby)
+            x = self._shallow_copy(x, groupby=self.groupby, _group_indices=self._groupby.indices)
 
             if isinstance(f, str):
                 return getattr(x, f)(**kwargs)

From d4d02d84aea5d2571712832bd8ba9b5593f5a472 Mon Sep 17 00:00:00 2001
From: phofl <patrick_hoefler@gmx.net>
Date: Tue, 8 Sep 2020 18:38:03 +0200
Subject: [PATCH 5/9] Refactor code and parametrize test

---
 pandas/core/groupby/grouper.py                | 27 +++++--------------
 pandas/core/resample.py                       |  4 +--
 .../tests/resample/test_resampler_grouper.py  |  6 +++--
 3 files changed, 13 insertions(+), 24 deletions(-)

diff --git a/pandas/core/groupby/grouper.py b/pandas/core/groupby/grouper.py
index 959432bed7716..f92fddf6b76b0 100644
--- a/pandas/core/groupby/grouper.py
+++ b/pandas/core/groupby/grouper.py
@@ -311,7 +311,7 @@ def _get_grouper(self, obj, validate: bool = True):
         )
         return self.binner, self.grouper, self.obj
 
-    def _set_grouper(self, obj: FrameOrSeries, sort: bool = False, _group_indices: Dict = None):
+    def _set_grouper(self, obj: FrameOrSeries, sort: bool = False, group_indices: Dict = None):
         """
         given an object and the specifications, setup the internal grouper
         for this particular specification
@@ -327,7 +327,7 @@ def _set_grouper(self, obj: FrameOrSeries, sort: bool = False, _group_indices: D
         if self.key is not None and self.level is not None:
             raise ValueError("The Grouper cannot specify both a key and a level!")
 
-        # Keep self.grouper value before overriding
+        # Keep self.grouper and self.indexer value before overriding
         if self._grouper is None:
             self._grouper = self.grouper
             self._indexer = self.indexer
@@ -339,24 +339,11 @@ def _set_grouper(self, obj: FrameOrSeries, sort: bool = False, _group_indices: D
             if getattr(self.grouper, "name", None) == key and isinstance(
                 obj, ABCSeries
             ):
-                # In some cases the self._grouper may be sorted differently than obj.
-                # self.obj has the right order with the old index in the first go
-                # around. We align the index from obj with the self.obj index to
-                # select the correct values. Additionally we have to sort
-                # obj.index correctly to aggregate correctly.
-                # if not hasattr(self, "_index_mapping"):
-                #     self._index_mapping = DataFrame(
-                #         self.obj.index, columns=["index"]
-                #     ).sort_values(by="index")
-                # from pandas.core.sorting import get_indexer_dict
-                # obj.sort_index(inplace=True)
-                indices = _group_indices.get(obj.name)
-                ax = self._grouper.take(self._indexer.argsort()).take(indices)
-                # obj.index = self._index_mapping.loc[
-                #     self._index_mapping["index"].isin(obj.index)
-                # ].index
-                # obj.sort_index(inplace=True)
-                # ax = self._grouper.take(obj.index)
+                indices = group_indices.get(obj.name)
+                if self._indexer is not None:
+                    ax = self._grouper.take(self._indexer.argsort()).take(indices)
+                else:
+                    ax = self._grouper.take(indices)
             else:
                 if key not in obj._info_axis:
                     raise KeyError(f"The grouper name {key} is not found")
diff --git a/pandas/core/resample.py b/pandas/core/resample.py
index 45e5c18bfcac2..95ab505a0c3b5 100644
--- a/pandas/core/resample.py
+++ b/pandas/core/resample.py
@@ -91,7 +91,7 @@ def __init__(self, obj, groupby=None, axis=0, kind=None, **kwargs):
         self.grouper = None
 
         if self.groupby is not None:
-            self.groupby._set_grouper(self._convert_obj(obj), sort=True, _group_indices=kwargs.get("_group_indices"))
+            self.groupby._set_grouper(self._convert_obj(obj), sort=True, group_indices=kwargs.get("group_indices"))
 
     def __str__(self) -> str:
         """
@@ -980,7 +980,7 @@ def _apply(self, f, grouper=None, *args, **kwargs):
         """
 
         def func(x):
-            x = self._shallow_copy(x, groupby=self.groupby, _group_indices=self._groupby.indices)
+            x = self._shallow_copy(x, groupby=self.groupby, group_indices=self._groupby.indices)
 
             if isinstance(f, str):
                 return getattr(x, f)(**kwargs)
diff --git a/pandas/tests/resample/test_resampler_grouper.py b/pandas/tests/resample/test_resampler_grouper.py
index e90052eb941c1..1f701cc6c298d 100644
--- a/pandas/tests/resample/test_resampler_grouper.py
+++ b/pandas/tests/resample/test_resampler_grouper.py
@@ -124,10 +124,12 @@ def test_getitem_multiple():
     tm.assert_series_equal(result, expected)
 
 
-def test_groupby_resample_on_api_with_getitem():
+@pytest.mark.parametrize("index_values", [[0, 1, 2, 3, 4], ["a", "b", "c", "d", "e"]])
+def test_groupby_resample_on_api_with_getitem(index_values):
     # GH 17813
     df = pd.DataFrame(
-        {"id": list("aabbb"), "date": pd.date_range("1-1-2016", periods=5), "data": 1}
+        {"id": list("aabbb"), "date": pd.date_range("1-1-2016", periods=5), "data": 1},
+        index=pd.Index(index_values)
     )
     exp = df.set_index("date").groupby("id").resample("2D")["data"].sum()
     result = df.groupby("id").resample("2D", on="date")["data"].sum()

From 087c6828a5c44a11f249782dba81f2bdce3e6d90 Mon Sep 17 00:00:00 2001
From: phofl <patrick_hoefler@gmx.net>
Date: Tue, 8 Sep 2020 18:44:26 +0200
Subject: [PATCH 6/9] Run black pas

---
 pandas/core/groupby/grouper.py                  |  4 +++-
 pandas/core/resample.py                         | 10 ++++++++--
 pandas/tests/resample/test_resampler_grouper.py |  2 +-
 3 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/pandas/core/groupby/grouper.py b/pandas/core/groupby/grouper.py
index f92fddf6b76b0..c6b9b21f762e2 100644
--- a/pandas/core/groupby/grouper.py
+++ b/pandas/core/groupby/grouper.py
@@ -311,7 +311,9 @@ def _get_grouper(self, obj, validate: bool = True):
         )
         return self.binner, self.grouper, self.obj
 
-    def _set_grouper(self, obj: FrameOrSeries, sort: bool = False, group_indices: Dict = None):
+    def _set_grouper(
+        self, obj: FrameOrSeries, sort: bool = False, group_indices: Dict = None
+    ):
         """
         given an object and the specifications, setup the internal grouper
         for this particular specification
diff --git a/pandas/core/resample.py b/pandas/core/resample.py
index 95ab505a0c3b5..06e75e0d36a8f 100644
--- a/pandas/core/resample.py
+++ b/pandas/core/resample.py
@@ -91,7 +91,11 @@ def __init__(self, obj, groupby=None, axis=0, kind=None, **kwargs):
         self.grouper = None
 
         if self.groupby is not None:
-            self.groupby._set_grouper(self._convert_obj(obj), sort=True, group_indices=kwargs.get("group_indices"))
+            self.groupby._set_grouper(
+                self._convert_obj(obj),
+                sort=True,
+                group_indices=kwargs.get("group_indices"),
+            )
 
     def __str__(self) -> str:
         """
@@ -980,7 +984,9 @@ def _apply(self, f, grouper=None, *args, **kwargs):
         """
 
         def func(x):
-            x = self._shallow_copy(x, groupby=self.groupby, group_indices=self._groupby.indices)
+            x = self._shallow_copy(
+                x, groupby=self.groupby, group_indices=self._groupby.indices
+            )
 
             if isinstance(f, str):
                 return getattr(x, f)(**kwargs)
diff --git a/pandas/tests/resample/test_resampler_grouper.py b/pandas/tests/resample/test_resampler_grouper.py
index 1f701cc6c298d..4e1d80fd8a880 100644
--- a/pandas/tests/resample/test_resampler_grouper.py
+++ b/pandas/tests/resample/test_resampler_grouper.py
@@ -129,7 +129,7 @@ def test_groupby_resample_on_api_with_getitem(index_values):
     # GH 17813
     df = pd.DataFrame(
         {"id": list("aabbb"), "date": pd.date_range("1-1-2016", periods=5), "data": 1},
-        index=pd.Index(index_values)
+        index=pd.Index(index_values),
     )
     exp = df.set_index("date").groupby("id").resample("2D")["data"].sum()
     result = df.groupby("id").resample("2D", on="date")["data"].sum()

From 1e43de6151e1ccf9da993ee900fae2371b967860 Mon Sep 17 00:00:00 2001
From: phofl <patrick_hoefler@gmx.net>
Date: Tue, 8 Sep 2020 19:11:33 +0200
Subject: [PATCH 7/9] Fix wrong Type hint

---
 pandas/core/groupby/grouper.py | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/pandas/core/groupby/grouper.py b/pandas/core/groupby/grouper.py
index c6b9b21f762e2..ce138a2a126ee 100644
--- a/pandas/core/groupby/grouper.py
+++ b/pandas/core/groupby/grouper.py
@@ -312,7 +312,10 @@ def _get_grouper(self, obj, validate: bool = True):
         return self.binner, self.grouper, self.obj
 
     def _set_grouper(
-        self, obj: FrameOrSeries, sort: bool = False, group_indices: Dict = None
+        self,
+        obj: FrameOrSeries,
+        sort: bool = False,
+        group_indices: Optional[Dict] = None,
     ):
         """
         given an object and the specifications, setup the internal grouper

From f08d56eee02dd46379cc9090dace63ae2c2cb48b Mon Sep 17 00:00:00 2001
From: phofl <patrick_hoefler@gmx.net>
Date: Tue, 8 Sep 2020 20:50:21 +0200
Subject: [PATCH 8/9] Catch group_indices None case

---
 pandas/core/groupby/grouper.py | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/pandas/core/groupby/grouper.py b/pandas/core/groupby/grouper.py
index ce138a2a126ee..169e51bfab2a9 100644
--- a/pandas/core/groupby/grouper.py
+++ b/pandas/core/groupby/grouper.py
@@ -344,11 +344,14 @@ def _set_grouper(
             if getattr(self.grouper, "name", None) == key and isinstance(
                 obj, ABCSeries
             ):
-                indices = group_indices.get(obj.name)
-                if self._indexer is not None:
-                    ax = self._grouper.take(self._indexer.argsort()).take(indices)
+                if group_indices is None:
+                    ax = self._grouper.take(obj.index)
                 else:
-                    ax = self._grouper.take(indices)
+                    indices = group_indices.get(obj.name)
+                    if self._indexer is not None:
+                        ax = self._grouper.take(self._indexer.argsort()).take(indices)
+                    else:
+                        ax = self._grouper.take(indices)
             else:
                 if key not in obj._info_axis:
                     raise KeyError(f"The grouper name {key} is not found")

From 19569c4784f9271be9fb78e716496fe188b1c727 Mon Sep 17 00:00:00 2001
From: phofl <patrick_hoefler@gmx.net>
Date: Wed, 9 Sep 2020 09:42:11 +0200
Subject: [PATCH 9/9] Change whats new entry

---
 doc/source/whatsnew/v1.2.0.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/doc/source/whatsnew/v1.2.0.rst b/doc/source/whatsnew/v1.2.0.rst
index e82705c9a9273..1ddc46d4b0679 100644
--- a/doc/source/whatsnew/v1.2.0.rst
+++ b/doc/source/whatsnew/v1.2.0.rst
@@ -314,7 +314,7 @@ Groupby/resample/rolling
 - Bug in :meth:`DataFrameGroupby.tshift` failing to raise ``ValueError`` when a frequency cannot be inferred for the index of a group (:issue:`35937`)
 - Bug in :meth:`DataFrame.groupby` does not always maintain column index name for ``any``, ``all``, ``bfill``, ``ffill``, ``shift`` (:issue:`29764`)
 - Bug in :meth:`DataFrameGroupBy.apply` raising error with ``np.nan`` group(s) when ``dropna=False`` (:issue:`35889`)
-- Bug when combining methods :meth:`DataFrame.groupby` with :meth:`DataFrame.resample` and restricting to `Series` or using `agg` did miscalculate the aggregation (:issue:`27343`, :issue:`33548`, :issue:`35275`).
+- Bug in :meth:`DataFrame.groupby(...).resample(...)` when restricting to `Series` or using `agg` did miscalculate the aggregation (:issue:`27343`, :issue:`33548`, :issue:`35275`).
 -
 
 Reshaping