doc added & tests fix

Author: GYHHAHA

doc/source/user_guide/timeseries.rst

@@ -1897,6 +1897,50 @@ Those two examples are equivalent for this time series:
 
 Note the use of ``'start'`` for ``origin`` on the last example. In that case, ``origin`` will be set to the first value of the timeseries.
 
+Backward resample
+~~~~~~~~~~~~~~~~~
+
+.. versionadded:: 1.2.0
+
+``origin`` can not only make a foreward resample, namely grouping from the starting point with the given ``freq`` , but is also able to implement the backward resample. This method allows users to control bins of the grouping from the given origin with a backward direction. (:issue:`37804`)
+
+.. ipython:: python
+
+   start, end = "2000-10-01 23:30:00", "2000-10-02 00:26:00"
+   rng = date_range(start, end, freq="7min")
+   ts = Series(np.arange(len(rng)) * 3, index=rng)
+
+Setting ``offset='end'`` means using the max ``Timestamp`` as the ``origin`` with ``backward=True`` .
+
+   ts.index.max()
+   ts.resample("17min", origin="end").sum()
+
+Setting ``offset='end'`` means using the ceiling midnight of the max ``Timestamp`` as the ``origin`` with ``backward=True`` .
+
+.. ipython:: python
+
+   ts.resample("17min", origin="end").sum()
+
+If you want to make the backward resample from a Timestamp-like ``origin`` , ``backward=True`` should be set.
+
+.. ipython:: python
+
+   ts.resample("17min", origin="2000-10-02 00:40:00", backward=True).sum()
+
+You can implement ``offset='end_day'`` in the following method equivalently.
+
+.. ipython:: python
+
+   end_day_origin = ts.index.max().ceil("D")
+   end_day_origin
+   ts.resample("17min", origin=end_day_origin, backward=True).sum()
+
+By defualt, backward resample uses ``closed=right`` while ``closed=left`` is also available.
+
+.. ipython:: python
+
+   ts.resample("17min", closed="left", origin="end").sum()
+
 .. _timeseries.periods:
 
 Time span representation

doc/source/whatsnew/v1.2.0.rst

@@ -203,6 +203,50 @@ example where the index name is preserved:
 The same is true for :class:`MultiIndex`, but the logic is applied separately on a
 level-by-level basis.
 
+.. _whatsnew_120.backward_resample:
+
+Backward resample 
+^^^^^^^^^^^^^^^^^
+
+:class:`Grouper` and :meth:`DataFrame.resample` now support the argument ``backward`` . ``'end'`` and ``'end_day'`` are available in argument ``offset`` . Backward resample allows users to control bins of the grouping from the given origin with a backward direction. (:issue:`37804`)
+
+.. ipython:: python
+
+   start, end = "2000-10-01 23:30:00", "2000-10-02 00:26:00"
+   rng = date_range(start, end, freq="7min")
+   ts = Series(np.arange(len(rng)) * 3, index=rng)
+
+Setting ``offset='end'`` means using the max ``Timestamp`` as the ``origin`` with ``backward=True`` .
+
+   ts.index.max()
+   ts.resample("17min", origin="end").sum()
+
+Setting ``offset='end'`` means using the ceiling midnight of the max ``Timestamp`` as the ``origin`` with ``backward=True`` .
+
+.. ipython:: python
+
+   ts.resample("17min", origin="end").sum()
+
+If you want to make the backward resample from a Timestamp-like ``origin`` , ``backward=True`` should be set.
+
+.. ipython:: python
+
+   ts.resample("17min", origin="2000-10-02 00:40:00", backward=True).sum()
+
+You can implement ``offset='end_day'`` in the following method equivalently.
+
+.. ipython:: python
+
+   end_day_origin = ts.index.max().ceil("D")
+   end_day_origin
+   ts.resample("17min", origin=end_day_origin, backward=True).sum()
+
+By defualt, backward resample uses ``closed=right`` while ``closed=left`` is also available.
+
+.. ipython:: python
+
+   ts.resample("17min", closed="left", origin="end").sum()
+
 .. _whatsnew_120.enhancements.other:
 
 Other enhancements

pandas/tests/resample/test_resample_api.py

@@ -613,14 +613,12 @@ def test_resample_agg_readonly():
     tm.assert_series_equal(result, expected)
 
 
-# test data for backward resample GH#37804
-start, end = "2000-10-01 23:30:00", "2000-10-02 00:26:00"
-rng = date_range(start, end, freq="7min")
-ts = Series(np.arange(len(rng)) * 3, index=rng)
-
-
 def test_backward_origin_consistency():
 
+    start, end = "2000-10-01 23:30:00", "2000-10-02 00:26:00"
+    rng = date_range(start, end, freq="7min")
+    ts = Series(np.arange(len(rng)) * 3, index=rng)
+
     msg = "`start` or `start_day` origin isn't allowed when `backward` is True"
     with pytest.raises(ValueError, match=msg):
         ts.resample("1min", origin="start", backward=True)
@@ -631,6 +629,10 @@ def test_backward_origin_consistency():
 
 def test_end_origin():
 
+    start, end = "2000-10-01 23:30:00", "2000-10-02 00:26:00"
+    rng = date_range(start, end, freq="7min")
+    ts = Series(np.arange(len(rng)) * 3, index=rng)
+
     res = ts.resample("17min", origin="end").sum().astype("int64")
     data = [0, 18, 27, 63]
     expected = Series(
@@ -659,6 +661,10 @@ def test_end_origin():
 
 def test_end_day_origin():
 
+    start, end = "2000-10-01 23:30:00", "2000-10-02 00:26:00"
+    rng = date_range(start, end, freq="7min")
+    ts = Series(np.arange(len(rng)) * 3, index=rng)
+
     # 12 == 24 * 60 - 84 * 17 <= 26 (last value) <= 24 * 60 - 83 * 17 == 29
     res = ts.resample("17min", origin="end_day").sum().astype("int64")
     data = [3, 15, 45, 45]
@@ -676,6 +682,10 @@ def test_end_day_origin():
 
 def test_backward_resample_with_datetime_origin():
 
+    start, end = "2000-10-01 23:30:00", "2000-10-02 00:26:00"
+    rng = date_range(start, end, freq="7min")
+    ts = Series(np.arange(len(rng)) * 3, index=rng)
+
     res = (
         ts.resample(
             "17min",
@@ -721,6 +731,10 @@ def test_backward_resample_with_datetime_origin():
 
 def test_left_and_right_close_in_backward_resample():
 
+    start, end = "2000-10-01 23:30:00", "2000-10-02 00:26:00"
+    rng = date_range(start, end, freq="7min")
+    ts = Series(np.arange(len(rng)) * 3, index=rng)
+
     res = (
         ts.resample(
             "17min",