ENH: Use Kahan summation to calculate groupby.sum()

doc/source/whatsnew/v1.3.0.rst

@@ -289,7 +289,7 @@ Groupby/resample/rolling
 
 - Bug in :meth:`SeriesGroupBy.value_counts` where unobserved categories in a grouped categorical series were not tallied (:issue:`38672`)
 - Bug in :meth:`.GroupBy.indices` would contain non-existent indices when null values were present in the groupby keys (:issue:`9304`)
--
+- Fixed bug in :meth:`DataFrameGroupBy.sum` and :meth:`SeriesGroupBy.sum` causing loss of precision through using Kahan summation (:issue:`38778`)
 
 Reshaping
 ^^^^^^^^^

pandas/_libs/groupby.pyx

@@ -467,12 +467,12 @@ def _group_add(complexfloating_t[:, :] out,
                const int64_t[:] labels,
                Py_ssize_t min_count=0):
     """
-    Only aggregates on axis=0
+    Only aggregates on axis=0 using Kahan summation
     """
     cdef:
         Py_ssize_t i, j, N, K, lab, ncounts = len(counts)
-        complexfloating_t val, count
-        complexfloating_t[:, :] sumx
+        complexfloating_t val, count, t, y
+        complexfloating_t[:, :] sumx, compensation
         int64_t[:, :] nobs
         Py_ssize_t len_values = len(values), len_labels = len(labels)
 
@@ -481,6 +481,7 @@ def _group_add(complexfloating_t[:, :] out,
 
     nobs = np.zeros((<object>out).shape, dtype=np.int64)
     sumx = np.zeros_like(out)
+    compensation = np.zeros_like(out)
 
     N, K = (<object>values).shape
 
@@ -497,12 +498,10 @@ def _group_add(complexfloating_t[:, :] out,
                 # not nan
                 if val == val:
                     nobs[lab, j] += 1
-                    if (complexfloating_t is complex64_t or
-                            complexfloating_t is complex128_t):
-                        # clang errors if we use += with these dtypes
-                        sumx[lab, j] = sumx[lab, j] + val
-                    else:
-                        sumx[lab, j] += val
+                    y = val - compensation[lab, j]
+                    t = sumx[lab, j] + y
+                    compensation[lab, j] = t - sumx[lab, j] - y
+                    sumx[lab, j] = t
 
         for i in range(ncounts):
             for j in range(K):

pandas/tests/groupby/test_groupby.py

@@ -5,6 +5,7 @@
 import numpy as np
 import pytest
 
+from pandas.compat import IS64
 from pandas.errors import PerformanceWarning
 
 import pandas as pd
@@ -2174,3 +2175,15 @@ def test_groupby_series_with_tuple_name():
     expected = Series([2, 4], index=[1, 2], name=("a", "a"))
     expected.index.name = ("b", "b")
     tm.assert_series_equal(result, expected)
+
+
+@pytest.mark.xfail(not IS64, reason="GH#38778: fail on 32-bit system")
+def test_groupby_numerical_stability_sum():
+    # GH#38778
+    data = [1e16, 1e16, 97, 98, -5e15, -5e15, -5e15, -5e15]
+    df = DataFrame({"group": [1, 2] * 4, "a": data, "b": data})
+    result = df.groupby("group").sum()
+    expected = DataFrame(
+        {"a": [97.0, 98.0], "b": [97.0, 98.0]}, index=Index([1, 2], name="group")
+    )
+    tm.assert_frame_equal(result, expected)