ENH: Support mask in groupby sum

doc/source/whatsnew/v1.5.0.rst

@@ -1047,6 +1047,7 @@ Groupby/resample/rolling
 - Bug when using ``engine="numba"`` would return the same jitted function when modifying ``engine_kwargs`` (:issue:`46086`)
 - Bug in :meth:`.DataFrameGroupBy.transform` fails when ``axis=1`` and ``func`` is ``"first"`` or ``"last"`` (:issue:`45986`)
 - Bug in :meth:`DataFrameGroupBy.cumsum` with ``skipna=False`` giving incorrect results (:issue:`46216`)
+- Bug in :meth:`GroupBy.sum` with integer dtypes losing precision (:issue:`37493`)
 - Bug in :meth:`.GroupBy.cumsum` with ``timedelta64[ns]`` dtype failing to recognize ``NaT`` as a null value (:issue:`46216`)
 - Bug in :meth:`.GroupBy.cummin` and :meth:`.GroupBy.cummax` with nullable dtypes incorrectly altering the original data in place (:issue:`46220`)
 - Bug in :meth:`DataFrame.groupby` raising error when ``None`` is in first level of :class:`MultiIndex` (:issue:`47348`)

pandas/_libs/groupby.pyi

@@ -51,10 +51,12 @@ def group_any_all(
     skipna: bool,
 ) -> None: ...
 def group_sum(
-    out: np.ndarray,  # complexfloating_t[:, ::1]
+    out: np.ndarray,  # complexfloatingintuint_t[:, ::1]
     counts: np.ndarray,  # int64_t[::1]
-    values: np.ndarray,  # ndarray[complexfloating_t, ndim=2]
+    values: np.ndarray,  # ndarray[complexfloatingintuint_t, ndim=2]
     labels: np.ndarray,  # const intp_t[:]
+    mask: np.ndarray | None,
+    result_mask: np.ndarray | None = ...,
     min_count: int = ...,
     is_datetimelike: bool = ...,
 ) -> None: ...

pandas/_libs/groupby.pyx

@@ -513,6 +513,15 @@ ctypedef fused mean_t:
 
 ctypedef fused sum_t:
     mean_t
+    int8_t
+    int16_t
+    int32_t
+    int64_t
+
+    uint8_t
+    uint16_t
+    uint32_t
+    uint64_t
     object
 
 
@@ -523,6 +532,8 @@ def group_sum(
     int64_t[::1] counts,
     ndarray[sum_t, ndim=2] values,
     const intp_t[::1] labels,
+    const uint8_t[:, :] mask,
+    uint8_t[:, ::1] result_mask=None,
     Py_ssize_t min_count=0,
     bint is_datetimelike=False,
 ) -> None:
@@ -535,6 +546,8 @@ def group_sum(
         sum_t[:, ::1] sumx, compensation
         int64_t[:, ::1] nobs
         Py_ssize_t len_values = len(values), len_labels = len(labels)
+        bint uses_mask = mask is not None
+        bint isna_entry
 
     if len_values != len_labels:
         raise ValueError("len(index) != len(labels)")
@@ -572,7 +585,8 @@ def group_sum(
         for i in range(ncounts):
             for j in range(K):
                 if nobs[i, j] < min_count:
-                    out[i, j] = NAN
+                    out[i, j] = None
+
                 else:
                     out[i, j] = sumx[i, j]
     else:
@@ -590,11 +604,18 @@ def group_sum(
                     # With dt64/td64 values, values have been cast to float64
                     #  instead if int64 for group_sum, but the logic
                     #  is otherwise the same as in _treat_as_na
-                    if val == val and not (
-                        sum_t is float64_t
-                        and is_datetimelike
-                        and val == <float64_t>NPY_NAT
-                    ):
+                    if uses_mask:
+                        isna_entry = mask[i, j]
+                    elif (sum_t is float32_t or sum_t is float64_t
+                        or sum_t is complex64_t or sum_t is complex64_t):
+                        # avoid warnings because of equality comparison
+                        isna_entry = not val == val
+                    elif sum_t is int64_t and is_datetimelike and val == NPY_NAT:
+                        isna_entry = True
+                    else:
+                        isna_entry = False
+
+                    if not isna_entry:
                         nobs[lab, j] += 1
                         y = val - compensation[lab, j]
                         t = sumx[lab, j] + y
@@ -604,7 +625,23 @@ def group_sum(
             for i in range(ncounts):
                 for j in range(K):
                     if nobs[i, j] < min_count:
-                        out[i, j] = NAN
+                        # if we are integer dtype, not is_datetimelike, and
+                        #  not uses_mask, then getting here implies that
+                        #  counts[i] < min_count, which means we will
+                        #  be cast to float64 and masked at the end
+                        #  of WrappedCythonOp._call_cython_op. So we can safely
+                        #  set a placeholder value in out[i, j].
+                        if uses_mask:
+                            result_mask[i, j] = True
+                        elif (sum_t is float32_t or sum_t is float64_t
+                            or sum_t is complex64_t or sum_t is complex64_t):
+                            out[i, j] = NAN
+                        elif sum_t is int64_t:
+                            out[i, j] = NPY_NAT
+                        else:
+                            # placeholder, see above
+                            out[i, j] = 0
+
                     else:
                         out[i, j] = sumx[i, j]
 

pandas/core/groupby/ops.py

@@ -155,6 +155,7 @@ def __init__(self, kind: str, how: str, has_dropped_na: bool) -> None:
         "last",
         "first",
         "rank",
+        "sum",
     }
 
     _cython_arity = {"ohlc": 4}  # OHLC
@@ -217,7 +218,7 @@ def _get_cython_vals(self, values: np.ndarray) -> np.ndarray:
             values = ensure_float64(values)
 
         elif values.dtype.kind in ["i", "u"]:
-            if how in ["sum", "var", "prod", "mean", "ohlc"] or (
+            if how in ["var", "prod", "mean", "ohlc"] or (
                 self.kind == "transform" and self.has_dropped_na
             ):
                 # result may still include NaN, so we have to cast
@@ -578,6 +579,8 @@ def _call_cython_op(
                     counts=counts,
                     values=values,
                     labels=comp_ids,
+                    mask=mask,
+                    result_mask=result_mask,
                     min_count=min_count,
                     is_datetimelike=is_datetimelike,
                 )
@@ -613,7 +616,8 @@ def _call_cython_op(
             # need to have the result set to np.nan, which may require casting,
             # see GH#40767
             if is_integer_dtype(result.dtype) and not is_datetimelike:
-                cutoff = max(1, min_count)
+                # Neutral value for sum is 0, so don't fill empty groups with nan
+                cutoff = max(0 if self.how == "sum" else 1, min_count)
                 empty_groups = counts < cutoff
                 if empty_groups.any():
                     if result_mask is not None and self.uses_mask():

pandas/tests/groupby/test_groupby.py

@@ -2808,3 +2808,24 @@ def test_single_element_list_grouping():
     )
     with tm.assert_produces_warning(FutureWarning, match=msg):
         values, _ = next(iter(df.groupby(["a"])))
+
+
+def test_groupby_sum_avoid_casting_to_float():
+    # GH#37493
+    val = 922337203685477580
+    df = DataFrame({"a": 1, "b": [val]})
+    result = df.groupby("a").sum() - val
+    expected = DataFrame({"b": [0]}, index=Index([1], name="a"))
+    tm.assert_frame_equal(result, expected)
+
+
+def test_groupby_sum_support_mask(any_numeric_ea_dtype):
+    # GH#37493
+    df = DataFrame({"a": 1, "b": [1, 2, pd.NA]}, dtype=any_numeric_ea_dtype)
+    result = df.groupby("a").sum()
+    expected = DataFrame(
+        {"b": [3]},
+        index=Index([1], name="a", dtype=any_numeric_ea_dtype),
+        dtype=any_numeric_ea_dtype,
+    )
+    tm.assert_frame_equal(result, expected)