BUG: incorrect rounding in groupby.cummin near int64 implementation bounds

doc/source/whatsnew/v1.3.0.rst

@@ -791,7 +791,7 @@ Groupby/resample/rolling
 - Bug in aggregation functions for :class:`DataFrame` not respecting ``numeric_only`` argument when ``level`` keyword was given (:issue:`40660`)
 - Bug in :class:`core.window.RollingGroupby` where ``as_index=False`` argument in ``groupby`` was ignored (:issue:`39433`)
 - Bug in :meth:`.GroupBy.any` and :meth:`.GroupBy.all` raising ``ValueError`` when using with nullable type columns holding ``NA`` even with ``skipna=True`` (:issue:`40585`)
-
+- Bug in :meth:`GroupBy.cummin` and :meth:`GroupBy.cummax` incorrectly rounding integer values near the ``int64`` implementations bounds (:issue:`40767`)
 
 Reshaping
 ^^^^^^^^^

pandas/_libs/groupby.pyx

@@ -1139,6 +1139,7 @@ cdef group_min_max(groupby_t[:, ::1] out,
                    ndarray[groupby_t, ndim=2] values,
                    const intp_t[:] labels,
                    Py_ssize_t min_count=-1,
+                   bint is_datetimelike=False,
                    bint compute_max=True):
     """
     Compute minimum/maximum  of columns of `values`, in row groups `labels`.
@@ -1156,6 +1157,8 @@ cdef group_min_max(groupby_t[:, ::1] out,
     min_count : Py_ssize_t, default -1
         The minimum number of non-NA group elements, NA result if threshold
         is not met
+    is_datetimelike : bool
+        True if `values` contains datetime-like entries.
     compute_max : bint, default True
         True to compute group-wise max, False to compute min
 
@@ -1204,8 +1207,7 @@ cdef group_min_max(groupby_t[:, ::1] out,
             for j in range(K):
                 val = values[i, j]
 
-                if not _treat_as_na(val, True):
-                    # TODO: Sure we always want is_datetimelike=True?
+                if not _treat_as_na(val, is_datetimelike):
                     nobs[lab, j] += 1
                     if compute_max:
                         if val > group_min_or_max[lab, j]:
@@ -1237,9 +1239,18 @@ def group_max(groupby_t[:, ::1] out,
               int64_t[::1] counts,
               ndarray[groupby_t, ndim=2] values,
               const intp_t[:] labels,
-              Py_ssize_t min_count=-1) -> None:
+              Py_ssize_t min_count=-1,
+              bint is_datetimelike=False) -> None:
     """See group_min_max.__doc__"""
-    group_min_max(out, counts, values, labels, min_count=min_count, compute_max=True)
+    group_min_max(
+        out,
+        counts,
+        values,
+        labels,
+        min_count=min_count,
+        is_datetimelike=is_datetimelike,
+        compute_max=True,
+    )
 
 
 @cython.wraparound(False)
@@ -1248,9 +1259,18 @@ def group_min(groupby_t[:, ::1] out,
               int64_t[::1] counts,
               ndarray[groupby_t, ndim=2] values,
               const intp_t[:] labels,
-              Py_ssize_t min_count=-1) -> None:
+              Py_ssize_t min_count=-1,
+              bint is_datetimelike=False) -> None:
     """See group_min_max.__doc__"""
-    group_min_max(out, counts, values, labels, min_count=min_count, compute_max=False)
+    group_min_max(
+        out,
+        counts,
+        values,
+        labels,
+        min_count=min_count,
+        is_datetimelike=is_datetimelike,
+        compute_max=False,
+    )
 
 
 @cython.boundscheck(False)

pandas/core/groupby/ops.py

@@ -20,7 +20,6 @@
 
 from pandas._libs import (
     NaT,
-    iNaT,
     lib,
 )
 import pandas._libs.groupby as libgroupby
@@ -663,12 +662,7 @@ def _cython_operation(
         elif is_bool_dtype(dtype):
             values = ensure_int_or_float(values)
         elif is_integer_dtype(dtype):
-            # we use iNaT for the missing value on ints
-            # so pre-convert to guard this condition
-            if (values == iNaT).any():
-                values = ensure_float64(values)
-            else:
-                values = ensure_int_or_float(values)
+            values = ensure_int_or_float(values)
         elif is_numeric:
             if not is_complex_dtype(dtype):
                 values = ensure_float64(values)
@@ -686,20 +680,36 @@ def _cython_operation(
         result = maybe_fill(np.empty(out_shape, dtype=out_dtype))
         if kind == "aggregate":
             counts = np.zeros(ngroups, dtype=np.int64)
-            func(result, counts, values, comp_ids, min_count)
+            if how in ["min", "max"]:
+                func(
+                    result,
+                    counts,
+                    values,
+                    comp_ids,
+                    min_count,
+                    is_datetimelike=is_datetimelike,
+                )
+            else:
+                func(result, counts, values, comp_ids, min_count)
         elif kind == "transform":
             # TODO: min_count
             func(result, values, comp_ids, ngroups, is_datetimelike, **kwargs)
 
-        if is_integer_dtype(result.dtype) and not is_datetimelike:
-            mask = result == iNaT
-            if mask.any():
-                result = result.astype("float64")
-                result[mask] = np.nan
-
-        if kind == "aggregate" and self._filter_empty_groups and not counts.all():
-            assert result.ndim != 2
-            result = result[counts > 0]
+        if kind == "aggregate":
+            # i.e. counts is defined.  Locations where count<min_count
+            # 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)
+                empty_groups = counts < cutoff
+                if empty_groups.any():
+                    # Note: this conversion could be lossy, see GH#40767
+                    result = result.astype("float64")
+                    result[empty_groups] = np.nan
+
+            if self._filter_empty_groups and not counts.all():
+                assert result.ndim != 2
+                result = result[counts > 0]
 
         result = result.T
 

pandas/tests/groupby/test_function.py

@@ -4,6 +4,7 @@
 import numpy as np
 import pytest
 
+from pandas._libs.tslibs import iNaT
 from pandas.errors import UnsupportedFunctionCall
 
 import pandas as pd
@@ -591,6 +592,38 @@ def test_max_nan_bug():
     assert not r["File"].isna().any()
 
 
+def test_max_inat():
+    # GH#40767 dont interpret iNaT as NaN
+    ser = Series([1, iNaT])
+    gb = ser.groupby([1, 1])
+
+    result = gb.max(min_count=2)
+    expected = Series({1: 1}, dtype=np.int64)
+    tm.assert_series_equal(result, expected, check_exact=True)
+
+    result = gb.min(min_count=2)
+    expected = Series({1: iNaT}, dtype=np.int64)
+    tm.assert_series_equal(result, expected, check_exact=True)
+
+    # not enough entries -> gets masked to NaN
+    result = gb.min(min_count=3)
+    expected = Series({1: np.nan})
+    tm.assert_series_equal(result, expected, check_exact=True)
+
+
+def test_max_inat_not_all_na():
+    # GH#40767 dont interpret iNaT as NaN
+
+    # make sure we dont round iNaT+1 to iNaT
+    ser = Series([1, iNaT, 2, iNaT + 1])
+    gb = ser.groupby([1, 2, 3, 3])
+    result = gb.min(min_count=2)
+
+    # Note: in converting to float64, the iNaT + 1 maps to iNaT, i.e. is lossy
+    expected = Series({1: np.nan, 2: np.nan, 3: iNaT + 1})
+    tm.assert_series_equal(result, expected, check_exact=True)
+
+
 def test_nlargest():
     a = Series([1, 3, 5, 7, 2, 9, 0, 4, 6, 10])
     b = Series(list("a" * 5 + "b" * 5))
@@ -708,11 +741,13 @@ def test_cummin(numpy_dtypes_for_minmax):
 
     # Test w/ min value for dtype
     df.loc[[2, 6], "B"] = min_val
+    df.loc[[1, 5], "B"] = min_val + 1
     expected.loc[[2, 3, 6, 7], "B"] = min_val
+    expected.loc[[1, 5], "B"] = min_val + 1  # should not be rounded to min_val
     result = df.groupby("A").cummin()
-    tm.assert_frame_equal(result, expected)
+    tm.assert_frame_equal(result, expected, check_exact=True)
     expected = df.groupby("A").B.apply(lambda x: x.cummin()).to_frame()
-    tm.assert_frame_equal(result, expected)
+    tm.assert_frame_equal(result, expected, check_exact=True)
 
     # Test nan in some values
     base_df.loc[[0, 2, 4, 6], "B"] = np.nan