BUG: groupby.min with UInt64 and empty group (#46182)

Author: jbrockmendel

pandas/_libs/groupby.pyx

@@ -1344,18 +1344,17 @@ cdef group_min_max(iu_64_floating_t[:, ::1] out,
         for i in range(ngroups):
             for j in range(K):
                 if nobs[i, j] < min_count:
-                    if iu_64_floating_t is uint64_t:
+                    if uses_mask:
+                        result_mask[i, j] = True
+                        # set out[i, j] to 0 to be deterministic, as
+                        #  it was initialized with np.empty. Also ensures
+                        #  we can downcast out if appropriate.
+                        out[i, j] = 0
+                    elif iu_64_floating_t is uint64_t:
                         runtime_error = True
                         break
                     else:
-                        if uses_mask:
-                            result_mask[i, j] = True
-                            # set out[i, j] to 0 to be deterministic, as
-                            #  it was initialized with np.empty. Also ensures
-                            #  we can downcast out if appropriate.
-                            out[i, j] = 0
-                        else:
-                            out[i, j] = nan_val
+                        out[i, j] = nan_val
                 else:
                     out[i, j] = group_min_or_max[i, j]
 

pandas/tests/groupby/test_min_max.py

@@ -225,3 +225,20 @@ def test_groupby_min_max_nullable(dtype):
 
     res_max4 = gb2.max(min_count=100)
     tm.assert_frame_equal(res_max4, expected2)
+
+
+def test_min_max_nullable_uint64_empty_group():
+    # don't raise NotImplementedError from libgroupby
+    cat = pd.Categorical([0] * 10, categories=[0, 1])
+    df = DataFrame({"A": cat, "B": pd.array(np.arange(10, dtype=np.uint64))})
+    gb = df.groupby("A")
+
+    res = gb.min()
+
+    idx = pd.CategoricalIndex([0, 1], dtype=cat.dtype, name="A")
+    expected = DataFrame({"B": pd.array([0, pd.NA], dtype="UInt64")}, index=idx)
+    tm.assert_frame_equal(res, expected)
+
+    res = gb.max()
+    expected.iloc[0, 0] = 9
+    tm.assert_frame_equal(res, expected)