TST: follow-up ot pandas-dev#30318 (pandas-dev#30326)

Author: jbrockmendel

pandas/core/groupby/ops.py

@@ -663,10 +663,7 @@ def _aggregate_series_pure_python(self, obj: Series, func):
                     if len(res) == 1:
                         # e.g. test_agg_lambda_with_timezone lambda e: e.head(1)
                         # FIXME: are we potentially losing import res.index info?
-
-                        # TODO: use `.item()` if/when we un-deprecate it.
-                        # For non-Series we could just do `res[0]`
-                        res = next(iter(res))
+                        res = res.item()
                     else:
                         raise ValueError("Function does not reduce")
                 result = np.empty(ngroups, dtype="O")

pandas/tests/extension/base/getitem.py

@@ -272,3 +272,10 @@ def test_item(self, data):
         s = pd.Series(data)
         result = s[:1].item()
         assert result == data[0]
+
+        msg = "can only convert an array of size 1 to a Python scalar"
+        with pytest.raises(ValueError, match=msg):
+            s[:0].item()
+
+        with pytest.raises(ValueError, match=msg):
+            s.item()