TST: add test case for user-defined function taking correct path in groupby transform

pandas/core/groupby/generic.py

@@ -1380,7 +1380,7 @@ def _define_paths(self, func, *args, **kwargs):
             )
         return fast_path, slow_path
 
-    def _choose_path(self, fast_path, slow_path, group):
+    def _choose_path(self, fast_path: Callable, slow_path: Callable, group: DataFrame):
         path = slow_path
         res = slow_path(group)
 
@@ -1390,8 +1390,8 @@ def _choose_path(self, fast_path, slow_path, group):
         except AssertionError:
             raise
         except Exception:
-            # Hard to know ex-ante what exceptions `fast_path` might raise
-            # TODO: no test cases get here
+            # GH#29631 For user-defined function, we cant predict what may be
+            #  raised; see test_transform.test_transform_fastpath_raises
             return path, res
 
         # verify fast path does not change columns (and names), otherwise

pandas/tests/groupby/test_transform.py

@@ -1073,3 +1073,33 @@ def test_transform_lambda_with_datetimetz():
         name="time",
     )
     tm.assert_series_equal(result, expected)
+
+
+def test_transform_fastpath_raises():
+    # GH#29631 case where fastpath defined in groupby.generic _choose_path
+    #  raises, but slow_path does not
+
+    df = pd.DataFrame({"A": [1, 1, 2, 2], "B": [1, -1, 1, 2]})
+    gb = df.groupby("A")
+
+    def func(grp):
+        # we want a function such that func(frame) fails but func.apply(frame)
+        #  works
+        if grp.ndim == 2:
+            # Ensure that fast_path fails
+            raise NotImplementedError("Don't cross the streams")
+        return grp * 2
+
+    # Check that the fastpath raises, see _transform_general
+    obj = gb._obj_with_exclusions
+    gen = gb.grouper.get_iterator(obj, axis=gb.axis)
+    fast_path, slow_path = gb._define_paths(func)
+    _, group = next(gen)
+
+    with pytest.raises(NotImplementedError, match="Don't cross the streams"):
+        fast_path(group)
+
+    result = gb.transform(func)
+
+    expected = pd.DataFrame([2, -2, 2, 4], columns=["B"])
+    tm.assert_frame_equal(result, expected)