BUG: add the 'name' attribute to dataframes that go through apply_frame_axis0

Author: Joe Jevnik

pandas/src/reduce.pyx

@@ -497,7 +497,7 @@ def apply_frame_axis0(object frame, object f, object names,
     # Need to infer if our low-level mucking is going to cause a segfault
     if n > 0:
         chunk = frame.iloc[starts[0]:ends[0]]
-        shape_before = chunk.shape
+        object.__setattr__(chunk, 'name', names[0])
         try:
             result = f(chunk)
             if result is chunk:

pandas/tests/groupby/test_groupby.py

@@ -6022,6 +6022,21 @@ def test_cummin_cummax(self):
         result = base_df.groupby('A').B.apply(lambda x: x.cummax()).to_frame()
         tm.assert_frame_equal(expected, result)
 
+    def test_group_name_available_in_inference_pass(self):
+        df = pd.DataFrame({'a': [0, 0, 1, 1, 2, 2], 'b': np.arange(6)})
+
+        names = []
+
+        def f(group):
+            names.append(group.name)
+            return group.copy()
+
+        df.groupby('a', sort=False, group_keys=False).apply(f)
+        # we expect 2 zeros because we call ``f`` once to see if a faster route
+        # can be used.
+        expected_names = [0, 0, 1, 2]
+        tm.assert_equal(names, expected_names)
+
 
 def assert_fp_equal(a, b):
     assert (np.abs(a - b) < 1e-12).all()