gh-106510: Fix DEBUG output for atomic group (GH-106511)

Lib/re/_parser.py

@@ -113,7 +113,6 @@ class SubPattern:
         self.width = None
 
     def dump(self, level=0):
-        nl = True
         seqtypes = (tuple, list)
         for op, av in self.data:
             print(level*"  " + str(op), end='')
@@ -135,6 +134,9 @@ class SubPattern:
                 if item_no:
                     print(level*"  " + "ELSE")
                     item_no.dump(level+1)
+            elif isinstance(av, SubPattern):
+                print()
+                av.dump(level+1)
             elif isinstance(av, seqtypes):
                 nl = False
                 for a in av:

Lib/test/test_re.py

@@ -2489,7 +2489,10 @@ ELSE
 
     def test_atomic_group(self):
         self.assertEqual(get_debug_out(r'(?>ab?)'), '''\
-ATOMIC_GROUP [(LITERAL, 97), (MAX_REPEAT, (0, 1, [(LITERAL, 98)]))]
+ATOMIC_GROUP
+  LITERAL 97
+  MAX_REPEAT 0 1
+    LITERAL 98
 
  0. INFO 4 0b0 1 2 (to 5)
  5: ATOMIC_GROUP 11 (to 17)

Misc/NEWS.d/next/Library/2023-07-07-13-47-28.gh-issue-106510.9n5BdC.rst

@@ -0,0 +1 @@
+Improve debug output for atomic groups in regular expressions.