Add max_group_width and max_group_depth limits to IDLE's ExceptionGroup rendering

Match the traceback module defaults (max_group_width=15,
max_group_depth=10) to prevent pathological exception groups from
producing unbounded output or stack overflow via recursion.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: gpshead

Lib/idlelib/idle_test/test_run.py

@@ -145,6 +145,36 @@ def test_print_exception_group_seen(self):
         self.assertIn('ValueError: shared', tb)
         self.assertIn('<exception ValueError has printed>', tb)
 
+    @force_not_colorized
+    def test_print_exception_group_max_width(self):
+        excs = [ValueError(f'v{i}') for i in range(20)]
+        try:
+            raise ExceptionGroup('eg', excs)
+        except ExceptionGroup:
+            tb = self._capture_exception()
+
+        self.assertIn('+---------------- 15 ----------------', tb)
+        self.assertIn('+---------------- ... ----------------', tb)
+        self.assertIn('and 5 more exceptions', tb)
+        self.assertNotIn('+---------------- 16 ----------------', tb)
+
+    @force_not_colorized
+    def test_print_exception_group_max_depth(self):
+        def make_nested(depth):
+            if depth == 0:
+                return ValueError('leaf')
+            return ExceptionGroup(f'level{depth}',
+                                  [make_nested(depth - 1)])
+
+        try:
+            raise make_nested(15)
+        except ExceptionGroup:
+            tb = self._capture_exception()
+
+        self.assertIn('... (max_group_depth is 10)', tb)
+        self.assertIn('ExceptionGroup: level15', tb)
+        self.assertNotIn('ValueError: leaf', tb)
+
 # StdioFile tests.
 
 class S(str):

Lib/idlelib/run.py

@@ -251,9 +251,19 @@ def print_exception():
     seen = set()
     exclude = ("run.py", "rpc.py", "threading.py", "queue.py",
                "debugger_r.py", "bdb.py")
+    max_group_width = 15
+    max_group_depth = 10
+    group_depth = 0
 
     def print_exc_group(typ, exc, tb, prefix=""):
+        nonlocal group_depth
+        group_depth += 1
         prefix2 = prefix or "  "
+        if group_depth > max_group_depth:
+            print(f"{prefix2}| ... (max_group_depth is {max_group_depth})",
+                  file=efile)
+            group_depth -= 1
+            return
         if tb:
             if not prefix:
                 print("  + Exception Group Traceback (most recent call last):", file=efile)
@@ -267,13 +277,23 @@ def print_exc_group(typ, exc, tb, prefix=""):
         lines = get_message_lines(typ, exc, tb)
         for line in lines:
             print(f"{prefix2}| {line}", end="", file=efile)
-        for i, sub in enumerate(exc.exceptions, 1):
-            if i == 1:
-                first_line_pre = "+-"
-            else:
-                first_line_pre = "  "
-            print(f"{prefix2}{first_line_pre}+---------------- {i} ----------------", file=efile)
-            if id(sub) not in seen:
+        num_excs = len(exc.exceptions)
+        if num_excs <= max_group_width:
+            n = num_excs
+        else:
+            n = max_group_width + 1
+        for i, sub in enumerate(exc.exceptions[:n], 1):
+            truncated = (i > max_group_width)
+            first_line_pre = "+-" if i == 1 else "  "
+            title = str(i) if not truncated else '...'
+            print(f"{prefix2}{first_line_pre}+---------------- {title} ----------------", file=efile)
+            if truncated:
+                remaining = num_excs - max_group_width
+                plural = 's' if remaining > 1 else ''
+                print(f"{prefix2}  | and {remaining} more exception{plural}",
+                      file=efile)
+                need_print_underline = True
+            elif id(sub) not in seen:
                 if not prefix:
                     print_exc(type(sub), sub, sub.__traceback__, "    ")
                 else:
@@ -282,8 +302,9 @@ def print_exc_group(typ, exc, tb, prefix=""):
             else:
                 print(f"{prefix2}  | <exception {type(sub).__name__} has printed>", file=efile)
                 need_print_underline = True
-            if need_print_underline and i == len(exc.exceptions):
+            if need_print_underline and i == n:
                 print(f"{prefix2}  +------------------------------------", file=efile)
+        group_depth -= 1
 
     def print_exc(typ, exc, tb, prefix=""):
         seen.add(id(exc))