Issue #16491: IDLE now prints chained exception tracebacks.

Lib/idlelib/run.py

@@ -171,16 +171,35 @@ def print_exception():
     efile = sys.stderr
     typ, val, tb = excinfo = sys.exc_info()
     sys.last_type, sys.last_value, sys.last_traceback = excinfo
+    seen = set()
+
+    def print_exc(typ, exc, tb):
+        seen.add(exc)
+        context = exc.__context__
+        cause = exc.__cause__
+        if cause is not None and cause not in seen:
+            print_exc(type(cause), cause, cause.__traceback__)
+            print("\nThe above exception was the direct cause "
+                  "of the following exception:\n", file=efile)
+        elif (context is not None and
+              not exc.__suppress_context__ and
+              context not in seen):
+            print_exc(type(context), context, context.__traceback__)
+            print("\nDuring handling of the above exception, "
+                  "another exception occurred:\n", file=efile)
+        if tb:
             tbe = traceback.extract_tb(tb)
             print('Traceback (most recent call last):', file=efile)
             exclude = ("run.py", "rpc.py", "threading.py", "queue.py",
                        "RemoteDebugger.py", "bdb.py")
             cleanup_traceback(tbe, exclude)
             traceback.print_list(tbe, file=efile)
-    lines = traceback.format_exception_only(typ, val)
+        lines = traceback.format_exception_only(typ, exc)
         for line in lines:
             print(line, end='', file=efile)
 
+    print_exc(typ, val, tb)
+
 def cleanup_traceback(tb, exclude):
     "Remove excluded traces from beginning/end of tb; get cached lines"
     orig_tb = tb[:]

Misc/NEWS

@@ -209,6 +209,8 @@ Core and Builtins
 Library
 -------
 
+- Issue #16491: IDLE now prints chained exception tracebacks.
+
 - fcntl: add F_DUPFD_CLOEXEC constant, available on Linux 2.6.24+.
 
 - Issue #15972: Fix error messages when os functions expecting a file name or