bpo-43146: fix None-handling in single-arg traceback.print_exception(…

…None) (pythonGH-24629)

(The previous commit fixed print_exception(None, None, None).)

Lib/test/test_traceback.py

@@ -234,6 +234,10 @@ def test_format_exception_only_exc(self):
 
     def test_exception_is_None(self):
         NONE_EXC_STRING = 'NoneType: None\n'
+        excfile = StringIO()
+        traceback.print_exception(None, file=excfile)
+        self.assertEqual(excfile.getvalue(), NONE_EXC_STRING)
+
         excfile = StringIO()
         traceback.print_exception(None, None, None, file=excfile)
         self.assertEqual(excfile.getvalue(), NONE_EXC_STRING)
@@ -243,6 +247,7 @@ def test_exception_is_None(self):
         self.assertEqual(excfile.getvalue(), NONE_EXC_STRING)
 
         self.assertEqual(traceback.format_exc(None), NONE_EXC_STRING)
+        self.assertEqual(traceback.format_exception(None), [NONE_EXC_STRING])
         self.assertEqual(
             traceback.format_exception(None, None, None), [NONE_EXC_STRING])
         self.assertEqual(

Lib/traceback.py

@@ -91,7 +91,10 @@ def _parse_value_tb(exc, value, tb):
     if (value is _sentinel) != (tb is _sentinel):
         raise ValueError("Both or neither of value and tb must be given")
     if value is tb is _sentinel:
-        return exc, exc.__traceback__
+        if exc is not None:
+            return exc, exc.__traceback__
+        else:
+            return None, None
     return value, tb
 
 

Misc/NEWS.d/next/Library/2021-02-23-17-20-16.bpo-43146.JAFplg.rst

@@ -0,0 +1 @@
+Handle None in single-arg versions of :func:`~traceback.print_exception` and :func:`~traceback.format_exception`.