Issue #27348: Restore “Exception: None” formatting in traceback module

This fixes a regression caused by revision 73afda5a4e4c. Also reverts the
decimal test workaround added in revision 5f3dd0a2b1ab.

Remove test_without_exception(). According to revision ecaafc32c500, this was
added in Python 2 so that print_exc() would output “None” when called with no
exception set. However print_exc() never worked like this in Python 3, and
the use case is not documented.

Restore TracebackCases class name (instead of SyntaxTracebackCases), because
the class also tests other exceptions.

Lib/_pydecimal.py

@@ -4106,7 +4106,7 @@ class Context(object):
         >>> context.create_decimal_from_float(3.1415926535897932)
         Traceback (most recent call last):
             ...
-        decimal.Inexact
+        decimal.Inexact: None
 
         """
         d = Decimal.from_float(f)       # An exact conversion

Lib/test/test_traceback.py

@@ -19,7 +19,7 @@ test_frame = namedtuple('frame', ['f_code', 'f_globals', 'f_locals'])
 test_tb = namedtuple('tb', ['tb_frame', 'tb_lineno', 'tb_next'])
 
 
-class SyntaxTracebackCases(unittest.TestCase):
+class TracebackCases(unittest.TestCase):
     # For now, a very minimal set of tests.  I want to be sure that
     # formatting of SyntaxErrors works based on changes for 2.1.
 
@@ -106,10 +106,6 @@ class SyntaxTracebackCases(unittest.TestCase):
             str_name = '.'.join([X.__module__, X.__qualname__])
         self.assertEqual(err[0], "%s: %s\n" % (str_name, str_value))
 
-    def test_without_exception(self):
-        err = traceback.format_exception_only(None, None)
-        self.assertEqual(err, ['None\n'])
-
     def test_encoded_file(self):
         # Test that tracebacks are correctly printed for encoded source files:
         # - correct line number (Issue2384)
@@ -456,6 +452,17 @@ class BaseExceptionReportingTests:
         msg = self.get_report(e).splitlines()
         self.assertEqual(msg[-2], '              ^')
 
+    def test_message_none(self):
+        # A message that looks like "None" should not be treated specially
+        err = self.get_report(Exception(None))
+        self.assertIn('Exception: None\n', err)
+        err = self.get_report(Exception('None'))
+        self.assertIn('Exception: None\n', err)
+        err = self.get_report(Exception())
+        self.assertIn('Exception\n', err)
+        err = self.get_report(Exception(''))
+        self.assertIn('Exception\n', err)
+
 
 class PyExcReportingTests(BaseExceptionReportingTests, unittest.TestCase):
     #

Lib/traceback.py

@@ -140,7 +140,7 @@ def format_exception_only(etype, value):
 
 def _format_final_exc_line(etype, value):
     valuestr = _some_str(value)
-    if value == 'None' or value is None or not valuestr:
+    if value is None or not valuestr:
         line = "%s\n" % etype
     else:
         line = "%s: %s\n" % (etype, valuestr)

Misc/NEWS

@@ -77,6 +77,10 @@ Core and Builtins
 Library
 -------
 
+- Issue #27348: In the traceback module, restore the formatting of exception
+  messages like "Exception: None".  This fixes a regression introduced in
+  3.5a2.
+
 - Issue #25651: Allow falsy values to be used for msg parameter of subTest().
 
 - Issue #27932: Prevent memory leak in win32_ver().