Issue #24695: Fix a regression in traceback.print_exception()

If exc_traceback is None we shouldn't print a traceback header
like described in the documentation.

AUTHORS

@@ -1 +1,2 @@
 Robert Collins <rbtcollins@hp.com>
+Berker Peksag <berker.peksag@gmail.com>

traceback2/__init__.py

@@ -465,6 +465,7 @@ class TracebackException:
                 _seen=_seen)
         else:
             context = None
+        self.exc_traceback = exc_traceback
         self.__cause__ = cause
         self.__context__ = context
         self.__suppress_context__ = \
@@ -590,7 +591,8 @@ class TracebackException:
                 for line in self.__context__.format(chain=chain):
                     yield line
                 yield _context_message
-        yield u('Traceback (most recent call last):\n')
+        if self.exc_traceback is not None:
+            yield u('Traceback (most recent call last):\n')
         for line in self.stack.format():
             yield line
         for line in self.format_exception_only():

traceback2/tests/test_traceback.py

@@ -793,6 +793,12 @@ class TestTracebackException(unittest.TestCase):
         exc = traceback.TracebackException(Exception, e, tb)
         self.assertEqual(exc.stack[0].locals, None)
 
+    def test_traceback_header(self):
+        # do not print a traceback header if exc_traceback is None
+        # see issue #24695
+        exc = traceback.TracebackException(Exception, Exception("haven"), None)
+        self.assertEqual(list(exc.format()), [u("Exception: haven\n")])
+
     def test_syntax_no_extras(self):
         linecache.updatecache('/foo.py', fake_module)
         e = SyntaxError("uh oh")