Handle objects with broken __unicode__

Because Python 2 objects may have __unicode__ broken, and we need
unicode output to avoid implicit decodes, explicitly handle
__unicode__.

README.rst

@@ -6,7 +6,12 @@ Profit.
 
 Things to be aware of!
 
-In Python 2.x, unlike traceback, traceback2 creates unicode output.
+In Python 2.x, unlike traceback, traceback2 creates unicode output (because it
+depends on the linecache2 module).
 
 Exception frame clearing silently does nothing if the interpreter in use does
 not support it.
+
+traceback2._some_str, which while not an official API is so old its likely in
+use behaves similarly to the Python3 version - objects where unicode(obj) fails
+but str(object) works will be shown as b'thestrvaluerepr'.

traceback2/__init__.py

@@ -4,7 +4,7 @@ import sys
 import operator
 
 import linecache2 as linecache
-from six import u
+from six import u, PY2
 
 __all__ = ['extract_stack', 'extract_tb', 'format_exception',
            'format_exception_only', 'format_list', 'format_stack',
@@ -141,14 +141,24 @@ 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:
-        line = "%s\n" % etype
+        line = u("%s\n") % etype
     else:
-        line = "%s: %s\n" % (etype, valuestr)
+        line = u("%s: %s\n") % (etype, valuestr)
     return line
 
 def _some_str(value):
     try:
-        return str(value)
+        if PY2:
+            # If there is a working __unicode__, great.
+            # Otherwise see if we can get a bytestring...
+            # Otherwise we fallback to unprintable.
+            try:
+                return unicode(value)
+            except:
+                return "b%s" % repr(str(value))
+        else:
+            # For Python3, bytestrings don't implicit decode, so its trivial.
+            return str(value)
     except:
         return '<unprintable %s object>' % type(value).__name__
 
@@ -508,7 +518,7 @@ class TracebackException:
 
         stype = getattr(self.exc_type, '__qualname__', self.exc_type.__name__)
         smod = u(self.exc_type.__module__)
-        if smod not in ("__main__", "builtins"):
+        if smod not in ("__main__", "builtins", "exceptions"):
             stype = smod + u('.') + stype
 
         if not issubclass(self.exc_type, SyntaxError):

traceback2/tests/test_traceback.py

@@ -12,7 +12,7 @@ import contextlib2 as contextlib
 import fixtures
 import linecache2 as linecache
 import six
-from six import text_type, u
+from six import b, text_type, u
 try:
     from six import raise_from
 except ImportError:
@@ -153,6 +153,14 @@ class SyntaxTracebackCases(testtools.TestCase):
             str_name = '.'.join([X.__module__, qualname(X)])
         self.assertEqual(err[0], "%s: %s\n" % (str_name, str_value))
 
+    def test_format_exception_only_undecodable__str__(self):
+        # This won't decode via the ascii codec.
+        X = Exception(u('\u5341').encode('shift-jis'))
+        err = traceback.format_exception_only(type(X), X)
+        self.assertEqual(len(err), 1)
+        str_value = "b'\\x8f\\\\'"
+        self.assertEqual(err[0], "Exception: %s\n" % str_value)
+
     def test_without_exception(self):
         err = traceback.format_exception_only(None, None)
         self.assertEqual(err, ['None\n'])
@@ -619,7 +627,7 @@ class TestStack(unittest.TestCase):
                 traceback.walk_stack(None), capture_locals=True, limit=1)
         s = some_inner(3, 4)
         self.assertEqual(
-            ['  File "' + FNAME + '", line 619, '
+            ['  File "' + FNAME + '", line 627, '
              'in some_inner\n'
              '    traceback.walk_stack(None), capture_locals=True, limit=1)\n'
              '    a = 1\n'