Fix exception_to_unicode() for oslo_i18n Message

Message instances created by oslo_i18n are subclasses of the Unicode
type (unicode on Python 2, str on Python 3) and have no __unicode__()
method. exception_to_unicode() raises an AttributeError when trying to
convert it to Unicode.

This change fixes this issue and adds an unit test.

Change-Id: Ica67429ac64f74e5c636b6d74d71910a26511378

oslo_utils/encodeutils.py

@@ -107,13 +107,23 @@ def exception_to_unicode(exc):
     """
     msg = None
     if six.PY2:
-        # Don't call directly unicode(exc), because it fails with
-        # UnicodeDecodeError on Python 2 if exc.__unicode__() returns a bytes
-        # string not decodable from the default encoding (ASCII)
+        # First try by calling the unicode type constructor. We should try
+        # unicode() before exc.__unicode__() because subclasses of unicode can
+        # be easily casted to unicode, whereas they have no __unicode__()
+        # method.
         try:
-            msg = exc.__unicode__()
+            msg = unicode(exc)
         except UnicodeError:
-            pass
+            # unicode(exc) fail with UnicodeDecodeError on Python 2 if
+            # exc.__unicode__() or exc.__str__() returns a bytes string not
+            # decodable from the default encoding (ASCII)
+            if hasattr(exc, '__unicode__'):
+                # Call directly the __unicode__() method to avoid
+                # the implicit decoding from the default encoding
+                try:
+                    msg = exc.__unicode__()
+                except UnicodeError:
+                    pass
 
     if msg is None:
         # Don't call directly str(exc), because it fails with

oslo_utils/tests/tests_encodeutils.py

@@ -20,6 +20,7 @@ from oslotest import base as test_base
 import six
 import testtools
 
+import oslo_i18n.fixture
 from oslo_utils import encodeutils
 
 
@@ -231,3 +232,9 @@ class ExceptionToUnicodeTest(test_base.BaseTestCase):
         exc = UnicodeOnlyException(b'utf-8 \xc3\xa9\xe2\x82\xac')
         self.assertEqual(encodeutils.exception_to_unicode(exc),
                          u'utf-8 \xe9\u20ac')
+
+    def test_oslo_i18n_message(self):
+        # use the lazy translation to get a Message instance of oslo_i18n
+        exc = oslo_i18n.fixture.Translation().lazy("test")
+        self.assertEqual(encodeutils.exception_to_unicode(exc),
+                         u"test")