make "ustr.ustr" Python 3 compatible

CHANGES.rst

@@ -4,6 +4,9 @@ Changelog
 4.0 (unreleased)
 ----------------
 
+- Make ``ustr.ustr`` Python 3 compatible
+  (`Zope#921 <https://github.com/zopefoundation/Zope/issues/921>`_)
+
 - Restore ``sql_quote`` behavior of always returning native strings
   (`#54 <https://github.com/zopefoundation/DocumentTemplate/issues/54>`_)
 

src/DocumentTemplate/tests/testustr.py

@@ -34,6 +34,12 @@ class Bar(str):
     pass
 
 
+class RequestLike(object):
+    def __str__(self): return "RequestLike"
+
+    args = () # to confuse `ustr._exception_str`
+
+
 class UnicodeTests(unittest.TestCase):
 
     def test_bare_string_literall(self):
@@ -46,6 +52,10 @@ def test_with_force_str(self):
         a = ustr(force_str('hello'))
         self.assertEqual(a, 'hello')
 
+    def test_RequestLike(self):
+        from DocumentTemplate.ustr import ustr
+        self.assertEqual(ustr(RequestLike()), "RequestLike")
+
     def test_with_non_ascii_char(self):
         from DocumentTemplate.ustr import ustr
         a = ustr(chr(200))

src/DocumentTemplate/ustr.py

@@ -12,7 +12,7 @@
 ##############################################################################
 """ustr function."""
 
-nasty_exception_str = getattr(Exception.__str__, 'im_func', None)
+nasty_exception_str = getattr(Exception.__str__, '__func__', None)
 
 
 def ustr(v):
@@ -31,7 +31,7 @@ def ustr(v):
             # they all constrain the type which potentially raises an
             # exception.
             # To avoid exceptions we have to call __str__ direct.
-            if getattr(fn, 'im_func', None) == nasty_exception_str:
+            if getattr(fn, '__func__', None) == nasty_exception_str:
                 # Exception objects have been optimised into C, and their
                 # __str__ function fails when given a unicode object.
                 # Unfortunately this scenario is all too common when