be as helpful as we can in giving a hint for a failing test against t…

…raceback output.

src/zope/testing/renormalizing.py

@@ -16,6 +16,13 @@
 
 
 EXCEPTION_2TO3 = doctest.register_optionflag('EXCEPTION_2TO3')
+EXCEPTION_2TO3_HINT = """
+===============================================================
+HINT:
+  You seem to test traceback output.
+  The optionflag EXCEPTION_2TO3 is set.
+  Do you use the full dotted name for the exception class name?
+==============================================================="""
 
 
 class OutputChecker(doctest.OutputChecker):
@@ -71,6 +78,12 @@ def output_difference(self, example, got, optionflags):
 
         # temporarily hack example with normalized want:
         example.want = want
+
+        if sys.version < '3':
+            if optionflags & EXCEPTION_2TO3:
+                if maybe_a_traceback(got) is not None:
+                    got += EXCEPTION_2TO3_HINT
+
         result = doctest.OutputChecker.output_difference(
             self, example, got, optionflags)
         example.want = original
@@ -80,19 +93,28 @@ def output_difference(self, example, got, optionflags):
 RENormalizing = OutputChecker
 
 
-def strip_dottedname_from_traceback(string):
-    # We might want to confirm more strictly were dealing with a traceback.
-    # We'll assume so for now.
+def maybe_a_traceback(string):
     if not string:
-        return string
+        return None
 
     lines = string.splitlines()
     last = lines[-1]
     words = last.split(' ')
     first = words[0]
     if not first.endswith(':'):
+        return None
+
+    return lines, last, words, first
+
+
+def strip_dottedname_from_traceback(string):
+    # We might want to confirm more strictly were dealing with a traceback.
+    # We'll assume so for now.
+    maybe = maybe_a_traceback(string)
+    if maybe is None:
         return string
 
+    lines, last, words, first = maybe
     name = first.split('.')[-1]
     words[0] = name
     last = ' '.join(words)

src/zope/testing/renormalizing.txt

@@ -252,3 +252,38 @@ Using the 2to3 exception normalization:
     ...     expected = False
     >>> result == expected
     True
+
+When reporting a failing test and running in Python 2, the normalizer tries
+to be helpful by explaining how to test for exceptions in the traceback output.
+
+    >>> want = """\
+    ... Traceback (most recent call last):
+    ... foo.bar.FooBarErrorXX: requires at least one argument."""
+    >>> got = """\
+    ... Traceback (most recent call last):
+    ... FooBarError: requires at least one argument."""
+    >>> checker.check_output(want, got, EXCEPTION_2TO3)
+    False
+    >>> from doctest import Example
+    >>> example = Example('dummy', want)
+    >>> result = checker.output_difference(example, got, EXCEPTION_2TO3)
+    >>> output = """\
+    ... Expected:
+    ...     Traceback (most recent call last):
+    ...     foo.bar.FooBarErrorXX: requires at least one argument.
+    ... Got:
+    ...     Traceback (most recent call last):
+    ...     FooBarError: requires at least one argument."""
+    >>> hint = """
+    ...     ===============================================================
+    ...     HINT:
+    ...       You seem to test traceback output.
+    ...       The optionflag EXCEPTION_2TO3 is set.
+    ...       Do you use the full dotted name for the exception class name?
+    ...     ==============================================================="""
+    >>> if sys.version < '3':
+    ...     expected = output + hint
+    ... else:
+    ...     expected = output
+    >>> result == expected
+    True