A version that actually works.

testtools/_compat2x.py

@@ -0,0 +1,17 @@
+# Copyright (c) 2011 testtools developers. See LICENSE for details.
+
+"""Compatibility helpers that are valid syntax in Python 2.x.
+
+Only add things here if they *only* work in Python 2.x or are Python 2
+alternatives to things that *only* work in Python 3.x.
+"""
+
+__all__ = [
+    'reraise',
+    ]
+
+
+def reraise(exc_class, exc_obj, exc_tb, _marker=object()):
+    """Re-raise an exception received from sys.exc_info() or similar."""
+    raise exc_class, exc_obj, exc_tb
+

testtools/_compat3x.py

@@ -0,0 +1,17 @@
+# Copyright (c) 2011 testtools developers. See LICENSE for details.
+
+"""Compatibility helpers that are valid syntax in Python 3.x.
+
+Only add things here if they *only* work in Python 3.x or are Python 3
+alternatives to things that *only* work in Python 2.x.
+"""
+
+__all__ = [
+    'reraise',
+    ]
+
+
+def reraise(exc_class, exc_obj, exc_tb, _marker=object()):
+    """Re-raise an exception received from sys.exc_info() or similar."""
+    raise exc_class(*exc_obj.args).with_traceback(exc_tb)
+

testtools/compat.py

@@ -7,8 +7,14 @@ __all__ = [
     '_b',
     '_u',
     'advance_iterator',
+    'all',
+    'BytesIO',
+    'classtypes',
+    'isbaseexception',
+    'istext',
     'str_is_unicode',
     'StringIO',
+    'reraise',
     'unicode_output_stream',
     ]
 
@@ -25,6 +31,14 @@ from testtools.helpers import try_imports
 BytesIO = try_imports(['StringIO.StringIO', 'io.BytesIO'])
 StringIO = try_imports(['StringIO.StringIO', 'io.StringIO'])
 
+try:
+    from testtools import _compat2x as _compat
+    _compat
+except SyntaxError:
+    from testtools import _compat3x as _compat
+
+reraise = _compat.reraise
+
 
 __u_doc = """A function version of the 'u' prefix.
 
@@ -284,10 +298,3 @@ def _format_exc_info(eclass, evalue, tb, limit=None):
     else:
         list.append("%s\n" % sclass)
     return list
-
-
-def reraise(exc_class, exc_obj, exc_tb, _marker=object()):
-    """Re-raise an exception received from sys.exc_info() or similar."""
-    if getattr(exc_class, 'with_traceback', _marker) is _marker:
-        raise exc_class(exc_obj).with_traceback(exc_tb)
-    raise exc_class, exc_obj, exc_tb