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When running "2to3 -f raise" on the following code, which uses an old Python 2.4 raise of a string:
def f():
 raise ("message %s %s" % (1, 2))
try:
 f()
finally:
 pass
I obtain the following quite surprising result. I would have expected to get either an error or leave the original file unchanged, because "raise (s)" is syntactically valid although incorrect. 
$ 2to3 -f raise w.py
RefactoringTool: Refactored w.py
--- w.py	(original)
+++ w.py	(refactored)
@@ -1,5 +1,5 @@
 def f():
- raise ("message %s %s" % (1, 2))
+ raise "message %s %s"
 
 try:
 f()