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The following code raises an unexpected exception:
 class Foo(object):
 def __len__(self):
 raise Exception()
 def __iter__(self):
 return iter([])
 list(Foo())
In the optimizations being done in listextend, it appears len is getting called first for some reason (sorry, haven't gotten a chance to step through it carefully yet).
Tangentially, PyPy (correctly I guess) throws no such exception.