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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.

Oh, and, with apologies for the double post, tuple does the same, while set, dict, collections.deque do not.

It's an optimization. No one says we can't call __len__().