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I think the generator.__next__() documentation should say that it raises an exception if the generator is already executing:
http://docs.python.org/dev/reference/expressions.html#generator.__next__
I don't think this is currently mentioned anywhere in the section on yield expressions.
I think this is worth mentioning because this is different from the general situation for iterators, for example. One consequence of this is that, unlike for iterators, using a bare generator in a multithreaded situation will always result in a critical section (since an iterator can be made to take care of its own locking, etc).