Regarding the sample code in: http://docs.python.org/library/itertools#itertools.izip_longest

If an IndexError is raised inside an iterator in the "equivalent" code
from the docs, the generator stops and the error is swallowed.

If an IndexError is raised inside an iterator in the
"izip_longest_next" code in CPython's "Modules/itertoolsmodule.c", the
generator stops and the error propagates.

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Reported by Adam Forsyth on the docs mailing list. More details in this SO discussion: http://stackoverflow.com/questions/7392902/izip-longest-in-itertools-how-does-rasing-indexerror-inside-the-iterator-work