On Tue, Nov 19, 2019 at 4:41 AM Andrew Barnert via Python-ideas <python-ideas@python.org> wrote: > > On Nov 18, 2019, at 04:32, Ned Batchelder <n...@nedbatchelder.com> wrote: > > There are uses for comprehension syntax for making tuples, but they occur far > far less frequently than the comprehensions we have. To me, the existing way > to do it makes perfect sense, and exactly expresses what needs to happen: > > tuple(i for i in range(10)) > > > It’s worth noting that this isn’t 100% identical to what a tuple > comprehension would do—but only to know exactly what the differences are so > we can conclude they don’t matter. > > def stop(): raise StopIteration > a = list(stop() if i%3==2 else I for i in range(10)) > b = [stop() if i%3==2 else I for i in range(10)] > > The second line will construct a = [0,1], but the third will raise a > StopIteration to the user and not construct anything or bind b. You can also > put the stop() in an if clause. > > Back around 3.2-ish, the way comprehensions were defined implied that these > should be identical. I looked into whether we could add this behavior to > listcomps (or actually implement a listcomp on top of a genexpr without a > performance cost). But the overwhelming consensus was that this is an > antipattern and the right fix was to change the docs. (Also, there used to be > more varied opportunities for this kind of hackery, but now they raise > RuntimeError.) >
As of current versions of Python, the difference is now relatively insignificant: the third one halts with StopIteration, but the second will halt with RuntimeError chained to StopIteration. Neither of them will give you a truncated list :) ChrisA _______________________________________________ Python-ideas mailing list -- python-ideas@python.org To unsubscribe send an email to python-ideas-le...@python.org https://mail.python.org/mailman3/lists/python-ideas.python.org/ Message archived at https://mail.python.org/archives/list/python-ideas@python.org/message/W77ESTJMACUK5F7WHFADEGZDTZCBLWWE/ Code of Conduct: http://python.org/psf/codeofconduct/
