[Python-ideas] How assignment should work with generators?

Chris Angelico rosuav at gmail.com
Mon Nov 27 08:47:45 EST 2017

On Mon, Nov 27, 2017 at 11:31 PM, Kirill Balunov
<kirillbalunov at gmail.com> wrote:
>> In terms of language proposals, you can't just say "don't need values
>> for"; the semantics have to be EITHER "consume and discard" OR "don't
>> consume". We already have a perfectly good way of spelling "consume
>> and discard":
>> x, y, _ = iter
> You mean ( x, y, *_ = iter ) ?

Uhh, yeah, that's what I meant. Sorry. Anyhow, point is, there IS a
syntax for that, so we don't need another.

>> The proposed semantics, if I understand you correctly, are:
>> try:
>>     _iter = iter(it)
>>     x = next(_iter)
>>     y = next(_iter)
>> except StopIteration:
>>     raise ValueError
>> # no "else" clause, we're done here
> Yes, "roughly" this semantics is proposed, with some assumptions on _iter =
> iter(it).
> As I can see at the moment, these cases should behave differently:
>>>> x, y = [1,2,3,4]             # must raise ValueError
>>>> x, y = iter([1,2,3,4])      # should work
> But at the same time, it violates current situation. So maybe, as you have
> said we need special syntax. I will think about it.

That's the part I disagree with, but if you're the PEP author, you can
make the recommendation be anything you like. However, one very strong
piece of advice: it's easier to get a proposal accepted if the
backward compatibility section simply says "the proposed notation is a
SyntaxError in current versions of Python". Changing the semantics of
currently legal code requires that you demonstrate that the current
semantics are, in some way, faulty or buggy.


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