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

[Paul Moore]

On 27 November 2017 at 16:05, Chris Angelico <rosuav at gmail.com> wrote:
> On Tue, Nov 28, 2017 at 2:35 AM, Steven D'Aprano <steve at pearwood.info> wrote:
>> In this case, there is a small but real benefit to counting the
>> assignment targets and being explicit about the number of items to
>> slice. Consider an extension to this "non-consuming" unpacking that
>> allowed syntax like this to pass silently:
>>
>>     a, b = x, y, z
>>
>> That ought to be a clear error, right? I would hope you don't think that
>> Python should let that through. Okay, now we put x, y, z into a list,
>> then unpack the list:
>>
>>     L = [x, y, z]
>>     a, b = L
>>
>> That ought to still be an error, unless we explicity silence it. One way
>> to do so is with an explicit slice:
>>
>>     a, b = L[:2]
>
> I absolutely agree with this for the default case. That's why I am
> ONLY in favour of the explicit options. So, for instance:
>
> a, b = x, y, z # error
> a, b, ... = x, y, z # valid (evaluates and ignores z)

Agreed, only explicit options are even worth considering (because of
backward compatibility if for no other reason). However, the unpacking
syntax is already complex, and hard to search for. Making it more
complex needs a strong justification. And good luck in doing a google
search for "..." if you saw that code in a project you had to
maintain. Seriously, has anyone done a proper investigation into how
much benefit this proposal would provide? It should be reasonably easy
to do a code search for something like "=.*islice", to find code
that's a candidate for using the proposed syntax. I suspect there's
very little code like that.

I'm -1 on this proposal without a much better justification of the
benefits it will bring.
Paul