[Python-ideas] Generator unpacking

[Andrew Barnert]

On Feb 15, 2016, at 15:43, Steven D'Aprano <steve at pearwood.info> wrote:
> 
>> On Mon, Feb 15, 2016 at 06:53:45PM +0100, Sven R. Kunze wrote:
>>> On 15.02.2016 18:39, Ethan Furman wrote:
>>> I /think/ I've used that trick to confirm an iterable was empty; I 
>>> /know/ I've used
>>> 
>>> [some_var] = some_var
>> 
>> Is that an unknown Python idiom?
> 
> No, it's exactly the same Python idiom (assignment to a list of targets) 
> as we've been talking about for the last few posts. We've had examples 
> with four targets, three targets, two targets and zero targets. This is 
> an example with one target.
> 
> [a] = iterable
> 
> requires that the right-hand side be iterable, and after unpacking it 
> must contain exactly one item, to match the one assignment target given 
> on the left.

I see this more often written in tuple-ish form:

    index, value = struct.unpack_from('!HH', buf, off)
    namelen, = struct.unpack_from('!H', buf, off+4)

Somehow, that feels more natural and pythonic than using [index, value] and [namelen], despite the fact that without the brackets it's easy to miss the trailing comma and end up with (16,) instead of 16 as your length and a confusing TypeError a few lines down.