[Python-Dev] The `for y in [x]` idiom in comprehensions

[Guido van Rossum]

As to the validity or legality of this code, it's both, and working as
intended.

A list comprehension of the form

    [STUFF for VAR1 in SEQ1 for VAR2 in SEQ2 for VAR3 in SEQ3]

should be seen (informally) as

    for VAR1 in SEQ1:
        for VAR2 in SEQ2:
            for VAR3 in SEQ3:
                "put STUFF in the result"

(If there are `if COND` phrases too those get inserted into the nested set
of blocks where they occur in the sequence.)


On Fri, Feb 23, 2018 at 9:41 AM, Chris Barker - NOAA Federal <
chris.barker at noaa.gov> wrote:

> > Is it similar enough to
> >
> >    def f(x=[0]):
>
> No, not at all — it’s a very different use case.
>
> When I first saw this on the original thread, I needed to stare at it
> a good while, and then whip up some code to experiment with it to know
> what it did.
>
> And not because I don’t know what a single element list means, or what
> it means to iterate over a single element list, or what two fors mean
> in a comprehension.
>
> I was confused by the ‘x’ in the second iterable. I guess I’m (still)
> not really clear on the scope(s) inside a comprehension, and when the
> elements get evaluated in a list.
>
> I expected that the list would be created once, with the value x had
> initially, rather than getting the-evaluated each time through the
> outer loop.
>
> So I think that it is a very confusing use of comprehensions, and
> always will be. I’m still surprised it’s legal. Anyone know if this
> being allowed was deliberate or just kind of fell out of the
> implementation?
>
> So no, I don’t think it should be promoted as idiomatic.
>
> All that being said, it’s valid Python, so why not optimize it?
>
> -CHB
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-- 
--Guido van Rossum (python.org/~guido)
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