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Fri Feb 23 13:16:03 EST 2018 · https://mail.python.org/pipermail/python-dev/2018-February/152272.html

On Fri, Feb 23, 2018 at 9:51 AM, Guido van Rossum <guido at python.org> wrote:

> 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"
>

Thanks -- right after posting, I realized that was the way to unpack it to
understand it. I think my confusion came from two things:

1) I usually don't care in which order the loops are ordered -- i.e., that
could be:

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

As I usually don't care, I have to think about it (and maybe experiment to
be sure). (this is the old Fortran vs C order thing :-)

2) since it's a single expression, I wasn't sure of the evaluation order,
so maybe (in my head) it could have been (optimized) to be:

[STUFF for  VAR1 in Expression_that_evaluates_to_an_iterable1 for VAR2 in
Expression_that_evaluates_to_an_iterable2]

and that could translate to:

IT1 = Expression_that_evaluates_to_an_iterable1
IT2 = Expression_that_evaluates_to_an_iterable2
for VAR1 in IT1:
    for VAR2 in IT2:
        "put STUFF in the result"

In which case, VAR1 would not be available to
Expression_that_evaluates_to_an_iterable2.

Maybe that was very wrong headed -- but that's where my head went -- and
I'm not a Python newbie (maybe an oddity, though :-) )

-CHB

-- 

Christopher Barker, Ph.D.
Oceanographer

Emergency Response Division
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Chris.Barker at noaa.gov
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[Python-Dev] The `for y in [x]` idiom in comprehensions