On Fri, Feb 23, 2018 at 9:51 AM, Guido van Rossum <guido@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.
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