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