There are useful things you can only do with comprehensions if the second for-loop can use the variable in the first for-loop. E.g.
[(i, j) for i in range(10) for j in range(i)] On Fri, Feb 23, 2018 at 10:16 AM, Chris Barker <chris.bar...@noaa.gov> wrote: > 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 > NOAA/NOS/OR&R (206) 526-6959 voice > 7600 Sand Point Way NE > <https://maps.google.com/?q=7600+Sand+Point+Way+NE&entry=gmail&source=g> > (206) 526-6329 fax > Seattle, WA 98115 (206) 526-6317 main reception > > chris.bar...@noaa.gov > -- --Guido van Rossum (python.org/~guido)
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