Temporary variables in list comprehensions

Mon Jan 9 07:59:56 EST 2017

On Monday, January 9, 2017 at 5:54:15 PM UTC+5:30, Tim Chase wrote:
> On 2017年01月09日 02:46, Paul Rubin wrote:
> > > gen = (expensive_calculation(x) for x in data)
> > > result = [(tmp, tmp + 1) for tmp in gen] 
> > 
> > result = [(tmp, tmp+1) for tmp in map(expensive_calculation, data)]
>> As charmingly expressive as map() is, the wildly different behavior in
> py3 (it's a generator that evaluates lazily) vs py2 (it consumes the
> entire iterable in one go) leads me to avoid it in general,
> especially when Python gives me list-comprehensions and generators
> that can do what I intend explicitly. E.g., passing in
> itertools.count() as an iterable to map() will kill py2 but is
> perfectly fine in py3 (a horrible example):
>> import itertools as i
> for x in i.takewhile(
> lambda n: n < 100, 
> map(lambda g: g**2, i.count())
> ):
> print(x)
>> But yes, both ChrisA's pass-it-to-a-function and Serhiy's nested
> generate-the-tmp-values-then-operate-on-those are good solutions
> depending on how they feel in any particular context. If I need to
> use an "if" clause in the outer generator that tests the resulting
> temp values, I use Serhiy's solution:
>> [(tmp, tmp + 1)
> for tmp in (
> expensive_calculation(x)
> for x in data
> )
> if tmp > 42
> ]

What happens when the expensive is on an inner generator?
Something like:
[expensive₂(y) for x in data for y in foo(x)]
[The ₂ representing the 2 or more occurrences in Steven's eg]