[Tutor] Multiple for and if/else statements into a single list comprehension

[Peter Otten]

Jignesh Sutar wrote:

> Is it possible to get two nested for statements followed by a nested
> if/else statement all into a single list comprehension ie. the equivalent
> of the below:
> 
> 
> for i in xrange(1,20):
>     for j in xrange(1,10):
>         if j<6:
>             j=int("8"+str(j))
>         else:
>             j=int("9"+str(j))
>         print "%(i)02d_%(j)02d" % locals()

> Many thanks in advance.

Need I say that it is a bad idea to build overly complex list 
comprehensions? Under that proviso:

The first step is always to ensure that there is a single expression in the 
inner loop. I'm keeping as similar as possible to your loops:

for i in xrange(1, 20):
    for j in xrange(1, 10):
        print "%02d_%02d" % (i,
            int(("8" if j < 6 else "9") + str(j)))

Now the translation is mechanical

for x in a:
    for y in b:
        expr

becomes

[expr for x in a for y in b]

or (using a genexp rather than a listcomp as you are going to print it 
anyway)

print "\n".join(
    "%02d_%02d" % (i, int(("8" if j < 6 else "9") + str(j)))
    for i in xrange(1, 20)
    for j in xrange(1, 10))

But again: don't do that ;)

By the way, using locals() is another bad idea, plus it does not capture the 
loop vars of genexps (all pythons) and listcomps (python3).

PS: a simple alternative:

print "\n".join(
    "%02d_%02d" % (i, k) 
    for i in range(1, 20) 
    for k  in range(81, 86) + range(96, 100))