A while loop and temporary mutable variables are definitely not the Pythonic way of doing this. More idiomatic:

    $ time python -c 'print sum(xrange(100000000 + 1))'
    5000000050000000
    
    real    0m1.398s
    user    0m1.383s
    sys     0m0.012s

Comparison to baseline:

    $ time (echo -e 'n=0 \ni=0 \nwhile (i <= 100000000): \n  n += i \n  i += 1 \n\nprint(n)\n' | python)
    5000000050000000
    
    real    0m33.140s
    user    0m32.939s
    sys     0m0.023s

I see. That is good to know; I merely chose mutating global variables in a loop because I knew how to do that in both languages. (I am not very familiar with Python.) That is not the idiomatic way to do it in Arc, either. I would normally use a recursive function, like this:

  arc> (time:xloop (i 0 n 0) (if (> i 100000000) n (next (+ i 1) (+ n i))))
  time: 9121 cpu: 9130 gc: 0 mem: 480  ; the times are in msec
  5000000050000000

Or perhaps a "higher-order function":

  arc> (time:sum idfn 1 100000000)
  time: 19889 cpu: 19908 gc: 0 mem: 1224
  5000000050000000

Or use a deforestation macro that I wrote, which is closest to your Python example:

  arc> (time:thunkify:reduce + (range 1 100000000))
  time: 17971 cpu: 17985 gc: 0 mem: 3592
  5000000050000000

Also, here's what you can get by dropping into Racket:

  arc> (time:$:let loop ((i 0) (n 0)) (if (> i 100000000) n (loop (+ i 1) (+ n i))))
  time: 402 cpu: 403 gc: 0 mem: 920
  5000000050000000

I suppose Python has an analogue of that--dropping into C, or at least loading C libraries. Which Racket can do too. Mmm.