I can't agree with what you wrote in "Style vs Performance". Your example seems like a classic example of premature optimization.

Especially this sentence:

whenever you can, and style doesn’t matter

Style matters a lot. Assuming that what you write is not a single-use program that'll be forgotten after execution, your code will be read much, much more often than written or refactored. This is even more true when you collaborate with others - then you can either focus on style first, or write terribly styled code and spend thrice as time on writing comments or docs, or be a jerk to your coworkers and write just the terribly styled code.

And as for your example, I measured it:

$ python -m timeit 'array=[1,1,1,1,1,1,1,1,1,1]
sum=0
for a in range(0,10):
  sum+=array[a]'
1000000 loops, best of 3: 0.802 usec per loop

$ python -m timeit 'array=[1,1,1,1,1,1,1,1,1,1]
sum=array[0]+array[1]+array[2]+array[3]+array[4]+array[5]+array[6]+array[7]+array[8]+array[9]'
1000000 loops, best of 3: 0.333 usec per loop

0.802µs vs 0.333µs. Yeah, it's slower. Does it matter? Most likely not. What matters is that second example is much less readable and more error-prone (which you even admit yourself).