The first question to ask is - do you actually need to optimize the code ?

What problem are you trying to solve ?

1. Is the code too slow ?

• Do you have data that shows the speed problem?
• Do you have a performance goal you are trying to achieve?

2. Is the code readable ?

• Who is going to be reading the code
• Who needs to be able to troubleshoot and understand it ?

You can spend a lot of time tweaking and changing - but unless you actually have a goal, then you're more likely just going to spend lots of time doing more and more tweaking.

IF I was going to tweak this code - I'd iterate across the true/false list.

for index in len(b):   
  if b[index]:
    a[index] += 1

That way you're only touching list "a" when you need to, rather than every time.