How can I, in python, iterate over multiple 2d lists at once, cleanly?

Question

If I\'m making a simple grid based game, for example, I might have a few 2d lists. One might be for terrain, another might be for objects, etc. Unfortunately, when I need to ite

Answer 1

If a.isWhatever is rarely true you could build an "index" once:

a_index = set((i,j) 
              for i,arow in enumerate(a) 
              for j,a in enumerate(arow) 
              if a.IsWhatever())

and each time you want something to be done:

for (i,j) in a_index:
    b[i][j].doSomething()

If a changes over time, then you will need to keep the index up-to-date. That's why I used a set, so items can be added and removed fast.