Question

  print liveCoords                                      # displays [(0,0),(1,0),(2,0)]
  if population > 3 or population < 2:
      if (j,i) in liveCoords:
          try:
              del liveCoords2[liveCoords2.index((j,i))]
          except:
              pass
  elif population == 3:
      if (j,i) in liveCoords:
          pass
      else:
          liveCoords2.append((j,i))
  print liveCoords                                       # displays [(1,0),(2,0)]

My question is not about how to do something specific, it is how my list liveCoords could possible be changed between those 2 points, from what I can see, nothing is being done that could possibly alter liveCoords, yet it is being altered.

Answer 1

Did you create liveCoords2 with the line liveCoords2 = liveCoords? If so then they are both references to the same list, so adding or removing elements in liveCoords2 would affect liveCoords as well.

If this is the case, then changing the assignment to liveCoords2 = list(liveCoords) or liveCoords2 = liveCoords[:] would fix this. Note that both of these methods will create a shallow copy, if any elements in listCoords are mutable objects then listCoords2 will have references to the same objects as listCoords, this should be fine for your current code but if you do end up needing to create a deep copy you can use the copy module:

import copy
liveCoords2 = copy.deepcopy(liveCoords)