The for index in range(len(list)) loop executes the loop body with index first set to 0, then 1, then 2, etc. up to len(list) - 1. The previous value of index is ignored and overwritten. If you want index to start at iteration + 1, use the 2-argument form of range:

for index in range(iteration + 1, len(list)):

You can avoid index errors with sum(v[i+1:i+4]). You have to compare each element (if v[i] == 0) not the whole list.

v = [0, 5, 5, 0, 6, 6, 0, 7, 7 ]

for i in range(len(v) - 1, -1, -1):       
    if v[i] == 0 :
        v[i] = sum(v[i+1:i+4])
print(v)

If you put one more print before the for loop of x

def mover():
global z,coordy
if loops % 250 == 0 and z<len(coordx)-1:
    print("loops = ".format(loops))
    z = z+2
print("range(0,z) = {}".format(range(0,z))) #<----Put this for debugging
for x in range(0,z):
    print ("x = {}".format(x))
    if coordy[x] <= screeny+10:  #<---------- Problem is here
        coordy[x] += 2  
    else:
        ...do something...

You will get the following output just before the index error:

You can see that the loop is actually counting from 0, but the if condition is not satisfied, therefore it prints the 'pop' inside the else condition. After that it prints another x. Since the previous value was 0, this time it prints 1. And you thought the x starts with 1 here, which is not true.

The real cause of the index error is after popping out the final 2 values from the coordy list, the length of the coordy list is only 4. However, the range of x is from 0 to 5. So when x=4, your coordy[4] is trying to get the 5th element from the list which is non-existent. That's why you get your index error.

From the documentation:

range([start], stop[, step])

The start defaults to 0, the step can be whatever you want, except 0 and stop is your upper bound, it is not the number of iterations. So declare n to be whatever your upper bound is correctly and you will not have to add 1 to it.

e.g.

>>> for i in range(1, 7, 1): print(i)
... 
1
2
3
4
5
6
>>> for i in range(1, 7, 2): print(i)
... 
1
3
5

A nice feature, is that it works in reverse as well.

>>> for i in range(7, 0, -1): print(i)
... 
7
6
5
4
3
2
1

If you aren't using it as an index but for something that can have positive or negative values, it still comes in handy:

>>> for i in range(2, -3, -1): print(i)
... 
2
1
0
-1
-2
>>> for i in range(-2, 3, 1): print(i)
... 
-2
-1
0
1
2

You can start a loop at whatever you want. The reason you see loops starting at zero often, is because they are looping through an array. Since the first item in an array is at index '0', it makes sense to start looping from 0 to access every item in an array.

I think you want something like this:

upper_limit = int(input("What's the upper limit? "))

total = 0

for num in range(upper_limit):
    total = total + num

print("The result is:", total)

Be aware that, for range calls, the upper limit is excluded: if you type 3 as the input, it will iterate three times (with num set to 0, 1, and 2) - having upper_limit+1 as the argument of range will also include the upper_limit itself (which is why in your case the output was 6 instead of 3).

When i=0, range(i) is empty, so it skips the inner loop:

>>> list(range(0))
[]