The problem here is that you are trying to modify the loop variable of a for loop. Python doesn't stop you from doing i = i + 1 inside the loop, but as soon as control goes back to the top of the loop i gets reset to whatever value is next in the sequence. That's just the way for loops work in Python, it also happens if you loop over a list or tuple rather than a range. Eg,

for i in (2, 3, 5, 7, 11):
    print(i)
    i = 10 * i
    print(i)

output

2
20
3
30
5
50
7
70
11
110

To get the output you want is easy, though. In the following code I replaced your infinite while loop with a small for loop.

i = 0 
for j in range(3):
    print("\ni initialized as", i)
    for i in range(i, i + 3):
        print ("inside for", i)
    i += 1

output

i initialized as 0
inside for 0
inside for 1
inside for 2

i initialized as 3
inside for 3
inside for 4
inside for 5

i initialized as 6
inside for 6
inside for 7
inside for 8

If the outer loop isn't infinite, you can do this sort of thing with a single loop and an if test. It's more compact, but slightly slower, since it has to perform the test on every loop iteration.

outer_loops = 4
inner_loops = 3
for i in range(outer_loops * inner_loops):
    if i % inner_loops == 0:
        print("\nIteration", i // inner_loops)
    print ("inside for", i)

output

Iteration 0
inside for 0
inside for 1
inside for 2

Iteration 1
inside for 3
inside for 4
inside for 5

Iteration 2
inside for 6
inside for 7
inside for 8

Iteration 3
inside for 9
inside for 10
inside for 11

The code you need is this:

for i in range(1, 100, 10):
    print(i, i+10)

By using list comprehension you can do this in a one-liner

s = 'teststring'

r = ' '.join([s[i:i+2] for i in range(len(s)-1)])

print(r)

You need to understand the purpose of variable index you are trying to increment.

Currently, index and item and assigned new values every time the loop iterates over read_list. index and task in the for loop with enumerate basically resolves to getting the index and value from read_list one by one.

You are trying to increase the same variable index inside if condition.

Even if you increment it, next time the loop runs, it will overwrite the value.

If you are trying to count the number of tasks that are incomplete, you can use a new variable and name it different that one you are using in the for loop.

Try this :

incomplete_count = 0
for index, item in enumerate(read_list, 1):
    item = item.rstrip('\n')
    if item[0] == '':
        print(f'{index}. {item}')
        incomplete_count += 1

itertools.islice:

lines = iter(cdata.splitlines())
for line in lines:
    if exp.match(line):
       #increment the position of the iterator by 5
       for _ in itertools.islice(lines, 4):
           pass
       continue # skip 1+4 lines
    print line

For example, if exp, cdata are:

exp = re.compile(r"skip5")
cdata = """
before skip
skip5
1 never see it
2 ditto
3 ..
4 ..
5 after skip
6 
"""

Then the output is:

before skip
5 after skip
6 

Python implementation of the C example

i = 0
while i < 100:
    if i == 50:
       i += 10
    print i
    i += 1

As @[Glenn Maynard] pointed out in the comment if you need to do a very large jumps such as i += 100000000 then you should use explicit while loop instead of just skipping steps in a for loop.

Here's the example that uses explicit while loop instead islice:

lines = cdata.splitlines()
i = 0
while i < len(lines):
    if exp.match(lines[i]):
       #increment the position of the iterator by 5
       i += 5
    else:
       print lines[i]
       i += 1

This example produces the same output as the above islice example.

In the provided code when you try to use i in a for loop with range, it always changes to the number provided by range in the function without bothering to look at the increment made to i. so basically if you try list(range(0, 10)) this will give you [0, 2, 3, 4, 5, 6, 7, 8, 9]. so for goes through that list one by one without thinking if any changes were made to i or not.

which if seen

loop_1: i=0
loop_2: i=1
loop_3: i=2
loop_4: i=3 (here now you increment value by 1), i=4
loop_5: i=4 (but the for loop was going though the list from range function so the change didn't do anything)
loop_6: i=5 (and so on until 9)

A little more background on why the loop in the question does not work as expected.

A loop

for i in iterable: 
    # some code with i

is basically a shorthand for

iterator = iter(iterable)
while True:
    try:
        i = next(iterator)            
    except StopIteration:
        break
    # some code with i

So the for loop extracts values from an iterator constructed from the iterable one by one and automatically recognizes when that iterator is exhausted and stops.

As you can see, in each iteration of the while loop i is reassigned, therefore the value of i will be overridden regardless of any other reassignments you issue in the # some code with i part.

For this reason, for loops in Python are not suited for permanent changes to the loop variable and you should resort to a while loop instead, as has already been demonstrated in Volatility's answer.