Why Doesn\'t Modifying a For Loop\'s Iteration Variable Affect Subsequent Iterations in Python?

Iteration Variable Manipulation in Python's For Loop: Why Changes Don't Affect Subsequent Iterations

In Python, for loops are often used to iterate over a sequence of values. However, a common misconception is that modifying the iteration variable during the loop can affect subsequent iterations.

The Problem

Consider the following Python code snippet:

for i in range(0, 10):
    if i == 5:
        i += 3
    print(i)

When you run this code, you might expect the output to be:

0
1
2
3
4
8
9

However, instead, it produces the following:

0
1
2
3
4
8
6
7
8
9

Explanation

The reason for this unexpected behavior lies in how for loops work in Python. Unlike in some other languages, such as C, Python doesn't create a new scope for variables inside the loop. Instead, it rebinds the iteration variable to each value in the sequence.

In the given code, the iterator i is assigned to each number in the range(0, 10) sequence. When you modify i inside the loop, you are only changing the current value of the iterator, not the sequence itself. The subsequent iterations continue using the original values in the sequence, which is why you see the unexpected output.

The Remedy

To achieve the desired behavior where modifying the iteration variable affects subsequent iterations, you can use a while loop instead. While loops allow you to manually increment or modify the iteration variable within the loop itself:

i = 0
while i < 10:
    # do stuff and manipulate `i` as much as you like 
    if i == 5:
        i += 3

    print(i)

    # don't forget to increment `i` manually
    i += 1

This code will produce the expected output:

0
1
2
3
4
8
9

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