Changing Iterables in Loop

We know it’s wrong, but why?

Sources

Example 1

Here we demonstrate the effect of changing an iterable in an iterator loop body on the traversing iterator. We print everything for convenience.

l = list(range(10))

index = 1
for i in l:
    print(f'Iteration {index}')
    index += 1
    
    print(f'List: \n{l}')
    print(f'Iterator value: {i}')
    print(f'Element pop: {l.pop(0)}')

    print(f'List: \n{l}')
    print(f'Element pop: {l.pop(0)}')

    print(f'List: \n{l}')
    print()

Iteration 1
List: 
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
Iterator value: 0
Element pop: 0
List: 
[1, 2, 3, 4, 5, 6, 7, 8, 9]
Element pop: 1
List: 
[2, 3, 4, 5, 6, 7, 8, 9]

Iteration 2
List: 
[2, 3, 4, 5, 6, 7, 8, 9]
Iterator value: 3
Element pop: 2
List: 
[3, 4, 5, 6, 7, 8, 9]
Element pop: 3
List: 
[4, 5, 6, 7, 8, 9]

Iteration 3
List: 
[4, 5, 6, 7, 8, 9]
Iterator value: 6
Element pop: 4
List: 
[5, 6, 7, 8, 9]
Element pop: 5
List: 
[6, 7, 8, 9]

Iteration 4
List: 
[6, 7, 8, 9]
Iterator value: 9
Element pop: 6
List: 
[7, 8, 9]
Element pop: 7
List: 
[8, 9]

Analysis

Notice:

The iterator i does not communicate with the list l.
Thus, when the iterable l changed in iteration 2, the iterator does not care - it still points to the second element in the iterable.
This means that in the second iteration, our iterator points at 3 even though it was originally the 4th element in l, simply because it is now the second element in l, and the iterator proceeds according to the order of iteration.

Example 2: Variation

Let’s examine the effect of changing the iterable inside the iterator loop body on the traversing iterator using a variant iterator.

l = list(range(10))

for index, elem in enumerate(l):
    print(f'Iteration {index}')
    
    print(f'List: \n{l}')
    print(f'Iterator index: {index}')
    print(f'Iterator value: {elem}')
    print(f'Element pop: {l.pop(0)}')

    print(f'List: \n{l}')
    print(f'Element pop: {l.pop(0)}')

    print(f'List: \n{l}')
    print()

Iteration 0
List: 
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
Iterator index: 0
Iterator value: 0
Element pop: 0
List: 
[1, 2, 3, 4, 5, 6, 7, 8, 9]
Element pop: 1
List: 
[2, 3, 4, 5, 6, 7, 8, 9]

Iteration 1
List: 
[2, 3, 4, 5, 6, 7, 8, 9]
Iterator index: 1
Iterator value: 3
Element pop: 2
List: 
[3, 4, 5, 6, 7, 8, 9]
Element pop: 3
List: 
[4, 5, 6, 7, 8, 9]

Iteration 2
List: 
[4, 5, 6, 7, 8, 9]
Iterator index: 2
Iterator value: 6
Element pop: 4
List: 
[5, 6, 7, 8, 9]
Element pop: 5
List: 
[6, 7, 8, 9]

Iteration 3
List: 
[6, 7, 8, 9]
Iterator index: 3
Iterator value: 9
Element pop: 6
List: 
[7, 8, 9]
Element pop: 7
List: 
[8, 9]

Analysis

Here we see exactly the “ignorance” of the iterator according to the index variable:

The iterator does as its supposed to: Element 1 -> Element 2 -> … -> Iterable End
Changing l does not affect the iterator traversal, but it can affect what the iterator finds in those positions.