Using the same iterator in nested for loops

Question

Consider the following code:

from itertools import chain

lst = ['a', 1, 2, 3, 'b', 4, 5, 'c', 6]

def nestedForLoops():
    it = iter(lst)
    for item0 in it:
        if isinstance(item0, str):
            print(item0)
        else:
            # this shouldn't happen because of
            # 1. lst[0] is a str, and
            # 2. line A
            print(f"this shouldn't happen: {item0=}")
            pass
        for item1 in it:
            if not isinstance(item1, int):
                break
            print(f'\t{item1}')
        else: # no-break
            # reached end of iterator
            return
        # reached a str
        assert isinstance(item1, str)
        it = chain(item1, it) # line A

nestedForLoops()

I was expecting it to print

a
    1
    2
    3
b
    4
    5
c
    6

but instead it printed

a
    1
    2
    3
this shouldn't happen: item0=4
this shouldn't happen: item0=5
c
this shouldn't happen: item0=6

I wrote what I thought was equivalent code using while loops instead of for loops:

from itertools import chain

lst = ['a', 1, 2, 3, 'b', 4, 5, 'c', 6]

def nestedWhileLoops():
    it = iter(lst)
    while True:
        try:
            item0 = next(it)
        except StopIteration:
            break
        if isinstance(item0, str):
            print(item0)
        else:
            # this shouldn't happen because of
            # 1. lst[0] is a str, and
            # 2. line B
            print(f"this shouldn't happen: {item0=}")
            pass
        while True:
            try:
                item1 = next(it)
            except StopIteration:
                # reached end of iterator
                return
            if not isinstance(item1, int):
                break
            print(f'\t{item1}')
        # reached a str
        assert isinstance(item1, str)
        it = chain(item1, it) # line B

nestedWhileLoops()

and this while loop version does print what I expected, namely

a
    1
    2
    3
b
    4
    5
c
    6

So why does nestedForLoops behave differently than nestedWhileLoops?

Answer 1

Once a for loop is entered, the current iterator it is using cannot be re-assigned until the outside of that scope is reached.

Here:

for item0 in it:

You start iterating over it and continue in the scope of this for loop until the end of the function.

When you reassign it within its scope:

it = chain(item1, it) # line A

It has no effect on the iterator you are already iterating over.

The reason it "kind of" works for the inner for loop is because you exit the scope of that for loop on each string.

So in short, your for loop example does the following:

1. Enter the outer for loop and start iterating the original it
2. Enter the inner for loop and continue iterating the original it
3. Exit inner for loop scope and re-assign it
4. In outer scope, continue iterating the original it
5. Re-enter the inner for loop and start iterating over the newly assigned it
6. Repeat 3 through 6

Doing something like this may give you a better understanding:

it = [1, 2, 3, 4]
for item in it:
    print(item)
    it = None
# 1
# 2
# 3
# 4