Why can I use an iterator once but get nothing the second time?

You may be reusing an iterator that has already been consumed. An iterator supplies values successively; after its items have been retrieved, it is exhausted. Starting another for loop does not rewind it. [S1]

This runnable example shows the first pass and the empty second pass:

numbers = iter([10, 20, 30])

print("First pass:")
for number in numbers:
    print(number)

print("Second pass:")
for number in numbers:
    print(number)

The first loop prints 10, 20, and 30; the second prints nothing. Both loops use the same iterator, and the first loop retrieved all its values. A list behaves differently, so the important distinction is what the variable refers to.

Yes. A list is a reusable container: each loop over it can get a fresh iterator, so both passes can visit the values. [S1]

numbers = [10, 20, 30]

for number in numbers:
    print("First pass:", number)

for number in numbers:
    print("Second pass:", number)

Both loops print all three numbers. In contrast, numbers = iter([10, 20, 30]) makes numbers an iterator, which the first loop consumes. You can check what a variable refers to with print(type(numbers)); use that as a debugging clue, then check how the object is being used.

For a finite set of values you want to reuse, put them in a list before the first pass. If you already have an iterator, converting it to a list retrieves its available values, so do this before consuming values you need to keep:

source = iter(["red", "green", "blue"])
values = list(source)

for value in values:
    print("First pass:", value)

for value in values:
    print("Second pass:", value)

Both loops use the reusable list values. If the iterator was partly consumed before list(source), the list contains only values still available from it.

If you can produce the values again, you can instead create a fresh iterator for each pass:

def make_numbers():
    return iter([10, 20, 30])

for number in make_numbers():
    print("First:", number)

for number in make_numbers():
    print("Second:", number)

Use a list when you need to retain the values; use a fresh iterator when you can readily produce them again.

Check the exact variable used in each loop. Make sure the second pass uses the list or newly created iterator you intended, rather than the same exhausted iterator. A common mistake is converting the iterator to a list after the first pass; then only values still available can be copied.

Also check whether an assignment such as values = iter(values) changed the variable you reuse. If you need the complete set for both passes, create the list before the first loop. The practical rule is: an iterator is consumed as you retrieve its values; use a reusable list or create a fresh iterator when you need another pass. [S1]