是否可以防止迭代器到达文件末尾？

Question

I tried to find an answer to this question, but I guess I can't really pinpoint the source of my problem. If this has been answered elsewhere, I'd appreciate it if you could take a moment to explain what I'm missing.

Basically, I thought you could do something like this:

ref = open(ref_file)
with ref:
    for row in ref:
         break

with ref:
    for row in ref:
        print(row)

Since I used a break statement in the first loop, I thought it'd stop the iterator, put a pause on reading the file, and then I could pick up where I left off with the second loop. This doesn't turn out to be the case- it instead throws me an error saying I am trying to work on a closed file. I reasoned through it, and I came to the conclusion that, perhaps, the with statement is what pushes the iterator to EOF. So I tried this:

with ref:
    for row in ref:
         break

    for row in ref:
        print(row)

This seemed to work as expected. Awesome. So, the next thing I tried is this:

with ref:
    for i in range(0,10):
        for row in vcf:
             break

        for row in vcf:
            print(row)

What I'd expect to happen here is, essentially, to print alternating rows until the range of 0,10 is exhausted. Instead, I print the second row of the file, and it stops.

Is there a better way to prevent an iterator from reaching EOF? What am I missing? Am I somehow breaking out of the with statement and missing the point?

EDIT:

Okay, the last example isn't exemplary of the issue I'm having.

with ref:
    for i in range(0,10):
        for row in vcf:
             break

        for row in vcf:
            print(row)
            break

This works as expected... Which really leaves me thinking I'm missing something in my real code. I'd still appreciate commentary on better ways to prevent an iterator from reaching EOF.

Answer 1

The indentation here is incorrect: it reads one line then breaks then reads the rest of the file, 10 times:

with ref:
    for i in range(0,10):
        for row in vcf:
             break

        for row in vcf:
            print(row)

note that you can iterate manually using next instead of breaking out of a for loop. More pythonic:

for _ in range(10):
    next(vcf,None)

# unindent here
for row in vcf:
    print(row)

the None argument in next ensures that even if the file is shorter than 10 lines, it doesn't raise StopIteration but yields a default value (that you're ignoring)