readline with for loop issue

Question

I have a .txt file with 10 sort lines of text (three comma separated words each), but the following only reads four of the lines in the text file:

def main():
    path = '/path/to/file.txt'
    f  = open(path, 'r')
    for line in f:
        s = f.readline()
        print(s)
    f.close
main()

but this will read all the lines but into a list:

def main():
    path = '/path/to/file.txt'
    f  = open(path, 'r')
    s = f.readlines()
    print(s)
    f.close
main()

Why doesn't the for loop work?

I think `for line in f` also reads a line, `f.readline()` reads the next line in line — Arjen Dijkstra, Mar 23 '17 at 09:33
ValueError: Mixing iteration and read methods would lose data I'm stunned it works at all. — Rolf of Saxony, Mar 23 '17 at 09:35
That's because the file objects are iterator-like objects. That means when you iterate over your object you're consuming its lines once, and in `readline` part your reading the next line. — Mazdak, Mar 23 '17 at 09:38
Thanks for your input, for your information Rolf, it certainly didn't work well, I had no idea that the for loop already read in a line, nice to know. — iFunction, Mar 23 '17 at 19:18

score 3 · Answer 1 · edited Mar 23 '17 at 09:38

3

When using for line in f you're already reading a line. So it's useless to do readline() since it will read the next line which explains why you get 4 lines read instead of 10.

edited Mar 23 '17 at 09:38

Mazdak

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answered Mar 23 '17 at 09:37

YGouddi

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Thanks, yes looking at it again now, I can see my loop is now reading two lines and only printing one. Taking out the s = f.readline() solved the problem. – iFunction Mar 23 '17 at 19:15

score 1 · Answer 2 · answered Mar 23 '17 at 09:34

1

This would work to get all the lines:

with open('/path/to/file.txt') as fp:
    for line in fp.read().split('/n'):
        print(line)

answered Mar 23 '17 at 09:34

Richy

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Maxime de Pachtere · Answer 3 · 2018-03-29T15:15:23.697

When you open your file.txt, you got an _io.TextIOWrapper object as f.

for line in f: will iterate on the f iterator to yield each line of your file successively,line by line, into your line variable. You can see how iterators work here.

When the f iterator moves one line after the start of your loop, you read another line with your s = f.readline() and that moves your iterator one more line ahead. When you end your first loop, another line of f is read with your for line in f: then, you skip that line by reading the next line with s = f.readline().

Your code will work with

def main():
    path = '/path/to/file.txt'
    with open(path, 'r') as f:
        for line in f:
            print(line)

main()

score 0 · Answer 4 · answered Mar 23 '17 at 09:38

0

for line in f: is already iterating over the lines in the file, you are then attempting to read a line within that iteration. Just use the value of line and forget about s

answered Mar 23 '17 at 09:38

Rolf of Saxony

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readline with for loop issue

4 Answers4