Index out of range for simple `readlines` operation in Python

Question

Basically I am trying to read the last few lines. I am baffled what leads to this error:

nrows = 10
with open(filename, "r") as f:
    for ii in xrange(-nrows, 0, 1):
        last_line = f.readlines()[ii].strip().split(",")

When I tried it in the IDE, it has no problem when I use mere numbers, like this:

with open(filename, "r") as f:
    last_line = f.readlines()[-10].strip().split(",")

Somehow it looks like the xrange generated indices cannot be "used". Is there a way around this?

Thank you very much for your attention and time!

Save the output of `f.readlines()` in a variable, and then do the indexing. A file object is an iterator, after the first `.readlines()` call rest of the calls will return an empty list. — Ashwini Chaudhary, Feb 17 '15 at 06:56

score 2 · Accepted Answer · edited May 23 '17 at 11:50

The reason it works in your second example is because you call readlines() only once.

you can read file only once, so once you read all lines the file iterator points at EOF and returns empty list which will fail your [ii] attempt.

To read file more than once you would have to call file.seek(0) after each round to move the iterator to the start again, but it will be much more efficient to read the lines into variable.

you may be interested in file like objects and also may consider slice instead of the xrange

Index out of range for simple `readlines` operation in Python

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