Reading specific lines from a file in a loop (optimization)

Question

I'm reading specific lines from a text file. Here is my code:

file.seek(0)
for j, ln in enumerate(file):
   if j == line_number
      line = ln   # i'm getting the line
      break

It tooks a long time when I use this code in a "loop" where the line_number is random every turn. I also tried linecache.getline() but it needs to be call linecache.clearcache() every turn, so its not better.

Is there a faster solution for that? Memory doesn't matter.

Answer 1

If memory truly doesn't matter:

lines = file.readlines()

line = lines[line_number]

You read the file in once, store it in memory, and access it from there.

Edit: Where memory matters:

from itertools import dropwhile

line = next(dropwhile(lambda x: x[0]==line_number, enumerate(file)))[1]

We can use itertools.dropwhile() to ignore the lines until we get to the one we need.

Answer 2

use this above the loop:

with open('in.txt') as f:
 lines=[ln for ln in f]

now access any line number inside the loop:

lines[line_number]

Answer 3

def getLine(fileObj, lineNo):
    [fileObj.readline() for i in xrange(lineNo - 1)]
    return fileObj.readline()

Answer 4

Other way of doing

import linecache
linecache.getline('myfile', line_number)