Working with text file in Python

Question

I am reading a text file with >10,000 number of lines.

results_file = open("Region_11_1_micron_o", 'r')

I would like to skip to the line in the file after a particular string "charts" which occurs at around line no. 7000 (different for different files). Is there a way to conveniently do that without having to read each single line of the file?

Answer 1

If you know the precise line number then you can use python's linecache module to read a particular line. You don't need to open the file.

import linecache

line = linecache.getline("test.txt", 3)
print(line)

Output:

chart

---

If you want to start reading from that line, you can use islice.

from itertools import islice

with open('test.txt','r') as f:
    for line in islice(f, 3, None):
        print(line)

Output:

chart
dang!
It
Works

---

If you don't know the precise line number and want to start after the line containing that particular string, use another for loop.

with open('test.txt','r') as f:
    for line in f:
        if "chart" in line:
            for line in f:
                # Do your job
                print(line) 

Output:

dang!
It    
Works

---

test.txt contains:

hello
world!
chart
dang!
It
Works

I don't think you can directly skip to a particular line number. If you want to do that, then certainly you must have gone through the file and stored the lines in some format or the other. In any case, you need to traverse atleast once through the file.