Working with a text file in Python

Question

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

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

I would like to go to the line in the file after the specific line of the “chart” that appears around line No. 7000 (different for different files). Is there a way to do this without having to read every single line of the file?

Answer 1

If you know the exact line number, you can use the python module to read a specific line. You do not need to open the file. linecache

import linecache

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

Conclusion:

chart

---

If you want to start reading from this 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)

Conclusion:

chart
dang!
It
Works

---

If you do not know the exact line number and want to start after the line containing this particular line, use another for the 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) 

Conclusion:

dang!
It    
Works

---

test.txt contains:

hello
world!
chart
dang!
It
Works

, . , , , . .

Answer 2

itertools.dropwhile, .

from itertools import dropwhile, islice

with open(fname) as fin:
    start_at = dropwhile(lambda L: 'Abstract' not in L.split(), fin)
    for line in islice(start_at, 1, None):
        print line

Answer 3

If your text file has lines whose length is evenly distributed across your file, you can try to find in the file

from os import stat
size = stat(your_file).st_size
start = int(0.65*size)
f = open(your_file)
f.seek(start)
buff = f.read() 
n = buff.index('\nchart\n')
start = n+len('\nchart\n')
buff = buff[start:]