Copying from one text file to another using Python

Question

I would like to copy certain lines of text from one text file to another. In my current script when I search for a string it copies everything afterwards, how can I copy just a certain part of the text? E.g. only copy lines when it has “tests/file/myword” in it?

current code:

#!/usr/bin/env python
f = open('list1.txt')
f1 = open('output.txt', 'a')

doIHaveToCopyTheLine=False

for line in f.readlines():

    if 'tests/file/myword' in line:
        doIHaveToCopyTheLine=True

    if doIHaveToCopyTheLine:
        f1.write(line)

f1.close()
f.close()

Asked By: DevCon

||

Source

Answer 1

The oneliner:

open("out1.txt", "w").writelines([l for l in open("in.txt").readlines() if "tests/file/myword" in l])

Recommended with with:

with open("in.txt") as f:
    lines = f.readlines()
    lines = [l for l in lines if "ROW" in l]
    with open("out.txt", "w") as f1:
        f1.writelines(lines)

Using less memory:

with open("in.txt") as f:
    with open("out.txt", "w") as f1:
        for line in f:
            if "ROW" in line:
                f1.write(line)

Answered By: ATOzTOA

Answer 2

readlines() reads the entire input file into a list and is not a good performer. Just iterate through the lines in the file. I used ‘with’ on output.txt so that it is automatically closed when done. That’s not needed on ‘list1.txt’ because it will be closed when the for loop ends.

#!/usr/bin/env python
with open('output.txt', 'a') as f1:
    for line in open('list1.txt'):
        if 'tests/file/myword' in line:
            f1.write(line)

Answered By: tdelaney

Answer 3

Just a slightly cleaned up way of doing this. This is no more or less performant than ATOzTOA’s answer, but there’s no reason to do two separate with statements.

with open(path_1, 'a') as file_1, open(path_2, 'r') as file_2:
    for line in file_2:
        if 'tests/file/myword' in line:
            file_1.write(line)

Answered By: sage88

Answer 4

f = open('list1.txt')
f1 = open('output.txt', 'a')

# doIHaveToCopyTheLine=False

for line in f.readlines():
    if 'tests/file/myword' in line:
        f1.write(line)

f1.close()
f.close()

Now Your code will work. Try This one.

Answered By: pranky

Answer 5

Safe and memory-saving:

with open("out1.txt", "w") as fw, open("in.txt","r") as fr: 
    fw.writelines(l for l in fr if "tests/file/myword" in l)

It doesn’t create temporary lists (what readline and [] would do, which is a non-starter if the file is huge), all is done with generator comprehensions, and using with blocks ensure that the files are closed on exit.

Answered By: Jean-François Fabre

Answer 6

f=open('list1.txt')  
f1=open('output.txt','a')
for x in f.readlines():
    f1.write(x)
f.close()
f1.close()

this will work 100% try this once

Answered By: Pavan Biradar

Answer 7

in Python 3.10 with parenthesized context managers, you can use multiple context managers in one with block:

with (open('list1.txt', 'w') as fout, open('output.txt') as fin):
    fout.write(fin.read())

Answered By: SuperNova

Copying from one text file to another using Python

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