Copying from one text file to another using Python

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) 

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 pythonwith open('output.txt', 'a') as f1:    for line in open('list1.txt'):        if 'tests/file/myword' in line:            f1.write(line)

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)