Replace Line in Textfile

[Larz60+]

it's better to use a with statement, makes code easier to read, and closes files automatically:

import os
# assure in script directory
os.chdir(os.path.abspath(os.path.dirname(__file__)))

def replace_a_line(original_line, new_line, filein, fileout):    
    with open(filein) as fin, open(fileout, 'w') as fout:
        for line in fin:
            lineout = line
            if line.strip() == original_line:
                lineout = f"{new_line}\n"
            fout.write(lineout)

def testit():
    replace_a_line('line 2', 'line xxx', 'testfilein.txt', 'testfileout.txt')

if __name__ == '__main__':
    testit()

testfilein.txt:

Output:
line 1
line 2
line 3

testfileout.txt

Output:
line 1
line xxx
line 3