How Can I Efficiently Replace Lines in a Text File Using Python?

Replacing Lines in a Text File Efficiently

To replace specific lines within a text file in Python, consider the use of the fileinput module. This method avoids loading the entire file into memory, making it efficient for large files.

Using fileinput

import fileinput

with fileinput.input("test.txt", inplace=True) as f:
    for line in f:
        if "foo" in line:
            print(line.replace("foo", "bar"), end='')

With this code, the original file is renamed to a backup, and the standard output is redirected to write into the original file within the loop. Any print statements inside the loop modify the file's contents directly.

Explicit File Manipulation

For readability, consider these explicit options:

Option 1 (For Small Files):

with open("test.txt", "r") as f:
    lines = f.readlines()

with open("test.txt", "w") as f:
    for line in lines:
        if "foo" in line:
            f.write(line.replace("foo", "bar"))
        else:
            f.write(line)

Option 2 (For Large Files):

import tempfile

with open("test.txt") as original, tempfile.NamedTemporaryFile(delete=False) as temp:
    for line in original:
        if "foo" in line:
            temp.write(line.replace("foo", "bar").encode())
        else:
            temp.write(line.encode())

temp.flush()
os.replace(temp.name, "test.txt")

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