如何写入文件的特定行？

Question

Let's say I have a text file

Line 1
Line 2
Line 3

I read through it and decide to change Line 2 to Line Two. Can I do that elegantly in Python without simply rewriting the file with the changes? I tried with file.seek() but I didn't really get anywhere with it nor I understand what it does exactly.

Answer 1

A file is a sequence of bytes. If you want to change something in the middle that requires more or fewer bytes to express, the rest of the file needs to resize.

Because a file is a physical sequence of bytes on a storage medium, that means you need to rewrite the entire rest of the file. In other words, you need to move over everything following line 2 .

In practice, that means rewriting the file, as that is much easier to achieve.

Answer 2

You want the power of in-place editing, which the fileinput module offers:

inplace-edit.py:

import sys
import fileinput

for line in fileinput.input(sys.argv[1], inplace=1):
    line = line.rstrip() # Remove the new line
    if line == 'Line 2':
        line = 'Line two'
    print line

data.txt:

Line 1
Line 2
Line 3

To run it:

python inplace-edit.py data.txt

The resulting data.txt:

Line 1
Line two
Line 3