Replacing text from one file from another file

Question

The f1.write(line2) works but it does not replace the text in the file, it just adds it to the file. I want the file1 to be identical to file2 if they are different by overwriting the text from file1 with the text from file2

Here is my code:

 with open("file1.txt", "r+") as f1, open("file2.txt", "r") as f2:
     for line1 in f1:
         for line2 in f2:
             if line1 == line2:
                 print("same")
             else:
                 print("different")
                 f1.write(line2)
             break
 f1.close()
 f2.close()

https://stackoverflow.com/questions/254350/in-python-is-there-a-concise-way-of-comparing-whether-the-contents-of-two-text — Maddy, Oct 01 '18 at 17:09
Wouldn't it be easier to simply delete file2 and create a new copy of file1, named file2? — Kevin, Oct 01 '18 at 17:09
Why are you comparing them line by line when you can just copy as @Kevin mentioned. — Evya, Oct 01 '18 at 17:12
Did you try append mode? If your problem is that `f1.write` is overwriting you can use `open('f1.txt', "a") as f1` and then when you `f1.write()` it will append instead of overwrite. However if you use `"w"` instead of `"r+"` `f1.write` will overwrite the entire file instead of appending. Also if you want to overwrite the entire file instead of line by line, just use `f1.read()` to get the file as string. — devssh, Oct 01 '18 at 17:25
@devssh append mode only adds to the end of the file, so it won't work. — nosklo, Oct 01 '18 at 17:45

score 0 · Answer 1 · answered Oct 01 '18 at 17:17

I want the file1 to be identical to file2

import shutil
with open('file2', 'rb') as f2, open('file1', 'wb') as f1:
    shutil.copyfileobj(f2, f1)

This will be faster as you don't have to read file1.

Your code is not working because you'd have to position the file current pointer (with f1.seek() in the correct position to write the line.

In your code, you're reading a line first, and that positions the pointer after the line just read. When writing, the line data will be written in the file in that point, thus duplicating the line.

Since lines can have different sizes, making this work won't be easy, because even if you position the pointer correctly, if some line is modified to get bigger it would overwrite part of the next line inside the file when you write it. You would end up having to cache at least part of the file contents in memory anyway.

Better truncate the file (erase its contents) and write the other file data directly - then they will be identical. That's what the code in the answer does.

score 0 · Accepted Answer · answered Oct 01 '18 at 17:43

I would read both files create a new list with the different elements replaced and then write the entire list to the file

with open('file2.txt', 'r') as f:
    content = [line.strip() for line in f]

with open('file1.txt', 'r') as j:
    content_a = [line.strip() for line in j]
    for idx, item in enumerate(content_a):
        if content_a[idx] == content[idx]:
            print('same')
            pass
        else:
            print('different')
            content_a[idx] = content[idx]

with open('file1.txt', 'w') as k:
    k.write('\n'.join(content_a))

file1.txt before:

chrx@chrx:~/python/stackoverflow/9.28$ cat file1.txt
this
that
this
that
who #replacing
that
what
blah

code output:

same
same
same
same
different
same
same
same

file1.txt after:

chrx@chrx:~/python/stackoverflow/9.28$ cat file1.txt
this
that
this
that
vash #replaced who
that
what
blah

Replacing text from one file from another file

2 Answers2