Replace given line in files in Python

Question

I have several files, and I need to replace third line in them:

files = ['file1.txt', 'file2.txt']
new_3rd_line = 'new third line'

What is the best way to do this?

Files are big enough, several 100mb's files.

What would you like it to do when there's an empty file, or a file with only one line? — MoshiBin
– MoshiBin, Commented Nov 5, 2011 at 9:28
best in terms of what? Speed/Scalability (what about doing this 100,000 times) Flexibility (what about replacing the 10,000th line)? Memory? (what about performing this on 4GB log files)? Need moar info. — Kenan Banks
– Kenan Banks, Commented Nov 5, 2011 at 9:30
I tired for line in file with counter, saving every line if counter == 3, but I want to see more or less optimal way for this. — Qiao
– Qiao, Commented Nov 5, 2011 at 9:31
Someone should also mention this is like a 60 character shell script... — Kenan Banks
– Kenan Banks, Commented Nov 5, 2011 at 9:31

Community · Accepted Answer · 2017-05-23 12:07:53Z

1

I used this solution: Search and replace a line in a file in Python

from tempfile import mkstemp
from shutil import move
from os import remove, close

def replace_3_line(file):
    new_3rd_line = 'new_3_line\n'
    #Create temp file
    fh, abs_path = mkstemp()
    new_file = open(abs_path,'w')
    old_file = open(file)
    counter = 0
    for line in old_file:
        counter = counter + 1
        if counter == 3:
            new_file.write(new_3rd_line)
        else:
            new_file.write(line)
    #close temp file
    new_file.close()
    close(fh)
    old_file.close()
    #Remove original file
    remove(file)
    #Move new file
    move(abs_path, file)

replace_3_line('tmp.ann')

But it does not work with files that contains non English charecters.

Traceback (most recent call last):
  File "D:\xxx\replace.py", line 27, in <module>
    replace_3_line('tmp.ann')
  File "D:\xxx\replace.py", line 12, in replace_3_line
    for line in old_file:
  File "C:\Python31\lib\encodings\cp1251.py", line 23, in decode
    return codecs.charmap_decode(input,self.errors,decoding_table)[0]
UnicodeDecodeError: 'charmap' codec can't decode byte 0x98 in position 32: character maps to <undefined>

That is bad. Where's python unicode? (file is utf8, python3).

File is:

фвыафыв
sdadf
试试
阿斯达а
阿斯顿飞

edited May 23, 2017 at 12:07

CommunityBot

11 silver badge

answered Nov 5, 2011 at 9:59

Qiao

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2 Comments

Qiao Over a year ago

ok, it is bug stackoverflow.com/questions/6109022/…. Quite frustrating.

Qiao Over a year ago

you should use open() with encoding='utf8'. Feels good now :)

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