Replace lines in file from For Loop

Question

Let's say that I have a file that contains different MAC address notations of multiple MAC addresses. I want to replace all the matching notations of one MAC address I have parsed from an argument input. So far my script generates all the notations I need and can loop through the lines of the text and show the lines that have to be changed.

import argparse, sys

parser = argparse.ArgumentParser()
parser.add_argument("-f", "--filename")
parser.add_argument("-m",  "--mac_address")
args = parser.parse_args()

mac = args.mac_address #In this case 00:1a:e8:31:71:7f

colon2 = mac #00:1a:e8:31:71:7f
dot2 = colon2.replace(":",".") # 00.1a.e8.31.71.7f
hyphen2 = colon2.replace(":","-") # 00-1a-e8-31-71-7f
nosymbol = colon2.replace(":","") # 001ae831717f
colon4 = ':'.join(nosymbol[i:i+4] for i in range(0, len(nosymbol), 4)) # 001a:e831:717f 
dot4 = colon4.replace(":",".") # 001a.e831.717f
hyphen4 = colon4.replace(":","-") # 001a-e831-717f


replacethis = [colon2,dot2,hyphen2,dot4,colon4,nosymbol,hyphen4]
with open(args.filename, 'r+') as f:
    text = f.read()
    for line in text.split('\n'):
        for n in replacethis:
            if line.replace(n, mac) != line:
                print line + '\n has to change to: \n'line.replace(n,mac)
            else:
                continue

If the file would look like this:

fb:76:03:f0:67:01

fb.76.03.f0.67.01

fb-76-03-f0-67-01

001a:e831:727f

001ae831727f

fb76.03f0.6701

001ae831727f

fb76:03f0:6701

001a.e831.727f

fb76-03f0-6701

fb7603f06701

it should change to:

fb:76:03:f0:67:01

fb.76.03.f0.67.01

fb-76-03-f0-67-01

00:1a:e8:31:71:7f

00:1a:e8:31:71:7f

fb76.03f0.6701

00:1a:e8:31:71:7f

fb76:03f0:6701

00:1a:e8:31:71:7f

fb76-03f0-6701

fb7603f06701

I am struggling at writing the new lines containing the changed MAC address notation back to the file replacing the previous line.

Is there a way to do this?

Wouldn't it be way easier to create a new file and write everything in sequence? Or is it some sort of a constraint? — Aleksei Maide, Jan 19 '18 at 09:08
Possible duplicate of [Correct way to write line to file?](https://stackoverflow.com/questions/6159900/correct-way-to-write-line-to-file) — Brandlingo, Jan 19 '18 at 09:09
You cannot "replace" parts of a text file in place, you have to create a new file, write the updated content to this file, and then remove the old file and rename the new one. — bruno desthuilliers, Jan 19 '18 at 09:11

Marcus Vinicius Melo · Answer 1 · 2018-01-19T11:43:10.407

A simple way to achieve what you are asking you can add a line to store the final values you get, and after that include another ‘with open’ statement to write it to a new file.

replacethis = [colon2, dot2, hyphen2, dot4, colon4, nosymbol, hyphen4]
final_values =[]
with open(args.filename, 'r+') as f:
    text = f.read()
    for line in text.split('\n'):
        for n in replacethis:
            if line.replace(n, mac) != line:
                print line + '\n has to change to: \n'line.replace(n,mac)
                final_values.append(line.replace(n, mac)
            else:
                continue
                final_values.append(line)
with open(new_file_name, ‘w’) as new_f:
    new_f.write(final_values)

Note that if new_file_name = your old file name, you will overwrite the original file.

I hope that answers your question.

Replace lines in file from For Loop

1 Answers1