edit text file using Python

Question

I need to update a text file whenever my IP address changes, and then run a few commands from the shell afterwards.

1. Create variable LASTKNOWN = "212.171.135.53" This is the ip address we have while writing this script.

2. Get the current IP address. It will change on a daily basis.

3. Create variable CURRENT for the new IP.

4. Compare (as strings) CURRENT to LASTKNOWN

5. If they are the same, exit()

6. If they differ,

   A. "Copy" the old config file (/etc/ipf.conf) containing LASTKNOWN IP address into /tmp B. Replace LASTKNOWN with CURRENT in the /tmp/ipf.conf file.
   C. Using subprocess "mv /tmp/ipf.conf /etc/ipf.conf"
   D. Using subprocess execute, "ipf -Fa -f /etc/ipf.conf"
   E. Using subprocess execute, "ipnat -CF -f /etc/ipnat.conf"

7. exit()

I know how to do steps 1 through 6. I fall down on the "file editing" part, A -> C. I can't tell what module to use or whether I should be editing the file in place. There are so many ways to do this, I can't decide on the best approach. I guess I want the most conservative one.

I know how to use subprocess, so you don't need to comment on that.

I don't want to replace entire lines; just a specific dotted quad.

Thanks!

Answer 1

fileinput module has very ugly API, I find beautiful module for this task - in_place, example for Python 3:

import in_place

with in_place.InPlace('data.txt') as file:
    for line in file:
        line = line.replace('test', 'testZ')
        file.write(line)

main difference from fileinput:

• Instead of hijacking sys.stdout, a new filehandle is returned for writing.
• The filehandle supports all of the standard I/O methods, not just readline().

for example - fileinput can line by line editing only, in_pace allow read whole file to memory (if it not big) and modify it.