Is there a way to replace a line within a Python script while keeping the script ongoing?

Question

This is just a simplified example, I know this isn't the best way to do it, but I'm trying it for something else. Is it possible to replace a line of a Python script using its own code, while having the code stay running?

I have this code:

import re

a = 0

f = open("main.py","r")
lines = f.read()
lines = re.sub(r"a = .+\n",r"a = " + str(a+1) + "\n",lines,count=1)
f.close()

f = open("main.py","w")
f.writelines(lines)
f.close()

Each time this is run, a is added to by 1. However, when I try to loop it:

import re
import time

a = 0

while True:
    f = open("main.py","r")
    lines = f.read()
    lines = re.sub(r"a = .+\n",r"a = " + str(a+1) + "\n",lines,count=1)
    f.close()

    f = open("main.py","w")
    f.writelines(lines)
    f.close()
    time.sleep(2)

It should add 1 to a every 2 seconds, but it only changes it once the code is finished (or when I end the code, since the while loop is infinite). Is there some way to continue changing the code without having to end the code?

No. That's a horrible, horrible practice called "self-modifying code". Some early languages allowed that (COBOL, for instance), but it was soon realized that it caused code to be unmaintainable. — Tim Roberts
– Tim Roberts, Commented Aug 14, 2023 at 1:14

Razzi Abuissa · Accepted Answer · 2023-08-14 01:22:18Z

Your code is writing the file every 2 seconds, however what it is writing is not changing. The in-memory python process reads a = 0 once (or a = 1 if it has run already), computes str(a+1), and writes that. The next loop, a+1 has not changed, so it writes the same value.

The simple "fix" here is to add the line a = a + 1 in the body of the while loop, if that's what you want.

If your goal is for the python program reloading itself based on its current value on the disk, look at How do I save and restore multiple variables in python? for an example.

A more robust approach than modifying the source code would be to store the value you're interested in (a here) on disk, separate from the source code. Look at the pickle module for a good example of how to serialize python data to disk, or use a simple plaintext format like json (or just a file containing a number, if that's all you want).

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