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How to Restart Python Script after Exception and Run it Forever

by 9 Comments

Here is a simple trick that I used to restart my python script after unhandled exception.

Let’s say I have this simple script called test.py that I want to run forever. It will just wait 2 seconds and throw an error.

import time

time.sleep(2)
raise Exception("Oh oh, this script just died")

I use the following script called forever in the same directory:


#!/usr/bin/python
from subprocess import Popen
import sys

filename = sys.argv[1]
while True:
    print("\nStarting " + filename)
    p = Popen("python " + filename, shell=True)
    p.wait()

It uses python to open test.py as a new subprocess. It does so in an infinite while loop, and whenever test.py fails, the while loop restarts test.py as a new subprocess.

I’ll have to make the forever script executable by running chmod +x forever. Optionally forever script can be moved to some location in the PATH variable, to make it available from anywhere.

Next, I can start my program with:

./forever test.py

Which will result in the following output:

Starting test.py
Traceback (most recent call last):
  File "test.py", line 4, in <module>
    raise Exception("Oh oh, this script just died")
Exception: Oh oh, this script just died

Starting test.py
Traceback (most recent call last):
  File "test.py", line 4, in <module>
    raise Exception("Oh oh, this script just died")
Exception: Oh oh, this script just died

Starting test.py

As you can tell, this script will run repeatedly, until it is killed with ctr+c.


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  2. Alex thanks for your guidance.

    It worked in Debian GNU/linux machine but I’m getting error on Linux 18.04. The error message:

    ./forever.py: /usr/bin/python: bad interpreter: No such file or directory

    When I removed first line the error was being like that:

    ./forever.py: line 2: from: command not found
    ./forever.py: line 3: import: command not found
    ./forever.py: line 4: import: command not found
    ./forever.py: line 6: filename: command not found
    ./forever.py: line 8: syntax error near unexpected token "\nStarting "'
    ./forever.py: line 8:     print(“\nStarting ” + filename)’

    What can be the problem?

    Reply

  3. Hello, I have an implementation where a python script calls two other scripts, is there a way to adapt your implementation with my example?

    Reply

  4. I have a flask framework implementation. I have a global variable which is being used in a route(ie. /xyz) and nowhere else.

    When I don’t run /xyz, the global variable gets vanished(my may or may not accurate #AfterDebuggingConclusion). How can I keep the variable in the memory if my conclusion is right?

    Reply

    • Hmm, I think if you declare it outside of scope of this /xyz method it should exist even if method did not run. Then you can reference it inside of function by declaring it global xyz.

      Reply

  7. Thanks for this great tip. I’m trying to output the subprocess script’s print statements to the screen, but so far no success :(. Here’s what I have so far:

    import subprocess
    import sys

    filename = ‘my_script.py’

    while True:
    print(‘Starting ‘ + filename + ‘ …’)
    p = subprocess.Popen(‘python ‘ + filename, shell=True, stdout=subprocess.PIPE)
    (output, err) = p.communicate()
    print(output)
    p.wait()

    It would be awesome if you could point me in the right direction. Thanks!

    Reply