Python - Catch signals and exceptions in remote script

Question

I am running a remote Python script on AWS (EC2 ubuntu) in background. The script performs some file manipulations, launches a long running simulation (subprocess run with os.system(...)) and writes some log files. I would like to manage the status of the running script and hopefully exit gracefully from various conditions. Specifically:

The sub-process is interrupted by the user with signal 15.
The simulation (sub-process) fails (signal 8 - Floating point exception)
The vm is rebooted
The vm is terminated. I am using Elastic File System, so even if the instance is destroyed, all the files are not.

I know how to handle basic exceptions, but I am a bit lost when I need to catch exceptions from subprocesses. Can you recommend a solid approach?

EDIT: Please notice the bold part.

Amey Dahale · Accepted Answer · 2017-05-26 09:02:51Z

2

For your given scenarios, try with signal handling. In given cases, case 1 (signal 15) and case 3 (vm is getting rebooted), are similar(generally signal 15/SIGTERM is part of shutdown sequence or maybe triggered by user with proper privileges. Nonetheless it serves the required purpose). signal 8 - SIGFPE

import signal

def signalHandler(sigNum, frameObject):
    if sigNum == 15:
       # Code for handling signal 15 goes here
    elif sigNum == 8:
       # Code for handling signal 8 goes here

signal.signal(signal.SIGTERM, signalHandler) # signal 15
signal.signal(signal.SIGFPE, signalHandler)  # signal 8

answered May 26, 2017 at 9:02

Amey Dahale

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4 Comments

Rojj Over a year ago

Thanks this helped me to understand, but in this way I cannot handle the signals to the sub-process. If the sub-process is killed, the Python script just thinks that it is completed and it goes to the next line. My bad, I should have specified. See revised question.

Amey Dahale Over a year ago

Have you tried cathing Floating Point Exception? This might help.

Rojj Over a year ago

How would that help. FPE is more of an exception (no pun).

Rojj Over a year ago

This actually worked anyway. SIGTERM also kills the main Python script.

Mauricio · Accepted Answer · 2017-05-26 08:23:49Z

1

I might be misunderstanding you but just put all exception causing code in try-except blocks. You seem pretty knowledgeable but I'll give an example anyways

try:
    //some potentially error causing code
except (errorType): //need to know what type of exception it will throw
    //code for what to do if the error occurs

answered May 26, 2017 at 8:23

Mauricio

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