code desing and error handling: exit program with an error in python

Question

I am aware of a few ways to terminate a python script but here I am looking for a good and robust code design (sort of a recommended way to do this).

Most of my code is written in functions that are called from the main function.

I'd like to know what'd be the most recommendable way to stop running a python script/program from a given function (called from main) and give a message error to the user.

Example of my current design (please comment on better practices if have some ideas):

import sys


def run_function(x):
    if x == 4:
       print 'error, x cannot be 4'
       sys.exit(0)
    else:
       print 'good x'
    return 0


def main():
    x=4
    run_function(x)
    return 0

Martijn Pieters · Accepted Answer · 2013-05-11 17:17:39Z

4

Simply print the message, then use sys.exit() to end the program. The argparse module uses a utility function (adjusted to be more or less stand-alone):

def exit(status=0, message=None):
    if message:
        _print_message(message, sys.stderr)
    sys.exit(status)

where _print_message() writes the message to the designated file object, here sys.stderr; basically just using print.

answered May 11, 2013 at 17:17

Martijn Pieters

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Comments

marcoseu · Accepted Answer · 2013-05-11 17:35:20Z

4

In your example, I would suggest that you raise error from the functions and only call the exit in __main__. The message will be passed using raise. Example:

import sys

def run_function(x):
    if x == 4:
        raise ValueError('error, x cannot be 4')
    else:
        print 'good x'
    return 0

def main():
    x=4
    run_function(x)
    return 0

if __name__ == "__main__":
    try:
        main()
    except ValueError as e:
        sys.exit(e)

This way, your function indicate that it received a wrong value, and it is the caller that decide to call a sys.exit based on the error.

A little more detail on the sys.exit:

The optional argument arg can be an integer giving the exit status (defaulting to zero), or another type of object. If it is an integer, zero is considered “successful termination” and any nonzero value is considered “abnormal termination” by shells and the like. Most systems require it to be in the range 0-127, and produce undefined results otherwise. Some systems have a convention for assigning specific meanings to specific exit codes, but these are generally underdeveloped; Unix programs generally use 2 for command line syntax errors and 1 for all other kind of errors. If another type of object is passed, None is equivalent to passing zero, and any other object is printed to stderr and results in an exit code of 1. In particular, sys.exit("some error message") is a quick way to exit a program when an error occurs.

edited May 11, 2013 at 17:35

answered May 11, 2013 at 17:26

marcoseu

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2 Comments

Martijn Pieters Over a year ago

Why use ValueError when you have SystemExit?

marcoseu Over a year ago

You function should raise what "is wrong" and it's the caller that should make the decision to perform a sys.exit based on error it received. I will update the answer with this info.

sodd · Accepted Answer · 2013-05-11 17:26:32Z

1

Firstly, you might want to change your exit code when encountering an error. exit(0) normally means the process has terminated without errors. Then why are you returning something in your run_function, when you're not even checking it afterwards (in the else branch, I mean)?

Other than that, I don't see any questions here to be answered.

edited May 11, 2013 at 17:26

sodd

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answered May 11, 2013 at 17:21

hellerve

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