How do I terminate a script?

Another way is:

raise SystemExit

from sys import exit
exit()

As a parameter you can pass an exit code, which will be returned to OS. Default is 0.

While you should generally prefer sys.exit because it is more “friendly” to other code, all it actually does is raise an exception.

If you are sure that you need to exit a process immediately, and you might be inside of some exception handler which would catch SystemExit, there is another function – os._exit – which terminates immediately at the C level and does not perform any of the normal tear-down of the interpreter; for example, hooks registered with the “atexit” module are not executed.

A simple way to terminate a Python script early is to use the built-in quit() function. There is no need to import any library, and it is efficient and simple.

Example:

#do stuff
if this == that:
  quit()

You can also use simply exit().

Keep in mind that sys.exit(), exit(), quit(), and os._exit(0) kill the Python interpreter. Therefore, if it appears in a script called from another script by execfile(), it stops execution of both scripts.

See “Stop execution of a script called with execfile” to avoid this.

I’m a total novice but surely this is cleaner and more controlled

def main():
    try:
        Answer = 1/0
        print  Answer
    except:
        print 'Program terminated'
        return
    print 'You wont see this'

if __name__ == '__main__': 
    main()

…

Program terminated

than

import sys
def main():
    try:
        Answer = 1/0
        print  Answer
    except:
        print 'Program terminated'
        sys.exit()
    print 'You wont see this'

if __name__ == '__main__': 
    main()

…

Program terminated Traceback (most recent call last): File “Z:Directorytestdieprogram.py”, line 12, in
main() File “Z:Directorytestdieprogram.py”, line 8, in main
sys.exit() SystemExit

Edit

The point being that the program ends smoothly and peacefully, rather than “I’VE STOPPED !!!!”

I’ve just found out that when writing a multithreadded app, raise SystemExit and sys.exit() both kills only the running thread. On the other hand, os._exit() exits the whole process. This was discussed in “Why does sys.exit() not exit when called inside a thread in Python?“.

The example below has 2 threads. Kenny and Cartman. Cartman is supposed to live forever, but Kenny is called recursively and should die after 3 seconds. (recursive calling is not the best way, but I had other reasons)

If we also want Cartman to die when Kenny dies, Kenny should go away with os._exit, otherwise, only Kenny will die and Cartman will live forever.

import threading
import time
import sys
import os

def kenny(num=0):
    if num > 3:
        # print("Kenny dies now...")
        # raise SystemExit #Kenny will die, but Cartman will live forever
        # sys.exit(1) #Same as above

        print("Kenny dies and also kills Cartman!")
        os._exit(1)
    while True:
        print("Kenny lives: {0}".format(num))
        time.sleep(1)
        num += 1
        kenny(num)

def cartman():
    i = 0
    while True:
        print("Cartman lives: {0}".format(i))
        i += 1
        time.sleep(1)

if __name__ == '__main__':
    daemon_kenny = threading.Thread(name='kenny', target=kenny)
    daemon_cartman = threading.Thread(name='cartman', target=cartman)
    daemon_kenny.setDaemon(True)
    daemon_cartman.setDaemon(True)

    daemon_kenny.start()
    daemon_cartman.start()
    daemon_kenny.join()
    daemon_cartman.join()

In Python 3.5, I tried to incorporate similar code without use of modules (e.g. sys, Biopy) other than what’s built-in to stop the script and print an error message to my users. Here’s my example:

## My example:
if "ATG" in my_DNA: 
    ## <Do something & proceed...>
else: 
    print("Start codon is missing! Check your DNA sequence!")
    exit() ## as most folks said above

Later on, I found it is more succinct to just throw an error:

## My example revised:
if "ATG" in my_DNA: 
    ## <Do something & proceed...>
else: 
    raise ValueError("Start codon is missing! Check your DNA sequence!")

My two cents.

Python 3.8.1, Windows 10, 64-bit.

sys.exit() does not work directly for me.

I have several nexted loops.

First I declare a boolean variable, which I call immediateExit.

So, in the beginning of the program code I write:

immediateExit = False

Then, starting from the most inner (nested) loop exception, I write:

            immediateExit = True
            sys.exit('CSV file corrupted 0.')

Then I go into the immediate continuation of the outer loop, and before anything else being executed by the code, I write:

    if immediateExit:
        sys.exit('CSV file corrupted 1.')

Depending on the complexity, sometimes the above statement needs to be repeated also in except sections, etc.

    if immediateExit:
        sys.exit('CSV file corrupted 1.5.')

The custom message is for my personal debugging, as well, as the numbers are for the same purpose – to see where the script really exits.

'CSV file corrupted 1.5.'

In my particular case I am processing a CSV file, which I do not want the software to touch, if the software detects it is corrupted. Therefore for me it is very important to exit the whole Python script immediately after detecting the possible corruption.

And following the gradual sys.exit-ing from all the loops I manage to do it.

Full code: (some changes were needed because it is proprietory code for internal tasks):

immediateExit = False
start_date = '1994.01.01'
end_date = '1994.01.04'
resumedDate = end_date


end_date_in_working_days = False
while not end_date_in_working_days:
    try:
        end_day_position = working_days.index(end_date)

        end_date_in_working_days = True
    except ValueError: # try statement from end_date in workdays check
        print(current_date_and_time())
        end_date = input('>> {} is not in the list of working days. Change the date (YYYY.MM.DD): '.format(end_date))
        print('New end date: ', end_date, 'n')
        continue


    csv_filename = 'test.csv'
    csv_headers = 'date,rate,brandn' # not real headers, this is just for example
    try:
        with open(csv_filename, 'r') as file:
            print('***nOld file {} found. Resuming the file by re-processing the last date lines.nThey shall be deleted and re-processed.n***n'.format(csv_filename))
            last_line = file.readlines()[-1]
            start_date = last_line.split(',')[0] # assigning the start date to be the last like date.
            resumedDate = start_date

            if last_line == csv_headers:
                pass
            elif start_date not in working_days:
                print('***nn{} file might be corrupted. Erase or edit the file to continue.n***'.format(csv_filename))
                immediateExit = True
                sys.exit('CSV file corrupted 0.')
            else:
                start_date = last_line.split(',')[0] # assigning the start date to be the last like date.
                print('nLast date:', start_date)
                file.seek(0) # setting the cursor at the beginnning of the file
                lines = file.readlines() # reading the file contents into a list
                count = 0 # nr. of lines with last date
                for line in lines: #cycling through the lines of the file
                    if line.split(',')[0] == start_date: # cycle for counting the lines with last date in it.
                        count = count + 1
        if immediateExit:
            sys.exit('CSV file corrupted 1.')
        for iter in range(count): # removing the lines with last date
            lines.pop()
        print('n{} lines removed from date: {} in {} file'.format(count, start_date, csv_filename))



        if immediateExit:
            sys.exit('CSV file corrupted 1.2.')
        with open(csv_filename, 'w') as file:
            print('nFile', csv_filename, 'open for writing')
            file.writelines(lines)

            print('nRemoving', count, 'lines from', csv_filename)

        fileExists = True

    except:
        if immediateExit:
            sys.exit('CSV file corrupted 1.5.')
        with open(csv_filename, 'w') as file:
            file.write(csv_headers)
            fileExists = False
    if immediateExit:
        sys.exit('CSV file corrupted 2.')

Problem

In my practice, there was even a case when it was necessary to kill an entire multiprocessor application from one of those processes.

The following functions work well if your application uses the only main process. But no one of the following functions didn’t work in my case as the application had many other alive processes.

• quit()
• exit(0)
• os._exit(0)
• sys.exit(0)
• os.kill(os.getppid(), 9) – where os.getppid() is the pid of parent process

The last one killed the main process and itself but the rest processes were still alive.

Solution

I had to kill it by external command and finally found the solution using pkill.

import os

# This can be called even in process worker and will kill
# whole application included correlated processes as well
os.system(f"pkill -f {os.path.basename(__file__)}")

Just put at the end of your code quit() and that should close a python script.

In Python 3.9, you can also use: raise SystemExit("Because I said so").