How do I exit a script early, like the die()
command in PHP?
import sys
sys.exit()
details from the sys
module documentation:
sys.exit([arg])
Exit from Python. This is implemented by raising the
SystemExit
exception, so cleanup actions specified by finally clauses oftry
statements are honored, and it is possible to intercept the exit attempt at an outer level.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.Since
exit()
ultimately “only” raises an exception, it will only exit the process when called from the main thread, and the exception is not intercepted.
Note that this is the 'nice' way to exit. @glyphtwistedmatrix below points out that if you want a 'hard exit', you can use os._exit(*errorcode*)
, though it's likely os-specific to some extent (it might not take an errorcode under windows, for example), and it definitely is less friendly since it doesn't let the interpreter do any cleanup before the process dies. On the other hand, it does kill the entire process, including all running threads, while sys.exit()
(as it says in the docs) only exits if called from the main thread, with no other threads running.
sys.exit()
doesn't work (doesn't kill the process, just kills the thread) if raised by a background thread? –
Ramonaramonda sys.exit()
raises a SystemExit
exception in the current thread. –
Sterol UserWarning: To exit: use 'exit', 'quit', or Ctrl-D. warn("To exit: use 'exit', 'quit', or Ctrl-D.", stacklevel=1)
–
Gutty Tkinter
it will not stop the root process mainLoop
and the window it displays. For this, you need the additional line of root.destroy()
. –
Gondola sys.exit()
; the script will exit naturally when there's nothing else left to do. I guess the only time you'd need this is when something unfinished is taking longer that the user is willing to wait. –
Robson 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()
quit()
is "useful for the interactive interpreter shell and should not be used in programs." –
Compellation sys.exit()
will raise a SystemExit
exception which, if left uncaught, will terminate the interpreter. It doesn't terminate the script immediately and it doesn't affect scripts that were started in a different python process. Since it's an exception, if called within a try
-except
-finally
block, the finally
block will still be executed. –
Vibrant quit()
able to stop multithreading application? –
Juneberry Another way is:
raise SystemExit
sys.exit
wrapper –
Gamesmanship 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.
quit()
or exit()
in your code. These functions are intended only for the interactive Python. See Python exit commands - why so many and when should each be used? –
Gigantopithecus 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.
os._exit()
is the only way to terminate the job properly without errors. –
Kizzee os._exit()
exits the entire process without any cleanup. Since in AWS the running VM instance will be discarded at the end of the job, this is not a problem for me. –
Kizzee 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()
from sys import exit
exit()
As a parameter you can pass an exit code, which will be returned to OS. Default is 0.
exit()
and sys.exit()
are not the same thing. Don't use the built-in exit()
in scripts, this is just a helper for the interactive shell - use sys.exit()
–
Dilemma 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:\Directory\testdieprogram.py", line 12, in main() File "Z:\Directory\testdieprogram.py", line 8, in main sys.exit() SystemExit
Edit
The point being that the program ends smoothly and peacefully, rather than "I'VE STOPPED !!!!"
return
to terminate the script. All the return
is doing is returning a value and a flow of control to the calling function. There it continues with the execution right after the call of the function which called return
. Of course, if the return
is the last statement in your script as in your example, then the script is terminated right after it is called. –
Kunming exit
-- hence python programmers may not notice the stench of this code smell as much); and lastly, (3) multi-threaded code (which pythonistas have historically just ignored). –
Surmise main
method and run to completion. I think this approach should be preferred when available. –
Vibrant 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)
- whereos.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__)}")
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!")
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")
.
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,brand\n' # 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('***\n\n{} 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.')
except:
without exception type. If you just use except Exception:
(or even a more detailed exception type if possible), the sys.exit()
would work as intended and you would not need this work-around. –
Compellation sys.exit()
is really a sort of last-resort solution for critical errors. Just my two cents :) –
Compellation use exit
and quit
in .py files
and sys.exit
for exe files
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import os
os._exit()
– Epithelioma