Terminating a Python script

问题:

I am aware of the die() command in PHP which stops a script early.

How can I do this in Python?

回答1:

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 of try 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.

回答2:

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

Example:

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

回答3:

Another way is:

raise SystemExit

回答4:

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.

回答5:

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.

回答6:

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 here.

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()

回答7:

from sys import exit
exit()

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

回答8:

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 !!!!"

回答9:

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!");

回答10:

There are a few standard ways to do it:

import sys
sys.exit(404)

# or...
exit(404)

# or...
quit(404)

# or...
raise SystemExit(404)

I was dissatisfied that (on repl.it at least,) none of these completely shut down the interpreter; it could always run code in the repl, even after exit, etc. the only way I could think of that always stops the interpreter from proceeding is by simulating a system exit by sleeping.

import time
def forceStop(errmsg="emergency stop deployed. :("):
    print(f"\033[31m{errmsg}\033[0m")
    time.sleep(10000000)

This is obviously not recommended.