Python exit commands - why so many and when should each be used

Question

It seems that python supports many different commands to stop script execution.

The choices I've found are: 
quit()
exit()
sys.exit()
os._exit()

Have I missed any? What's the difference between them? When would you use each?

Answer 1

Let me give some information on them:

1. quit raises the SystemExit exception behind the scenes.

2. Furthermore, if you print it, it will give a message:

3. >>> print (quit) Use quit() or Ctrl-Z plus Return to exit >>>

4. This functionality was included to help people who do not know Python. After all, one of the most likely things a newbie will try to exit Python is typing in quit.

5. Nevertheless, quit should not be used in production code. This is because it only works if the site module is loaded. Instead, this function should only be used in the interpreter.

6. exit is an alias for quit (or vice-versa). They exist together simply to make Python more user-friendly.

7. Furthermore, it too gives a message when printed:

8. >>> print (exit) Use exit() or Ctrl-Z plus Return to exit >>>

9. However, like quit, exit is considered bad to use in production code and should be reserved for use in the interpreter. This is because it too relies on the site module.

10. sys.exit raises the SystemExit exception in the background. This means that it is the same as quit and exit in that respect.

11. Unlike those two however, sys.exit is considered good to use in production code. This is because the sys module will always be there.

12. os._exit exits the program without calling cleanup handlers, flushing stdio buffers, etc. Thus, it is not a standard way to exit and should only be used in special cases. The most common of these is in the child process(es) created by os.fork.

13. Note that, of the four methods given, only this one is unique in what it does.

Summed up, all four methods exit the program. However, the first two are considered bad to use in production code and the last is a non-standard, dirty way that is only used in special scenarios. So, if you want to exit a program normally, go with the third method: sys.exit.
Or, even better in my opinion, you can just do directly what sys.exit does behind the scenes and run:

raise SystemExit

This way, you do not need to import sys first.

However, this choice is simply one on style and is purely up to you.

Answer 2

The functions quit(), exit(), sys.exit() and os._exit() have almost same functionality as they raise the SystemExit exception by which the Python interpreter exits and no stack traceback is printed.
We can catch the exception to intercept early exits and perform cleanup activities; if uncaught, the interpreter exits as usual.

When we run a program in Python, we simply execute all the code in file, from top to bottom. Scripts normally exit when the interpreter reaches the end of the file, but we may also call for the program to exit explicitly with the built-in exit functions.

1. quit()

   It works only if the site module is imported so it should not be used in production code. Production code means the code is being used by the intended audience in a real-world situation. This function should only be used in the interpreter.

   It raises the SystemExit exception behind the scenes. If you print it, it will give a message:

   Example:

# Python program to demonstrate # quit()   for i in range(10):           # If the value of i becomes      # 5 then the program is forced     # to quit     if i == 5:                   # prints the quit message         print(quit)         quit()     print(i) Output: 0 1 2 3 4 Use quit() or Ctrl-D (i.e. EOF) to exit Hope this helps!! If you need to know more about Python, join Python online course today. Thanks!

Answer 3

The functions^* quit(), exit(), and sys.exit() function in the same way: they raise the SystemExit exception. So there is no real difference, except that sys.exit() is always available but exit() and quit() are only available if the site module is imported.

The os._exit() function is special, it exits immediately without calling any cleanup functions (it doesn't flush buffers, for example). This is designed for highly specialized use cases... basically, only in the child after an os.fork() call.

Conclusion

• Use exit() or quit() in the REPL.

• Use sys.exit() in scripts, or raise SystemExit() if you prefer.

• Use os._exit() for child processes to exit after a call to os.fork().

All of these can be called without arguments, or you can specify the exit status, e.g., exit(1) or raise SystemExit(1) to exit with status 1. Note that portable programs are limited to exit status codes in the range 0-255, if you raise SystemExit(256) on many systems this will get truncated and your process will actually exit with status 0.

Footnotes

^* Actually, quit() and exit() are callable instance objects, but I think it's okay to call them functions.