What’s the difference between exit(0) and exit(1) in Python?

I tried looking around but didn’t find a specific question on these lines. If it’s already been answered, a link would be sufficient.

0 and 1 are the exit codes.

exit(0) means a clean exit without any errors / problems

exit(1) means there was some issue / error / problem and that is why the program is exiting.

This is not Python specific and is pretty common. A non-zero exit code is treated as an abnormal exit, and at times, the error code indicates what the problem was. A zero error code means a successful exit.

This is useful for other programs, shell, caller etc. to know what happened with your program and proceed accordingly.

This determines the exit status of the program when it finishes running (generally, 0 for success and 1 for error).

It is not unique to Python, and the exact effect depends on your operating system and how the program is called (though 99% of the time, if you’re just running Python scripts, it doesn’t matter).

The standard convention for all C programs, including Python, is for exit(0) to indicate success, and exit(1) or any other non-zero value (in the range 1..255) to indicate failure. Any value outside the range 0..255 is treated modulo 256 (the exit status is stored in an 8-bit value). Sometimes, that will be treated as signed (so you might see -128, -127, etc) but more usually it is treated as unsigned.

This status is available to the code that invoked Python. This convention applies across platforms, though the meaning of non-zero exit status can vary on different platforms.

The number you pass to the exit() function is simply your program’s return code, which is given to the operating system. From your program’s point of view, there is no difference: execution will end in both cases, and the value supplied to the function will be given to the OS. But some tools and scripts take into account the program’s exit code. Most tools return 0 when they succeed and nonzero to indicate an error.

So, if your program will be run from a script, an automated tool or from some other software that takes into account the return code (such as an IDE), you must be careful on what you return.

When in doubt, just return 0 to indicate everything is OK.

exit(0): This causes the program to exit with a successful termination.

exit(1): This causes the program to exit with a system-specific meaning.

On many systems, exit(1) signals some sort of failure, however there is no guarantee.

As I recall, the C standard only recognizes three standard exit values:

• EXIT_SUCCESS — successful termination
• EXIT_FAILURE — unsuccessful termination
• 0 — same as EXIT_SUCCESS

I got a message saying script xyz.py returned exit code 0. What does this mean?

What do the exit codes in Python mean? How many are there? Which ones are important?

What you’re looking for in the script is calls to sys.exit(). The argument to that method is returned to the environment as the exit code.

It’s fairly likely that the script is never calling the exit method, and that 0 is the default exit code.

From the documentation for 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.

One example where exit codes are used are in shell scripts. In bash you can check the special variable $? for the last exit status:

me@mini:~$ python -c ""; echo $?
0
me@mini:~$ python -c "import sys; sys.exit(0)"; echo $?
0
me@mini:~$ python -c "import sys; sys.exit(43)"; echo $?
43

Personally I try to use the exit codes I find in /usr/include/asm-generic/errno.h (on a Linux system), but I don’t know if this is the right thing to do.

For the record, you can use POSIX standard exit codes defined here.

Example:

import sys, os

try:
    config()
except:
    sys.exit(os.EX_CONFIG) 
try:
    do_stuff()
except:
    sys.exit(os.EX_SOFTWARE)
sys.exit(os.EX_OK) # code 0, all ok

There is an errno module that defines standard exit codes:

For example, Permission denied is error code 13:

import errno, sys

if can_access_resource():
    do_something()
else:
    sys.exit(errno.EACCES)

Exit codes of 0 usually mean, “nothing wrong here.” However if the programmer of the script didn’t follow convention you may have to consult the source to see what it means. Usually a non-zero value is returned as an error code.

The answer is “Depends on what exit code zero means”

However, in most cases, this means “Everything is Ok”

I like POSIX:

So, on the shell, I would type:

python script.py && echo 'OK' || echo 'Not OK'

If my python script calls sys.exit(0) the shell returns ‘OK’

If my python script calls sys.exit(1) (or any non-zero integer), the shell returns ‘Not OK’

It’s your job to get clever with the shell, and read the documentation (or the source) for your script to see what the exit codes mean.

Operating system commands have exit codes. Look for linux exit codes to see some material on this. The shell uses the exit codes to decide if the program worked, had problems, or failed. There are some efforts to create standard (or at least commonly-used) exit codes. See this Advanced Shell Script posting.

I recommend a clean shutdown with the exit(0) builtin rather than using sys.exit()

I’m not sure if this is good advice.

The very documentation mentioned reads:

The site module (which is imported automatically during startup, except if the -S command-line option is given) adds several constants to the built-in namespace. They are useful for the interactive interpreter shell and should not be used in programs.

Then:

exit(code=None)

Objects that when printed, print a message like “Use quit() or Ctrl-D (i.e. EOF) to exit”, and when called, raise SystemExit with the specified exit code.

If we compare it to sys.exit() documentation, it’s using the same mechanism which is raising a SystemExit exception.

If you would like to portably use the standard POSIX exit codes, see the exitstatus package on PyPI.

Install the package:

$ pip install exitstatus

Use in your code:

import sys
from exitstatus import ExitStatus

sys.exit(ExitStatus.success)

The exit codes only have meaning as assigned by the script author. The Unix tradition is that exit code 0 means ‘success’, anything else is failure. The only way to be sure what the exit codes for a given script mean is to examine the script itself.

Exit codes in many programming languages are up to programmers. So you have to look at your program source code (or manual). Zero usually means “everything went fine”.

To allow exception handlers and other things execute I recommend a clean shutdown with the exit(0) builtin rather than using sys.exit() which terminates the process abruptly.