This question already has answers here:
How do I terminate a script?
(14 answers)
Closed 3 years ago.
I have a simple Python script that I want to stop executing if a condition is met.
For example:
done = True
if done:
# quit/stop/exit
else:
# do other stuff
Essentially, I am looking for something that behaves equivalently to the 'return' keyword in the body of a function which allows the flow of the code to exit the function and not execute the remaining code.
To exit a script you can use,
import sys
sys.exit()
You can also provide an exit status value, usually an integer.
import sys
sys.exit(0)
Exits with zero, which is generally interpreted as success. Non-zero codes are usually treated as errors. The default is to exit with zero.
import sys
sys.exit("aa! errors!")
Prints "aa! errors!" and exits with a status code of 1.
There is also an _exit() function in the os module. The sys.exit() function raises a SystemExit exception to exit the program, so try statements and cleanup code can execute. The os._exit() version doesn't do this. It just ends the program without doing any cleanup or flushing output buffers, so it shouldn't normally be used.
The Python docs indicate that os._exit() is the normal way to end a child process created with a call to os.fork(), so it does have a use in certain circumstances.
You could put the body of your script into a function and then you could return from that function.
def main():
done = True
if done:
return
# quit/stop/exit
else:
# do other stuff
if __name__ == "__main__":
#Run as main program
main()
import sys
sys.exit()
You can either use:
import sys
sys.exit(...)
or:
raise SystemExit(...)
The optional parameter can be an exit code or an error message. Both methods are identical. I used to prefer sys.exit, but I've lately switched to raising SystemExit, because it seems to stand out better among the rest of the code (due to the raise keyword).
Try
sys.exit("message")
It is like the perl
die("message")
if this is what you are looking for. It terminates the execution of the script even it is called from an imported module / def /function
exit() should do the trick
exit() should do it.
If the entire program should stop use sys.exit() otherwise just use an empty return.
Related
I Have a shell script which in turn runs a python script, I have to exit from the main shell script when an exception is caught in python. Can anyone suggest a way on how to achieve it.
In Python you can set the return value using sys.exit(). Typically when execution completed successfully you return 0, and if not then some non-zero number.
So something like this in your Python will work:
import sys
try:
....
except:
sys.exit(1)
And then, as others have said, you need to make sure your bash script catches the error by either checking the return value explicitly (using e.g. $?) or using set -e.
I am somewhat new to Python, so I imagine this question has a simple answer. But I cannot seem to find a solution anywhere.
I have a Python script that continually accepts input from a streaming API and saves the data out to a file.
My problem when I need to stop the script to modify the code. If I use ctrl-f2, I sometime catch the script while it is in the process of writing to the output file, and the file ends up corrupted.
Is there a simple way to stop Python manually that allows it to finish executing the current line of code?
You can catch the SIGTERM or SIGINT signal and set a global variable that your script routinely checks to see if it should exit. It may mean you need to break your operations up into smaller chunks so that you can check the exit variable more frequently
import signal
EXIT = False
def handler(signum, frame):
global EXIT
EXIT = True
signal.signal(signal.SIGINT, handler)
def long_running_operation():
for i in range(1000000):
if EXIT:
# do cleanup or raise exception so that cleanup
# can be done higher up.
return
# Normal operation.
This question already has answers here:
How do I capture SIGINT in Python?
(12 answers)
Closed 7 years ago.
I am using Python's default cmd module to create a command-line-based app.
According to the docs CTRL+D (which signals EOF to the command line) can be captured by implementing a function called do_EOF, which works pretty well.
However, is there a way to capture CTRL+C as well? I want to execute a specific function (which saves all unsaved changes) before terminating the Python-script, which is the main motivation for doing this.
Currently, I capture KeyboardInterrupts using a try-except-block like the following:
if __name__ == '__main__':
try:
MyCmd().cmdloop()
except KeyboardInterrupt:
print('\n')
I tried to extend this into:
if __name__ == '__main__':
try:
app = MyCmd().cmdloop()
except KeyboardInterrupt:
app.execute_before_quit()
print('\n')
Which leads to the following NameError:
name 'app' is not defined
How can I capture CTRL+C?
I think signal could somehow do the trick, but I am not sure...
Remark: The script needs to be running on both Unix- and Windows-based systems.
Ctrl C activates system exit which is like an exception, I'm pretty sure you can catch it with try and except. Note: to catch SystemExit you must specify it in the except explicitly.
Heres the scenario I have a password that must be entered if entered wrong the script will not proceed and just exit itself? But how can I tell the script to safely exit itself?
I tried sys.exit() but that gives a traceback error and doesn't seem like a very clean exit method.
In fact, sys.exit() will only throw a SystemExit exception, which would make the program exit if this exception wasn't catched, but I don't think it's poor coding.
Anyway, another possibility is to use os._exit(0) if you need to exit immediately at any cost. (cf. http://docs.python.org/2/library/exceptions.html#exceptions.SystemExit)
you are using the sys module, but you haven't imported it.
add this before the sys.exit() line :
import sys
In Python, there are two similarly-named functions, exit() and sys.exit(). What's the difference and when should I use one over the other?
exit is a helper for the interactive shell - sys.exit is intended for use in programs.
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 (e.g. exit). They are useful for the interactive interpreter shell and should not be used in programs.
Technically, they do mostly the same: raising SystemExit. sys.exit does so in sysmodule.c:
static PyObject *
sys_exit(PyObject *self, PyObject *args)
{
PyObject *exit_code = 0;
if (!PyArg_UnpackTuple(args, "exit", 0, 1, &exit_code))
return NULL;
/* Raise SystemExit so callers may catch it or clean up. */
PyErr_SetObject(PyExc_SystemExit, exit_code);
return NULL;
}
While exit is defined in site.py and _sitebuiltins.py, respectively.
class Quitter(object):
def __init__(self, name):
self.name = name
def __repr__(self):
return 'Use %s() or %s to exit' % (self.name, eof)
def __call__(self, code=None):
# Shells like IDLE catch the SystemExit, but listen when their
# stdin wrapper is closed.
try:
sys.stdin.close()
except:
pass
raise SystemExit(code)
__builtin__.quit = Quitter('quit')
__builtin__.exit = Quitter('exit')
Note that there is a third exit option, namely os._exit, which exits without calling cleanup handlers, flushing stdio buffers, etc. (and which should normally only be used in the child process after a fork()).
If I use exit() in a code and run it in the shell, it shows a message asking whether I want to kill the program or not. It's really disturbing.
See here
But sys.exit() is better in this case. It closes the program and doesn't create any dialogue box.
Solution, Origins, Differences & Speed
Why do we need the exit() /
sys.exit() commands?
Usually, the code runs through the lines until the end and the program exists automatically.
Occasionally, we would like to ask the program to close before the full cycle run.
An example case is when you implement authentication and a user fails to authenticate, in some cases you would like to exit the program.
The exit()
Exits Python.
Maybe you didn't know this, but it's a synonym of quit() and was added after quit() to make python more user friendly.
Designed to work with interactive shells.
Usage:
Use the built-in exit() out of the box, as is, without importing any library.
Just type this:
exit()
Execution Time: 0.03s
Pros:
Faster to use (built-in)
Works both with python 2.x and python 3.x
Fast
Can be used exactly like sys.exit() (with the exception)
Cons:
No exception message
The sys.exit()
Exits Python and raising the SystemExit exception (requires an import).
Designed to work inside programs.
Usage:
import sys
sys.exit()
Execution Time (of just the import and sys.exit()): 0.07s
Or you can use a message for the SystemExit exception:
Added finally block to illustrate code cleanup clause. Inspired by #Nairum.
import sys
try:
sys.exit("This is an exit!")
except SystemExit as error:
print(error)
finally:
print("Preforming cleanup in 3, 2, 1..")
# Do code cleanup on exit
Output:
This is an exit!
Preforming cleanup in 3, 2, 1..
Pros:
Triggers SystemExit exception
You can use an exception message
Closes without a dialog
Utilizes finally clause of try
Works both with python 2.x and python 3.x
Cons:
Needs an import
Slower by 57.1% than exit()
Conclusion:
If you don't need an exception with an optional message, then use exit(), this is faster and built-in.
If you require more functionality of an exception with an optional message, use sys.exit().
In the code examples I am using Python 3.x