Ending an infinite while loop

Question

I currently have code that basically runs an infinite while loop to collect data from users. Constantly updating dictionaries/lists based on the contents of a text file. For reference:

while (True): IDs2=UpdatePoints(value,IDs2) time.sleep(10)

Basically, my problem is that I do not know when I want this to end, but after this while loop runs I want to use the information collected, not lose it by crashing my program. Is there a simple, elegant way to simply exit out of the while loop whenever I want? Something like pressing a certain key on my keyboard would be awesome.

Melebius · Accepted Answer · 2017-06-06 08:07:55Z

139

You can try wrapping that code in a try/except block, because keyboard interrupts are just exceptions:

try: while True: IDs2=UpdatePoints(value,IDs2) time.sleep(10) except KeyboardInterrupt: print('interrupted!')

Then you can exit the loop with CTRL-C.

edited Jun 6, 2017 at 8:07

Melebius

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answered Sep 25, 2013 at 1:54

Jess

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3 Comments

reem Over a year ago

I've got to say, this is one of the neater uses for a try/except block I've seen. I might try this pattern for debugging.

Alex Alifimoff Over a year ago

try... except... blocks are not only for "stuff that isn't supposed to happen". In fact, "Python generally recommends the EAFP policy" (easier to ask forgiveness than permission)

user136036 Over a year ago

Be aware that depending on the IDE you use, Ctrl+C will not work. For example PyDev does not support it. To test this, run the program in a normal console.

Tristan · Accepted Answer · 2016-10-29 16:19:29Z

You could use exceptions. But you only should use exceptions for stuff that isn't supposed to happen. So not for this.

That is why I recommand signals:

import sys, signal def signal_handler(signal, frame): print("\nprogram exiting gracefully") sys.exit(0) signal.signal(signal.SIGINT, signal_handler)

you should put this on the beginning of your program and when you press ctrl+c wherever in your program it will shut down gracefully

Code explanation:

You import sys and signals. Then you make a function that executes on exit. sys.exit(0) stops the programming with exit code 0 (the code that says, everything went good).

When the program get the SIGINT either by ctrl-c or by a kill command in the terminal you program will shutdown gracefully.

This worked for me when the try-except did not. Don't know why, and I'm afraid I've not got time to investigate.
@WillC Perhaps this helps: the try-except only defines a way to handle a KeyboardInterrupt once it managed to actually interrupt the program. But the program can still be blocking and not register your interrupt signal. That's where the signal handler is useful: when the corresponding signal arrives, the handler is directly executed regardless - even if your program is blocking/looping in some C code. (for example an expensive regex.match call)

eandersson · Accepted Answer · 2013-09-25 02:00:17Z

I think the easiest solution would be to catch the KeyboardInterrupt when the interrupt key is pressed, and use that to determine when to stop the loop.

except KeyboardInterrupt: break

The disadvantage of looking for this exception is that it may prevent the user from terminating the program while the loop is still running.

GabrieleMartini · Accepted Answer · 2019-02-12 01:44:53Z

I use python to track stock prices and submit automated buy/sell commands on my portfolio. Long story short, I wanted my tracking program to ping the data server for info, and place trades off of the information gathered, but I also wanted to save the stock data for future reference, on top of being able to start/stop the program whenever I wanted.

What ended up working for me was the following:

trigger = True while trigger == True: try: (tracking program and purchasing program conditions here) except: trigger = False print('shutdown initialized') df = pd.DataFrame... save all the datas print('shutdown complete')

etc.

From here, while the program is in the forever loop spamming away requests for data from my broker's API, using the CTRL-C keyboard interrupt function toggles the exception to the try loop, which nullifies the while loop, allowing the script to finalize the data saving protocol without bringing the entire script to an abrupt halt.

Hope this helps!

Resultant

Ambesh Shekhar · Accepted Answer · 2020-06-24 12:03:16Z

I would suggest using the try, except syntax within a loop if you are running on an IPYNB file in Google Colab or Jupyter, like:

while True: try: IDs2=UpdatePoints(value,IDs2) time.sleep(10) except KeyboardInterrupt: break except: continue

the last except is for any other error if occurs the loop will resume

topsoftwarepro · Accepted Answer · 2023-05-08 06:05:44Z

You can catch the KeyboardInterrupt error in Python:

try: while 1>0: IDs2=UpdatePoints(value,IDs2) time.sleep(10) except KeyboardInterrupt: print('While loop ended!')

To read more about Python Error handling (try, except, etc.):

https://www.w3schools.com/python/python_try_except.asp

or:

https://www.w3schools.com/python/gloss_python_try_finally.asp

How on Earth this could be more professional if you just obfuscating boolean for no reason?
I'm not sure how the innards of Python works, but as such it looks like wasted CPU cycles to me, aside from being less readable.

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