How to stop an infinite loop safely in Python?

I’ve got a script that runs an infinite loop and adds things to a database and does things that I can’t just stop halfway through, so I can’t just press Ctrl+C and stop it.

I want to be able to somehow stop a while loop, but let it finish it’s last iteration before it stops.

Let me clarify:

My code looks something like this:

while True:
    do something
    do more things
    do more things

I want to be able to interrupt the while loop at the end, or the beginning, but not between doing things because that would be bad.

And I don’t want it to ask me after every iteration if I want to continue.

---

Thanks for the great answers, I’m super grateful but my implementation doesn’t seem to be working:

def signal_handler(signal, frame):
    global interrupted
    interrupted = True

class Crawler():
    def __init__(self):
        # not relevant

    def crawl(self):
        interrupted = False
        signal.signal(signal.SIGINT, signal_handler)
        while True:
            doing things
            more things

            if interrupted:
                print("Exiting..")
                break

When I press Ctrl+C the program just keeps going ignoring me.

Answer

What you need to do is catch the interrupt, set a flag saying you were interrupted but then continue working until it’s time to check the flag (at the end of each loop). Because python’s try-except construct will abandon the current run of the loop, you need to set up a proper signal handler; it’ll handle the interrupt but then let python continue where it left off. Here’s how:

import signal

import time   # For the demo only

def signal_handler(signal, frame):
    global interrupted
    interrupted = True

signal.signal(signal.SIGINT, signal_handler)


interrupted = False
while True:
    print("Working hard...")
    time.sleep(3)
    print("All done!")

    if interrupted:
        print("Gotta go")
        break

Notes:

1. Use this from the command line. In the IDLE console, it’ll trample on IDLE’s own interrupt handling.

2. A better solution would be to “block” KeyboardInterrupt for the duration of the loop, and unblock it when it’s time to poll for interrupts. This is a feature of some Unix flavors but not all, hence python does not support it (see the third “General rule”)

3. The OP wants to do this inside a class. But the interrupt function is invoked by the signal handling system, with two arguments: The signal number and a pointer to the stack frame– no place for a self argument giving access to the class object. Hence the simplest way to set a flag is to use a global variable. You can rig a pointer to the local context by using closures (i.e., define the signal handler dynamically in __init__(), but frankly I wouldn’t bother unless a global is out of the question due to multi-threading or whatever.

Caveat: If your process is in the middle of a system call, handling an signal may interrupt the system call. So this may not be safe for all applications. Safer alternatives would be (a) Instead of relying on signals, use a non-blocking read at the end of each loop iteration (and type input instead of hitting ^C); (b) use threads or interprocess communication to isolate the worker from the signal handling; or (c) do the work of implementing real signal blocking, if you are on an OS that has it. All of them are OS-dependent to some extent, so I’ll leave it at that.