How to stop an infinite loop safely in Python?

Question

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.

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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 1

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.