Python Idle and KeyboardInterrupts

KeyboardInterrupts in Idle work for me 90% of the time, but I was wondering why they don't always work. In Idle, if I do

import time
time.sleep(10)

and then attempt a KeyboardInterrupt using Ctrl+开发者_如何学CC, it does not interrupt the process until after sleeping for 10 seconds.

The same code and a KeyboardInterrupt via Ctrl+C works immediately in the shell.

A quick glance at the IDLE source reveals that KeyboardInterrupts have some special case handling: http://svn.python.org/view/python/tags/r267/Lib/idlelib/PyShell.py?annotate=88851

On top of that, code is actually executed in a separate process which the main IDLE gui process communicates with via RPC. You're going to get different behavior under that model - it's best to just test with the canonical interpreter (via command line, interactive, etc.)

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Digging deeper...

The socket on the RPC server is managed in a secondary thread which is supposed to propagate a KeyboardInterrupt using a call to thread.interrupt_main() ( http://svn.python.org/view/python/tags/r267/Lib/idlelib/run.py?annotate=88851 ). Behavior is not as expected there... This posting hints that for some reason, interrupt_main doesn't provide the level of granularity that you would expect: http://bytes.com/topic/python/answers/38386-thread-interrupt_main-doesnt-seem-work

Async API functions in cPython are a little goofy (from my experience) due to how the interpreter loop is handled, so it doesn't surprise me. interrupt_main() calls PyErr_SetInterrupt() to asynchronously notify the interpreter to handle a SIGINT in the main thread. From http://docs.python.org/c-api/exceptions.html#PyErr_SetInterrupt:

This function simulates the effect of a SIGINT signal arriving — the next time PyErr_CheckSignals() is called, KeyboardInterrupt will be raised

That would require the interpreter to go though whatever number of bytecode instructions before PyErr_CheckSignals() is called again - something that probably doesn't happen during a time.sleep(). I would venture to say that's a wart of simulating a SIGINT rather than actually signaling a SIGINT.

See This article:

I quote:

If you try to stop a CPython program using Control-C, the interpreter throws a KeyboardInterrupt exception.

It makes some sense, because the thread is asleep for 10 seconds and so exceptions cannot be thrown until the 10 seconds pass. However, ctrl + c always work in the shell because you are trying to stop a process, not throw a python KeyboardInterrupt exception.

Also, see this previously answered question.

I hope this helps!