[Python-ideas] Interrupting threads (original) (raw)

Charles-François Natali cf.natali at gmail.com
Sun Jan 27 09:58:03 CET 2013

It's possible to interrupt the main thread using KeyboardInterrupt, so why shouldn't it be possible to do something similar to a thread?

Because it's unsafe. Allowing asynchronous interruptions at any point in the code is calling for trouble: in a multi-threaded program, if you interrupt a thread in the middle of a critical section, there's a high chance that the invariants protected in this critical section won't hold. So basically, the object/structure will be in an unusable state, which will lead to random failures at some point in the future.

Actually, there's more to it than that because sometimes you don't want a section of code to be interrupted.

Actually it's exactly the opposite: you only want to handle interruption at very specific points in the code, so that the rollback and interruption logic is tractable.

Also, as noted by Guido, it's basically useless because neither sleep() nor lock acquisition can be interrupted - at least in the current implementation - and those are likely the calls you'd like to interrupt.

FWIW, Java has a Thread.Stop() method that more or less does what you're suggesting. It was quickly depreciated because it's inherently unsafe: the right way to do it is through a cooperative form of interruption, with an interruption exception that can be thrown at specific points in the code (and a per-thread interrupt status flag that can be checked explicitly, and which is checked implicitly when entering an interruptible method). See the rationale here: http://docs.oracle.com/javase/1.5.0/docs/guide/misc/threadPrimitiveDeprecation.html

So here's what I'd like to suggest:

1. There's a private thread-specific flag called 'interruptoccurred'. 2. There's a private thread-specific flag called 'heedinginterrupt'. 3. There's a context manager called 'heedinterrupt'.

I'm not a native speaker, and I had never heard about the 'heed' verb before, had to look it up in the dictionary :-)

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