Ctrl-C doesn’t work when using threading.Timer

I’m writing a multithreaded Python app on Windows.

I used to terminate the app using ctrl-c, but once I added threading.Timer instances ctrl-c stopped working (or sometimes takes a very long time).

How could this be?
What’s the relation between having Timer threads and ctrl-c?

I found the following in Python’s thread documentation:

Threads interact strangely with
interrupts: the KeyboardInterrupt
exception will be received by an
arbitrary thread. (When the signal
module is available, interrupts always
go to the main thread.)


The way threading.Thread (and thus threading.Timer) works is that each thread registers itself with the threading module, and upon interpreter exit the interpreter will wait for all registered threads to exit before terminating the interpreter proper. This is done so threads actually finish execution, instead of having the interpreter brutally removed from under them. So when you hit ^C, the main thread receives the signal, decides to terminate and waits for the timers to finish.

You can set threads daemonic (with the setDaemon method) to make the threading module not wait for these threads, but if they happen to be executing Python code while the interpreter exits, you get confusing errors during exit. Even if you cancel the threading.Timer (and set it daemonic) it can still wake up while the interpreter is being destroyed — because threading.Timer‘s cancel method just tells the threading.Timer not to execute anything when it wakes up, but it has to actually execute Python code to make that determination.

There is no graceful way to terminate threads (other than the current one), and no reliable way to interrupt a thread that’s blocked. A more manageable approach to timers is usually an event loop, like the ones GUIs and other event-driven systems offer you. What to use depends entirely on what else your program will be doing.