CODEWITHSUNDEEP
pythonmultithreadingpython-3.x
candria malone
candria malone

Reputation: 41

How to reference a thread in Python 3?

I am trying to call a thread I define in a function from another function. Here is the first function, its purpose is to create and start a thread: 

def startThread(func):

    listen = threading.Thread(target = func)
    listen.start()

I am trying to implement a function that will close the thread created in that first function, how should I go about it? I don't know how to successfully pass the thread. 

def endThread(thread):
    thread.exit()

Thank you!

Upvotes: 4

Views: 676

Answers (1)

Hannu
Hannu

Reputation: 12205

This problem is almost FAQ material.

To summarise, there is no way to kill a thread from the outside. You can of course pass the thread object to any function you want, but threading library is missing kill and exit calls.

There are more or less two distinct ways around this, depending on what your thread does.

The first method is to make it so that your thread co-operates. This approach is discussed here: Is there any way to kill a Thread in Python? This method adds a check to your thread loop and a way to raise a "stop signal", which will then cause the thread to exit from the inside when detected.

This method works fine if your thread is a relatively busy loop. If it is something that is blocking in IO wait, not so much, as your thread could be blocking in a read call for days or weeks before receiving something and executing the signal check part. Many IO calls accept a timeout value, and if it is acceptable to wait a couple of seconds before your thread exits, you can use this to force the exit check every N seconds without making your thread a busy loop.

The other approach is to replace threads with processes. You can force kill a subprocess. If you can communicate with your main program with queues instead of shared variables, this is not too complicated, either. If your program relies heavily on sharing global variables, this would require a major redesign.

If your program is waiting in IO loops, you need instantaneous termination and you are using shared global variables, then you are somewhat out of luck, as you either need to accept your threads not behaving nicely or you need to redesign some parts of your code to untangle either the IO wait or shared variables.

Upvotes: 2

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