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What is "thread local storage" in Python, and why do I need it?

Asked 2008-09-19T19:53:19.447
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In Python specifically, how do variables get shared between threads?

Although I have used threading.Thread before I never really understood or saw examples of how variables got shared. Are they shared between the main thread and the children or only among the children? When would I need to use thread local storage to avoid this sharing?

I have seen many warnings about synchronizing access to shared data among threads by using locks but I have yet to see a really good example of the problem.

python multithreading thread-local
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Just like in every other language, every thread in Python has access to the same variables. There's no distinction between the 'main thread' and child threads.

One difference with Python is that the Global Interpreter Lock means that only one thread can be running Python code at a time. This isn't much help when it comes to synchronising access, however, as all the usual pre-emption issues still apply, and you have to use threading primitives just like in other languages. It does mean you need to reconsider if you were using threads for performance, however.

answered 2008-09-19T20:03:30.640

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