Thread local storage in Python

Question

How do I use thread local storage in Python?

Related

• What is “thread local storage” in Python, and why do I need it? - This thread appears to be focuse

Answer 1

Thread-local storage can simply be thought of as a namespace (with values accessed via attribute notation). The difference is that each thread transparently gets its own set of attributes/values, so that one thread doesn't see the values from another thread.

Just like an ordinary object, you can create multiple threading.local instances in your code. They can be local variables, class or instance members, or global variables. Each one is a separate namespace.

Here's a simple example:

import threading

class Worker(threading.Thread):
    ns = threading.local()
    def run(self):
        self.ns.val = 0
        for i in range(5):
            self.ns.val += 1
            print("Thread:", self.name, "value:", self.ns.val)

w1 = Worker()
w2 = Worker()
w1.start()
w2.start()
w1.join()
w2.join()

Output:

Thread: Thread-1 value: 1
Thread: Thread-2 value: 1
Thread: Thread-1 value: 2
Thread: Thread-2 value: 2
Thread: Thread-1 value: 3
Thread: Thread-2 value: 3
Thread: Thread-1 value: 4
Thread: Thread-2 value: 4
Thread: Thread-1 value: 5
Thread: Thread-2 value: 5

Note how each thread maintains its own counter, even though the ns attribute is a class member (and hence shared between the threads).

The same example could have used an instance variable or a local variable, but that wouldn't show much, as there's no sharing then (a dict would work just as well). There are cases where you'd need thread-local storage as instance variables or local variables, but they tend to be relatively rare (and pretty subtle).