Python "Thread-locality"

Python threading module and thread-local objects

The __init__() body of Python threading.Threads runs in the main thread. Any Thread-local setup must be done inside of the threading.Thread.run() body; for example, SQLAlchemy`s thread-local scoped_session must be invoked inside of each thread to avoid conflicts/race conditions/etc.

Here is a simple example:

### threading-example.py (python 2.6+)
from threading import Thread, current_thread

class BadThread(Thread):
    def __init__(self):
        Thread.__init__(self)
        self.current_thread = current_thread()

    def run(self):
        pass

class GoodThread(Thread):
    def __init__(self):
        Thread.__init__(self)

    def run(self):
        self.current_thread = current_thread()

if __name__ == '__main__':
    b1 = BadThread()
    b2 = BadThread()
    b1.start()
    b2.start()
    b1.join()
    b2.join()
    print 'BadThreads are the different : %s' % \
          (b1.current_thread is not b2.current_thread)

    g1 = GoodThread()
    g2 = GoodThread()
    g1.start()
    g2.start()
    g1.join()
    g2.join()
    print 'GoodThreads are the different : %s' % \
          (g1.current_thread is not g2.current_thread)

~$ python threading-example.py
BadThreads are the different : False
GoodThreads are the different : True