Start() vs run() for threads in Python?

Question

I'm a little bit confused.

I'm trying to start a thread in a loop, i.e.:

while True:
  my_thread.start()

I'm a little confused because I've had it working with my_thread.run(), but when I swapped it to start() it fails to start more than one thread. Is my .run() not actually a separate thread, and if not what should I be doing? Lastly, can I pass variables into start()?

Answer 1

You are correct in that run() does not spawn a separate thread. It runs the thread function in the context of the current thread.

It is not clear to me what you are trying to achieve by calling start() in a loop.

• If you want your thread to repeatedly do something, then move the loop into the thread function.
• If you want multiple threads, then create multiple Thread objects (and call start() once on each of them).

Finally, to pass arguments to a thread, pass args and kwargs to the Thread constructor.

Answer 2

Spawn threads

You can not spawn multiple threads like this:

while True:
   my_thread.start()     # will start one thread, no matter how many times you call it

Use instead:

while True:
   ThreadClass( threading.Thread ).start() # will create a new thread each iteration
   threading.Thread( target=function, args=( "parameter1", "parameter2" ))

def function( string1, string2 ):
  pass # Just to illustrate the threading factory. You may pass variables here.

Answer 3

Please read threading code and docs. start() must be called at most once per thread object. It arranges for the object’s run() method to be invoked in a separate thread of control. run() will be called by start() in the context as follow:

    def start(self):
        ....
        _start_new_thread(self._bootstrap, ())
        ....

    def _bootstrap(self):
        ....
        self._bootstrap_inner()
        ....

    def _bootstrap_inner(self):
        ...
        self.run()
        ...

Let's a demo for start() and run().

class MyThread(threading.Thread):

    def __init__(self, *args, **kwargs):
        super(MyThread, self).__init__(*args, **kwargs)

    def run(self):
        print("called by threading.Thread.start()")


if __name__ == '__main__':
    mythread = MyThread()
    mythread.start()
    mythread.join()

---

$ python3 threading.Thread.py
called by threading.Thread.start()