Python creates 2 times more threads than expected

Question

I'm new to Python so my apologies if this is something obvious.

I'm trying to build multi threaded application, however when I want to create a thread I get two instead of one.

MyThread.py

from threading import Thread
import time

class MyThreadClass(Thread):

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

    def run(self):
        print "starting " + self.getName() + "\n"
        from main import var1
        while True:
            print self.getName() + " is running\n"
            print "value: " + var1 + "\n"
            time.sleep(1)

main.py

from MyThread import MyThreadClass
var1 = "Test"
MyThreadClass().start()

The output I get

 Thread-1 is running
 Thread-2 is running
 Thread-1 is running
 Thread-2 is running
 Thread-1 is running
 Thread-2 is running
 .....

Why is it happening? I noticed that if I replace MyThreadClass().start() with MyThreadClass().run() I get 2 threads but only one of them keeps running

 Thread-1 is running
 Thread-2 is running
 Thread-2 is running
 Thread-2 is running
 Thread-2 is running
 .....

Any idea what's wrong with the code?

Answer 1

When you import main.py in MyThread.py, the line

MyThreadClass().start()

gets executed once again (since the module gets loaded), hence a second thread is started.

---

You could create a guard clause in main.py by replacing that line with

if __name__ == "__main__":
    MyThreadClass().run()

or better, just pass var1 to MyThreadClass as a parameter to avoid the circular dependency.

MyThread.py:

from threading import Thread
import time

class MyThreadClass(Thread):

    def __init__(self, var1):
        Thread.__init__(self)
        self.var1 = var1

    def run(self):
        print "starting " + self.getName() + "\n"
        while True:
            print self.getName() + " is running\n"
            print "value: " + self.var1 + "\n"
            time.sleep(1)

main.py

from MyThread import MyThreadClass

if __name__ == "__main__":
    MyThreadClass("Test").run()