How to access global variables from a thread in Python

Question

Consider the following code:

from threading import Thread 

def main():
    number = 5

    class my_thread(Thread):
        def __init__(self, range):
            Thread.__init__(self)
            self.range = range

        def run(self):
            global number
            for i in self.range:
                number += 1

    t1 = my_thread(range(4))
    t1.start()
    t1.join()
    print number

main()

The output from this program is

Exception in thread Thread-1:
Traceback (most recent call last):
  File "C:\Tools\Python27\lib\threading.py", line 801, in __bootstrap_inner
    self.run()
  File "C:\Dev\Workspace\Hello World\Hello.py", line 14, in run
    number += 1
NameError: global name 'number' is not defined

5

Evidently, my_thread does not have access to number. Why is this, and how can I access it correctly?

Dartmouth · Accepted Answer · 2016-08-10 08:48:50Z

2

You need to make number a global at its first definition, like this:

def main():
    global number
    number = 5

...etc...

answered Aug 10, 2016 at 8:48

Dartmouth

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