Python creating a shared variable between threads
multithreading, python
Solution
We can define the variable outside the thread classes and declare it global inside the methods of the classes.
Please see below trivial example which prints AB alternatively. Two variables `flag` and `val` are shared between two threads `Thread_A` and `Thread_B`. `Thread_A` prints `val=20` and then sets `val` to 30. `Thread_B` prints `val=30`, since `val` is modified in `Thread_A`. `Thread_B` then sets `val` to 20 which is again used in `Thread_A`. This demonstrates that variable `val` is shared between two threads. Similarly variable `flag` is also shared between two threads.
import threading
import time
c = threading.Condition()
flag = 0 #shared between Thread_A and Thread_B
val = 20
class Thread_A(threading.Thread):
def __init__(self, name):
threading.Thread.__init__(self)
self.name = name
def run(self):
global flag
global val #made global here
while True:
c.acquire()
if flag == 0:
print "A: val=" + str(val)
time.sleep(0.1)
flag = 1
val = 30
c.notify_all()
else:
c.wait()
c.release()
class Thread_B(threading.Thread):
def __init__(self, name):
threading.Thread.__init__(self)
self.name = name
def run(self):
global flag
global val #made global here
while True:
c.acquire()
if flag == 1:
print "B: val=" + str(val)
time.sleep(0.5)
flag = 0
val = 20
c.notify_all()
else:
c.wait()
c.release()
a = Thread_A("myThread_name_A")
b = Thread_B("myThread_name_B")
b.start()
a.start()
a.join()
b.join()
Output looks like
A: val=20
B: val=30
A: val=20
B: val=30
A: val=20
B: val=30
A: val=20
B: val=30
Each thread prints the value which was modified in another thread.
Problem
I'm working on a project in Python using the "thread" module. How can I make a global variable (in my case I need it to be True or False) that all the threads in my project (about 4-6) can access?