线程内部的python全局变量

Question

The story begin with two threads and a global variable that change.. a lot of time :)

Thread number one (for simplicity we will call t1) generates a random number and store it in a global variable GLB.

Thread number two (aka t2) check the value of the global variable and when it reaches a value starts to print his value until a period of time.

BUT if t1 changes the value of that global variable, also change the value inside the loop and I don't want this!

I try to write pseudocode:

import random
import time
import threading


GLB = [0,0]

#this is a thread 
def t1():
    while True:
        GLB[0] = random.randint(0, 100)
        GLB[1] = 1
        print GLB
        time.sleep(5)


#this is a thread 
def t2():
    while True:
        if GLB[0]<=30:
            static = GLB
            for i in range(50):
                print i," ",static
                time.sleep(1)




a = threading.Thread(target=t1)
a.start()


b = threading.Thread(target=t2)
b.start()

while True:
    time.sleep(1)

The question is: why variable static change inside the loop for? It should be remain constant unitl it escapes from loop! Could I create a lock to the variable? Or there is any other way to solve the problem?

Thanks regards.

Answer 1

GLB is a mutable object. To let one thread see a consistent value while another thread modifies it you can either protect the object temporarily with a lock (the modifier will wait) or copy the object. In your example, a copy seems the best option. In python, a slice copy is atomic so does not need any other locking.

import random
import time
import threading

GLB = [0,0]

#this is a thread 
def t1():
    while True:
        GLB[0] = random.randint(0, 100)
        GLB[1] = 1
        print GLB
        time.sleep(5)

#this is a thread 
def t2():
    while True:
        static = GLB[:]
        if static[0]<=30:
            for i in range(50):
                print i," ",static
                time.sleep(1)

a = threading.Thread(target=t1)
a.start()

b = threading.Thread(target=t2)
b.start()

while True:
    time.sleep(1)