Python在线程之间创建共享变量
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Python creating a shared variable between threads
提问by Shay
I'm working on a project in Python using the "thread" module.
我正在使用“线程”模块在 Python 中处理一个项目。
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?
如何创建一个全局变量（在我的情况下我需要它是 True 或 False），我的项目中的所有线程（大约 4-6 个）都可以访问？
采纳答案by user3343166
We can define the variable outside the thread classes and declare it global inside the methods of the classes.
我们可以在线程类之外定义变量，并在类的方法中将其声明为全局变量。
Please see below trivial example which prints AB alternatively. Two variables flagand valare shared between two threads Thread_Aand Thread_B. Thread_Aprints val=20and then sets valto 30. Thread_Bprints val=30, since valis modified in Thread_A. Thread_Bthen sets valto 20 which is again used in Thread_A. This demonstrates that variable valis shared between two threads. Similarly variable flagis also shared between two threads.
请参阅下面交替打印 AB 的简单示例。两个变量flag和val在两个线程Thread_A和之间共享Thread_B。Thread_A打印val=20，然后设置val为 30。Thread_B打印val=30，因为val在Thread_A. Thread_B然后设置val为 20，再次用于Thread_A. 这表明变量val在两个线程之间共享。同样变量flag也在两个线程之间共享。
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
输出看起来像
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.
每个线程打印在另一个线程中修改的值。
回答by doctorlove
With no clue as to what you are really trying to do, either go with nio's approach and use locks, or consider condition variables:
不知道您真正想要做什么，要么采用 nio 的方法并使用锁，要么考虑条件变量：
From the docs
从文档
# Consume one item
cv.acquire()
while not an_item_is_available():
cv.wait()
get_an_available_item()
cv.release()
# Produce one item
cv.acquire()
make_an_item_available()
cv.notify()
cv.release()
You can use this to let one thread tell another a condition has been met, without having to think about the locks explicitly. This example uses cvto signify that an item is available.
您可以使用它让一个线程告诉另一个线程已满足条件，而无需明确考虑锁。此示例用于cv表示项目可用。
