python之threading.local

简述：

threading.local是全局变量但是它的值却在当前调用它的线程当中

作用：

在threading module中，有一个非常特别的类local。一旦在主线程实例化了一个local，它会一直活在主线程中，并且又主线程启动的子线程调用这个local实例时，它的值将会保存在相应的子线程的字典中。可以为每个线程创建一块独立的空间，让他存放数据。

使用方式：

在使用threading.local()之前，先了解一下局部变量和全局变量。

import threading

import time

def worker():

    x = 0

    for i in range(100):

        time.sleep(0.0001)

        x += 1

    print(threading.current_thread(),x)

for i in range(10):

    threading.Thread(target=worker).start()

运行结果：

<Thread(Thread-2, started 123145372971008)> 100

<Thread(Thread-6, started 123145393991680)> 100

<Thread(Thread-1, started 123145367715840)> 100

<Thread(Thread-3, started 123145378226176)> 100

<Thread(Thread-5, started 123145388736512)> 100

<Thread(Thread-7, started 123145399246848)> 100

<Thread(Thread-4, started 123145383481344)> 100

<Thread(Thread-10, started 123145415012352)> 100

<Thread(Thread-8, started 123145404502016)> 100

<Thread(Thread-9, started 123145409757184)> 100

全局变量：

使用global:

import threading

import time

x = 0

def worker():

    global x

    x = 0

    for i in range(100):

        time.sleep(0.0001)

        x += 1

    print(threading.current_thread(),x)

for i in range(10):

    threading.Thread(target=worker).start()

运行结果：

<Thread(Thread-2, started 123145483571200)> 888

<Thread(Thread-5, started 123145499336704)> 908

<Thread(Thread-3, started 123145488826368)> 930

<Thread(Thread-4, started 123145494081536)> 937

<Thread(Thread-1, started 123145478316032)> 941

<Thread(Thread-6, started 123145504591872)> 947

<Thread(Thread-7, started 123145509847040)> 949

<Thread(Thread-8, started 123145515102208)> 955

<Thread(Thread-9, started 123145520357376)> 962

<Thread(Thread-10, started 123145525612544)> 964

上面例子中当主线程中x是全局变量时，就变成了公共资源(也就是同一个对象)，每个子线程互相干扰，最终导致错误的计算结果。

Python提供了 threading.local 类，将这个类实例化得到一个全局对象，但是不同的线程使用这个对象存储的数据其它线程不可见(本质上就是不同的线程使用这个对象时为其创建一个独立的字典)。

使用threading.local() :

import threading

import time

# class A:

#     def __init__(self,x):

#         self.x = x

# a = A(0)

a = threading.local()#全局对象

def worker():

    a.x = 0

    for i in range(100):

        time.sleep(0.0001)

        a.x += 1

    print(threading.current_thread(),a.x)

for i in range(10):

    threading.Thread(target=worker).start()

运行结果：

<Thread(Thread-4, started 123145570172928)> 100

<Thread(Thread-6, started 123145580683264)> 100

<Thread(Thread-1, started 123145554407424)> 100

<Thread(Thread-2, started 123145559662592)> 100

<Thread(Thread-8, started 123145591193600)> 100

<Thread(Thread-5, started 123145575428096)> 100

<Thread(Thread-3, started 123145564917760)> 100

<Thread(Thread-7, started 123145585938432)> 100

<Thread(Thread-10, started 123145601703936)> 100

<Thread(Thread-9, started 123145596448768)> 100

每个子线程使用全局对象a，但每个线程定义的属性a.x是该线程独有的。

举一个错误的例子:，主线程中使用threading.local定义本地变量x，x在主线程中是独有的，子线程中就访问不到主线程的x的属性。

import threading

X='abc'

ctx=threading.local()

ctx.x=123 #主线程中定义x本地属性

print(ctx,type(ctx),ctx.x)

def work():

    print(X)

    print(ctx)

    print(ctx.x) #子线程访问不到

    print('Good job')

threading.Thread(target=work).start()

运行结果：

<_thread._local object at 0x10407bd00> <class '_thread._local'> 123

abc

<_thread._local object at 0x10407bd00>

Exception in thread Thread-1:

Traceback (most recent call last):

  File "/Users/ihoney/Python/test_4.py", line 12, in work

    print(ctx.x)

AttributeError: '_thread._local' object has no attribute 'x'

ctx全局对象对主线程和子线程都是可以使用的，主线程定义了属性x，但子线程在尝试访问属性x时，就相当于访问自己线程内的属性x，而自己线程并没有定义，就会抛出AttributeError异常：'_thread._local' object has no attribute 'x'。