十二、threading.local

1、线程之间资源共享和threading.local

代码：

from threading import local, Thread


i = None


def func(num):
    global i
    i = num
    print(i)


threads = []
for i in range(10):
    threads.append(Thread(target=func, args=(i, )))

for thread in threads:
    thread.start()

for thread in threads:
    thread.join()

输出：

 由于没有阻塞操作，导致看不出没有给数据加锁会造成什么影响，

加入阻塞操作后的代码：

from threading import local, Thread
from time import sleep


i = None


def func(num):
    global i
    i = num
    sleep(2)
    print(i)


threads = []
for i in range(10):
    threads.append(Thread(target=func, args=(i, )))

for thread in threads:
    thread.start()

for thread in threads:
    thread.join()

输出：

 由于加入阻塞操作后，所有的线程在未输入i之前，争夺资源i，使得每个线程最终输出的i都是最后一次修改的数据，

那么有什么方法，可以使得每次在线程中访问同一个数据，但都是自己的资源，线程之间的数据相互隔离呢？

使用threading.local，

代码：

from threading import local as Local, Thread, RLock
from time import sleep


i = None
local = Local()


def func(num):
    local.x = num
    sleep(0.3)
    print(local.x)


threads = []
for i in range(10):
    threads.append(Thread(target=func, args=(i, )))

for thread in threads:
    thread.start()

for thread in threads:
    thread.join()

输出：

 2、threading.local原理

　　threading.local是一个类，当实例化后，使用点号给对象某个属性赋值时就会调用__setattr__方法，使用点号获取某个属性值时就会调用__getattr__，

我们可以在local对象中设置一个类似于字典的属性，每当要设置某个属性值或者访问某个属性时，就在__setattr__或__getattr__中找到对应线程存储的值（或者说为每个访问环境设置一个唯一标识符，对应访问环境中的值），

函数实现：

from threading import Thread, get_ident
from time import sleep

storage = {}


def set(k, v):
    # 获取当前线程或携程的唯一标识
    ident = get_ident()
    if ident in storage:
        storage[ident][k] = v
    else:
        storage[ident] = {k:v}

def get(k):
    ident = get_ident()
    try:
        res = storage[ident][k]
    except Exception as e:
        return None
    else:
        return res


def func(num):
    set("x", num)
    sleep(2)
    print(get("x"))


threads = []
for i in range(10):
    threads.append(Thread(target=func, args=(i, )))

for thread in threads:
    thread.start()

for thread in threads:
    thread.join()

输出：

 错误的类实现：

from threading import Thread, get_ident
from time import sleep


class Local(object):
    def __init__(self):
        self.storage = {}

    def __setattr__(self, key, value):
        ident = get_ident()
        if ident in self.storage:
            self.storage[ident][key] = value
        else:
            self.storage[ident] = {key: value}

    def __getattr__(self, item):
        ident = get_ident()
        try:
            res = self.storage[ident][item]
        except Exception as e:
            return None
        else:
            return res

local = Local()

def func(num):
    local.x = num
    print(local.x)


threads = []
for i in range(10):
    threads.append(Thread(target=func, args=(i, )))

for thread in threads:
    thread.start()

for thread in threads:
    thread.join()

报错信息：

 原因：

　　不可以直接在初始化对象时，直接将对象设置storage为字典，因为当使用self.storage={}时，就会调用self.__setattr__("storage", {})，而第11行的storage属性还没有被设置，因而为NoneType类型

正确的类实现：

from threading import Thread, get_ident
from time import sleep


class Local(object):
    def __init__(self):
        object.__setattr__(self, "storage", {})

    def __setattr__(self, key, value):
        ident = get_ident()
        if ident in self.storage:
            self.storage[ident][key] = value
        else:
            self.storage[ident] = {key: value}

    def __getattr__(self, item):
        ident = get_ident()
        try:
            res = self.storage[ident][item]
        except Exception as e:
            return None
        else:
            return res

local = Local()

def func(num):
    local.x = num
    print(local.x)


threads = []
for i in range(10):
    threads.append(Thread(target=func, args=(i, )))

for thread in threads:
    thread.start()

for thread in threads:
    thread.join()

输出：