[Python 多線程] threading.local類 (六)

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在使用threading.local()之前，先瞭解一下局部變數和全域變數。

 

局部變數：

import threadingimport timedef 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()運行結果：<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

　　上面例子使用多線程，每個子線程完成不同的計算任務，x是局部變數。

每個子線程都要壓棧，每個棧是獨立的空間。每次壓棧，局部變數x的範圍地址是不同的(線程獨享)，計算結果互不干擾。

 

全域變數：

使用global:

import threadingimport timex = 0def 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()運行結果：<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

　　上面例子中當主線程中x是全域變數時，就變成了公用資源(也就是同一個對象)，每個子線程互相干擾，最終導致錯誤的計算結果。

 

Python提供了 threading.local 類，將這個類執行個體化得到一個全域對象，但是不同的線程使用這個Object Storage Service的資料其它線程不可見(本質上就是不同的線程使用這個對象時為其建立一個獨立的字典)。

 

使用threading.local() :

import threadingimport time# class A:#     def __init__(self,x):#         self.x = x# a = A(0)a = threading.local()#全域對象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()運行結果：<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

　　每個子線程使用全域對象a，但每個線程定義的屬性a.x是該線程專屬的。

 

舉一個錯誤的例子:，主線程中使用threading.local定義本地變數x，x在主線程中是專屬的，子線程中就訪問不到主線程的x的屬性。

import threadingX=‘abc‘ctx=threading.local()ctx.x=123 #主線程中定義x區域屬性print(ctx,type(ctx),ctx.x)def work():    print(X)    print(ctx)    print(ctx.x) #子線程訪問不到    print(‘Good job‘)threading.Thread(target=work).start()運行結果：<_thread._local object at 0x10407bd00> <class ‘_thread._local‘> 123abc<_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全域對象對主線程和子線程都是可以使用的，主線程定義了屬性x，但子線程在嘗試訪問屬性x時，就相當於訪問自己線程內的屬性x，而自己線程並沒有定義，就會拋出AttributeError異常：‘_thread._local‘ object has no attribute ‘x‘。

 

[Python 多線程] threading.local類 (六)