Python多處理：在函數中調用pool.map

Question

我試圖使用mutltiprocessing包來使用函數中的多個CPU。當我在一個函數之外運行一個玩具例子時，它會在四分之一秒內運行，沒有任何問題（見下文）。

from multiprocessing import Pool 
import time 

start = time.clock() 

def f(x): 
    return x*x 

if __name__ == '__main__': 
    with Pool(processes=7) as pool:  
     result = pool.map(f, range(1000)) 



print(time.clock() - start) 

然而，當我適應了相同的代碼輸入功能（見下文），它打印True，表明__name__ == '__main__'，但隨後將永遠運行下去，永不返回結果。我在Windows 7上運行Python 3.3。

from multiprocessing import Pool 
import time 

start = time.clock() 

def f(x): 
    return x*x 

def testfunc(r): 
    if __name__ == '__main__': 
     print(True) 
     with Pool(processes=7) as pool:  
      result = pool.map(f, range(r)) 

    return result 

result = testfunc(1000) 
print(time.clock() - start) 

Answer 1

您在錯誤的地方使用if __name__ == '__main__'。

from multiprocessing import Pool 
import time 

start = time.clock() 

def f(x): 
    return x*x 

def testfunc(r): 
    print(True) 
    with Pool(processes=7) as pool:  
     result = pool.map(f, range(r)) 
    return result 

if __name__ == '__main__': 
    result = testfunc(1000) 
    print(time.clock() - start) 

根據multiprocessing - Programming guidelines：

Safe importing of main module

Make sure that the main module can be safely imported by a new Python interpreter without causing unintended side effects (such a starting a new process).

... one should protect the 「entry point」 of the program by using if __name__ == '__main__': as follows: