Python Multiprocessing 不允许在创建进程后在主进程中打印

Question

我试图确保在开始流程之前正确设置了此代码。添加一些打印语句后，我发现只有 'outer' 和 'inner' 正在打印，我不明白为什么其他打印语句没有执行。

import multiprocessing
from itertools import product

retailer_ids = [41, 499]  # defined retailers
product_ids = [4, 5, 10, 11, 12]  # ProductIDs to search on
NUMBER_OF_PROCESSES = 2

retailer_products = list(product(retailer_ids, product_ids))

# Start processing the retailer/product combinations
for i in range(0, len(retailer_products), NUMBER_OF_PROCESSES):
    print('outer')
    try:
        current_processes = []
        for j in range(0, NUMBER_OF_PROCESSES):
            print('inner')
            process = multiprocessing.Process(scrape_retailer_product, retailer_products[i+j])
            #process.start()
            current_processes.append(process)
        # wait for current process to finish before starting more
        print('waiting for processes to complete')
        for p in current_processes:
            p.join()

        print('completed')

    # something bad happened during process creation or a
    # a scrape process returned with an exception it could not handle
    except Exception as e:
        for p in current_processes:
            p.terminate()
            print('term')
            exit()

Answer 1

问题是您正在捕获所有异常。因此，您的代码没有将正确的参数传递给 Process 构造函数（生成 AssertionError），但您的 catch 语句正在静默处理异常。

当前异常是：

Traceback (most recent call last):
  File "C:\Users\MiguelAngel\Downloads\test.py", line 19, in <module>
    process = multiprocessing.Process(scrape_retailer_product, args=(retailer_products[i+j]))
  File "C:\Users\MiguelAngel\AppData\Local\Programs\Python\Python38-32\lib\multiprocessing\process.py", line 82, in __init__
    assert group is None, 'group argument must be None for now'
AssertionError: group argument must be None for now

我想scrape_retailer_product是应该在新进程中执行的功能。因此，根据documentation，对构造函数的调用应该是：

process = multiprocessing.Process(target=scrape_retailer_product, 
                                  args=(retailer_products[i+j],))

如果你想捕获所有多处理异常，你应该捕获 multiprocessing.ProcessError。根据 documentation，它是所有多处理异常的基础 class。

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