Python 3:多处理，EOFError:读取一行时的EOF [英] Python 3: multiprocessing, EOFError: EOF when reading a line

问题描述

我希望有一个进程持续监视 RPi 输入，并将变量(我选择了一个队列)设置为 True 或 False 以反映去抖动值.然后另一个进程将捕获图像(从流中).我写了一些代码只是为了检查我可以让多处理和信号(队列)工作正常(我是一个业余编码员......).

I wish to have a process continually monitoring RPi input, and set a variable (I have chosen a queue) to True or False to reflect the debounced value. Another process will then capture an image (from a stream). I have written some code just to check I can get multiprocessing and signalling (the queue) working ok (I'm an amature coder...).

线程处理一切正常，但多处理会出现奇怪的错误.特别是多处理，EOFError:读取一行时的EOF".代码输出:-

It all works fine with threading, but multiprocessing is giving an odd error. Specifically 'multiprocessing, EOFError: EOF when reading a line'. Code outputs:-

this computer has the following number of CPU's 6
OK, started thread on separate processor, now we monitor variable
enter something, True is the key word:
Process Process-1:
Traceback (most recent call last):
  File "c:\Python34\lib\multiprocessing\process.py", line 254, in _bootstrap
    self.run()
  File "c:\Python34\lib\multiprocessing\process.py", line 93, in run
    self._target(*self._args, **self._kwargs)
  File "C:\Users\Peter\Documents\NetBeansProjects\test_area\src\test4.py", line 16, in Wait4InputIsTrue
    ValueIs = input("enter something, True is the key word: ")
EOFError: EOF when reading a line

此模块监控端口"(我使用键盘作为输入):

#test4.py
from time import sleep
from multiprocessing import Lock

def Wait4InputIsTrue(TheVar, TheLock):
    while True:
        sleep(0.2)
        TheLock.acquire()
        #try:
        ValueIs = input("enter something, True is the key word: ")
        #except:
        #    ValueIs = False
        if ValueIs == "True":
            TheVar.put(True)
            print("changed TheVar to True")
        TheLock.release()

此模块监控状态，并对其采取行动:

#test5.py
if __name__ == "__main__":
    from multiprocessing import Process, Queue, Lock, cpu_count
    from time import sleep
    from test4 import Wait4InputIsTrue

    print("this computer has the following number of CPU's", cpu_count())
    LockIt = Lock()
    IsItTrue = Queue(maxsize = 3)

    Wait4 = Process(target = Wait4InputIsTrue, args = (IsItTrue, LockIt))
    Wait4.start()

    print("OK, started thread on separate processor, now we monitor variable")

    while True:
        if IsItTrue.qsize():
            sleep(0.1)
            print("received input from separate thread:", IsItTrue.get())

请注意，我已经尝试在 test4.py 中的输入语句中添加一个 try:，在这种情况下，它会一直打印输入内容，True 是关键字:"无限期地，没有 cr.

Note that I have tried adding a try: to the input statement in test4.py, in which case it keeps printing "enter something, True is the key word: " indefinitely, without a cr.

我疯狂地添加了 Lock 来修复它，没有任何区别

I added Lock in wild attempt to fix it, makes no difference

有人知道为什么会这样吗?

Anyone any idea why this is happening?

推荐答案

您的问题可以归结为一个更简单的脚本:

Your problem can be boiled down to a simpler script:

import multiprocessing as mp
import sys

def worker():
    print("Got", repr(sys.stdin.read(1)))

if __name__ == "__main__":
    process = mp.Process(target=worker)
    process.start()
    process.join()

运行时，它产生

$ python3 i.py
Got ''

读取零字节意味着管道已关闭，input(..) 将其转换为 EOFError 异常.

Reading zero bytes means the pipe is closed and input(..) turns that into an EOFError exception.

multiprocessing 模块不允许您阅读 stdin.这通常是有道理的，因为混合来自多个孩子的 stdin 阅读器是一项冒险的工作.实际上，深入研究实现，multiprocessing/process.py 显式地将 stdin 设置为 devnull:

The multiprocessing module doesn't let you read stdin. That makes sense generally because mixing stdin readers from multiple children is a risky business. In fact, digging into the implementation, multiprocessing/process.py explicitly sets stdin to devnull:

                sys.stdin.close()
                sys.stdin = open(os.devnull)

如果您只是使用 stdin 进行测试，那么解决方案很简单:不要那样做！如果你真的需要用户输入，生活就会困难得多.您可以在父级中使用额外的队列和代码来提示用户并获取输入.

If you are just using stdin for test, then the solution is simple: Don't do that! If you really need user input, life is quite a bit more difficult. You can use additional queues plus code in the parent to prompt users and get input.

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