Python 多处理将子进程的标准输出重定向到 Tkinter 文本

Question

我正在尝试使用 Tkinter GUI 来启动子进程并将其 stdout/stderr 输出显示到 Text 小部件。最初，我认为 sys.stdout 可以通过设置“sys.stdout = text_widget”轻松重定向到文本小部件，但似乎不是。报错：“Text instance has no attribute 'flush'”。

我在网上查了一下，得到了一些解决方案，比如使用队列与子进程通信。但是，由于我的特殊要求，它们都不适合我的情况：

1. 子进程最好由“multiprocessing.Process”启动，因为它更容易使用共享变量，这使得子进程解决方案可用。
2. 子进程的代码已经存在，里面有很多“打印”，所以我不想将它们修改为“Queue.put()”之类的东西。

在这种情况下，任何人都可以找到获得“multiprocessing.Process”的“打印”输出并显示到 Tkinter Text 的解决方案吗？非常感谢！

我的案例的示例代码如下：

import sys
import time
from multiprocessing import Process
from Tkinter import *

def test_child():
    print 'child running'

def test_parent():
    print 'parent running'
    time.sleep(0.5)
    Process(target=test_child).start()

def set_txt(msg):
    gui_txt.insert(END, str(msg))
    gui_txt.see(END)

if __name__ == '__main__':
    gui_root = Tk()
    gui_txt = Text(gui_root)
    gui_txt.pack()
    gui_btn = Button(gui_root, text='Test', command=test_parent)
    gui_btn.pack()

    gui_txt.write = set_txt
    sys.stdout = gui_txt

    gui_root.mainloop()

Answer 1

仍然可以使用队列，而不必删除所有 print 语句。您可以使用 Process 依赖 stdout 重定向来执行此操作。下面的解决方案使用Queue 子类来模仿stdout。然后，该队列由一个线程监视，该线程会查找被泵入文本小部件的新文本。

import sys
import time
from multiprocessing import Process
from multiprocessing.queues import Queue
from threading import Thread
from Tkinter import *

# This function takes the text widget and a queue as inputs.
# It functions by waiting on new data entering the queue, when it 
# finds new data it will insert it into the text widget 
def text_catcher(text_widget,queue):
    while True:
        text_widget.insert(END, queue.get())

# This is a Queue that behaves like stdout
class StdoutQueue(Queue):
    def __init__(self,*args,**kwargs):
        Queue.__init__(self,*args,**kwargs)

    def write(self,msg):
        self.put(msg)

    def flush(self):
        sys.__stdout__.flush()


def test_child(q):
    # This line only redirects stdout inside the current process 
    sys.stdout = q
    # or sys.stdout = sys.__stdout__ if you want to print the child to the terminal
    print 'child running'

def test_parent(q):
    # Again this only redirects inside the current (main) process
    # commenting this like out will cause only the child to write to the widget 
    sys.stdout = q                                                                                                                                                                                                                                                         
    print 'parent running'
    time.sleep(0.5)
    Process(target=test_child,args=(q,)).start()

if __name__ == '__main__':
    gui_root = Tk()
    gui_txt = Text(gui_root)
    gui_txt.pack()
    q = StdoutQueue()
    gui_btn = Button(gui_root, text='Test', command=lambda:test_parent(q),)
    gui_btn.pack()

    # Instantiate and start the text monitor
    monitor = Thread(target=text_catcher,args=(gui_txt,q))
    monitor.daemon = True
    monitor.start()

    gui_root.mainloop()

Answer 2

@ebarr 给出的解决方案是正确的。但它不适用于 Python V5 或更高版本。当您尝试对multiprocessing.queues.Queue 类进行子类化时，您将收到以下错误：

C:\Users\..\myFolder > python myTest.py

    Traceback (most recent call last):
        File "myTest.py", line 49, in <module>
          q = StdoutQueue()
        File "myTest.py", line 22, in __init__
          super(StdoutQueue,self).__init__(*args,**kwargs)
    TypeError: __init__() missing 1 required keyword-only argument: 'ctx'

您需要为您的子类队列显式提供“多处理上下文”。

这是更新后的代码：

import sys
import time
import multiprocessing as mp
import multiprocessing.queues as mpq

from threading import Thread
from tkinter import *

'''-------------------------------------------------------------------'''
'''                SUBCLASSING THE MULTIPROCESSING QUEUE              '''
'''                                                                   '''
'''         ..and make it behave as a general stdout io               '''
'''-------------------------------------------------------------------'''
# The StdoutQueue is a Queue that behaves like stdout.
# We will subclass the Queue class from the multiprocessing package
# and give it the typical stdout functions.
#
# (1) First issue
# Subclassing multiprocessing.Queue or multiprocessing.SimpleQueue
# will not work, because these classes are not genuine
# python classes.
# Therefore, you need to subclass multiprocessing.queues.Queue or
# multiprocessing.queues.SimpleQueue . This issue is known, and is not
# the reason for asking this question. But I mention it here, for
# completeness.
#
# (2) Second issue
# There is another problem that arises only in Python V5 (and beyond).
# When subclassing multiprocessing.queues.Queue, you have to provide
# a 'multiprocessing context'. Not doing that, leads to an obscure error
# message, which is in fact the main topic of this question. Darth Kotik
# solved it.
# His solution is visible in this code:
class StdoutQueue(mpq.Queue):

    def __init__(self,*args,**kwargs):
        ctx = mp.get_context()
        super(StdoutQueue, self).__init__(*args, **kwargs, ctx=ctx)

    def write(self,msg):
        self.put(msg)

    def flush(self):
        sys.__stdout__.flush()


'''-------------------------------------------------------------------'''
'''                           TEST SETUP                              '''
'''-------------------------------------------------------------------'''

# This function takes the text widget and a queue as inputs.
# It functions by waiting on new data entering the queue, when it
# finds new data it will insert it into the text widget.
def text_catcher(text_widget,queue):
    while True:
        text_widget.insert(END, queue.get())


def test_child(q):
    # This line only redirects stdout inside the current process
    sys.stdout = q
    # or sys.stdout = sys.__stdout__ if you want to print the child to the terminal
    print('child running')

def test_parent(q):
    # Again this only redirects inside the current (main) process
    # commenting this like out will cause only the child to write to the widget
    sys.stdout = q
    print('parent running')
    time.sleep(0.5)
    mp.Process(target=test_child,args=(q,)).start()

if __name__ == '__main__':
    gui_root = Tk()
    gui_txt = Text(gui_root)
    gui_txt.pack()
    q = StdoutQueue()
    gui_btn = Button(gui_root, text='Test', command=lambda:test_parent(q),)
    gui_btn.pack()

    # Instantiate and start the text monitor
    monitor = Thread(target=text_catcher,args=(gui_txt,q))
    monitor.daemon = True
    monitor.start()

    gui_root.mainloop()

更多详情请参考本主题：Cannot subclass multiprocessing Queue in Python 3.5