Redirecting stdout and stderr to a PyQt4 QTextEdit from a secondary thread

Firstly, +1 for realising how thread-unsafe many of the examples on stack overflow are!

The solution is to use a thread-safe object (like a Python Queue.Queue) to mediate the transfer of information. I’ve attached some sample code below which redirects stdout to a Python Queue. This Queue is read by a QThread, which emits the contents to the main thread through Qt’s signal/slot mechanism (emitting signals is thread-safe). The main thread then writes the text to a text edit.

Hope that is clear, feel free to ask questions if it is not!

EDIT: Note that the code example provided doesn’t clean up QThreads nicely, so you’ll get warnings printed when you quit. I’ll leave it to you to extend to your use case and clean up the thread(s)

import sys
from Queue import Queue
from PyQt4.QtCore import *
from PyQt4.QtGui import *

# The new Stream Object which replaces the default stream associated with sys.stdout
# This object just puts data in a queue!
class WriteStream(object):
    def __init__(self,queue):
        self.queue = queue

    def write(self, text):
        self.queue.put(text)

# A QObject (to be run in a QThread) which sits waiting for data to come through a Queue.Queue().
# It blocks until data is available, and one it has got something from the queue, it sends
# it to the "MainThread" by emitting a Qt Signal 
class MyReceiver(QObject):
    mysignal = pyqtSignal(str)

    def __init__(self,queue,*args,**kwargs):
        QObject.__init__(self,*args,**kwargs)
        self.queue = queue

    @pyqtSlot()
    def run(self):
        while True:
            text = self.queue.get()
            self.mysignal.emit(text)

# An example QObject (to be run in a QThread) which outputs information with print
class LongRunningThing(QObject):
    @pyqtSlot()
    def run(self):
        for i in range(1000):
            print i

# An Example application QWidget containing the textedit to redirect stdout to
class MyApp(QWidget):
    def __init__(self,*args,**kwargs):
        QWidget.__init__(self,*args,**kwargs)

        self.layout = QVBoxLayout(self)
        self.textedit = QTextEdit()
        self.button = QPushButton('start long running thread')
        self.button.clicked.connect(self.start_thread)
        self.layout.addWidget(self.textedit)
        self.layout.addWidget(self.button)

    @pyqtSlot(str)
    def append_text(self,text):
        self.textedit.moveCursor(QTextCursor.End)
        self.textedit.insertPlainText( text )

    @pyqtSlot()
    def start_thread(self):
        self.thread = QThread()
        self.long_running_thing = LongRunningThing()
        self.long_running_thing.moveToThread(self.thread)
        self.thread.started.connect(self.long_running_thing.run)
        self.thread.start()

# Create Queue and redirect sys.stdout to this queue
queue = Queue()
sys.stdout = WriteStream(queue)

# Create QApplication and QWidget
qapp = QApplication(sys.argv)  
app = MyApp()
app.show()

# Create thread that will listen on the other end of the queue, and send the text to the textedit in our application
thread = QThread()
my_receiver = MyReceiver(queue)
my_receiver.mysignal.connect(app.append_text)
my_receiver.moveToThread(thread)
thread.started.connect(my_receiver.run)
thread.start()

qapp.exec_()