PyQt program running from command line not routing the console output in real time

Question

Hi I am a newbie in PyQt. Can someone suggest why the below program is not populating the stdout of print.py in real time to the textbox widget.This problem exists when it is running from command line.If we are running the samecode in pycharm we will get realtime outputs. Command line execution will show the data as a chunk(approx 4K) to the text box.

from PyQt4 import QtGui,QtCore
import sys

class gui(QtGui.QMainWindow):
    def __init__(self):
        super(gui, self).__init__()
        self.initUI()

    def dataReady(self):
        cursor = self.output.textCursor()
        cursor.movePosition(cursor.End)
        cursor.insertText(str(self.process.readAll()))
        self.output.ensureCursorVisible()

    def initUI(self):
        # Layout are better for placing widgets
        layout = QtGui.QHBoxLayout()
        self.output = QtGui.QTextEdit()

        layout.addWidget(self.output)

        centralWidget = QtGui.QWidget()
        centralWidget.setLayout(layout)
        self.setCentralWidget(centralWidget)

        # QProcess object for external app
        self.process = QtCore.QProcess(self)
        # QProcess emits `readyRead` when there is data to be read
        self.process.readyRead.connect(self.dataReady)

        # Run program
        self.process.start('python', ['./print.py'])

#Function Main Start
def main():
    app = QtGui.QApplication(sys.argv)
    ui=gui()
    ui.show()
    sys.exit(app.exec_())
#Function Main END

if __name__ == '__main__':
    main()

Here is the code for print.py

import time
n=10
while(n):
    print "Hello world"
    time.sleep(1)
    n = n-1

Answer 1

You should explicitly flush the output buffer of the print.py script (the child script) running in the subprocess. The default behaviour of print is to send the output to standard out (stdout). As such, you should explicitly call sys.sydout.flush() in the child script, after each print statement, to ensure the content of the print statements from the child script are made available to the parent script in a timely manner.