Duplicate stdout, stderr in QTextEdit widget

Question

I've been having difficulty with my PySide program for a few days now. I don't think the problem is incredibly difficult because there are answer out there. Problem I have is none of them seem to work for me.

I want to 'listen' to the file objects stdout and stderr and output the contents to QText Edit widget while my PySide program is running. Now, I already realise this question (or something similar) has been asked before on here but like I said, can't get it to work for me for some reason and most other solutions out there are based on the one that I can't get working, so a very frustrating last few days for me. This solution (OutLog), is included in my code snippet below, just in case one of you guys can see a botched implementation on my part.

Things to remember:

1 I'm doing this on Windows 7(duuuh, da, da, duh)

2 I'm using eclipse and running it from inside the IDE(duh, da, da, duh, DUUUUH: It would be really handy if the suggestions worked with either commandline or an IDE)

3 I really just want to duplicate the output of stdout and stderr to the widget while the program runs. For this to happen line-by-line would be a dream but even if it all comes out as a chunk at the end of a loop or something, that would be fab.

4 Oh, and also regarding OutLog, could somebody tell me how, if self.out is set to 'None' in the init, this class can actually work? I mean, self.out is always a NoneType object, right???

Any help would be appreciated, even if it's just pointers to where I could find more information. I've been trying to build my own solution (I'm a bit of a sadist that way) but I've found it hard to find relevant info on how these objects work to do that.

Anyway, whine over. Here's my code:

#!/usr/bin/env python
import sys
import logging
import system_utilities

log = logging.getLogger()
log.setLevel("DEBUG")
log.addHandler(system_utilities.SystemLogger())

import matplotlib
matplotlib.use("Qt4Agg")
matplotlib.rcParams["backend.qt4"] = "PySide"
import subprocess
import plot_widget

from matplotlib.backends.backend_qt4agg import FigureCanvasQTAgg as FigureCanvas
from matplotlib.figure import Figure

from PySide import QtCore, QtGui


class MainWindow(QtGui.QMainWindow):
    """This is the main window class and displays the primary UI when launched.
    Inherits from QMainWindow.
    """

    def __init__(self):
        """Init function. 
        """
        super(MainWindow, self).__init__()
        self.x = None
        self.y = None
        self.data_plot = None
        self.plot_layout = None
        self.terminal = None
        self.setup_plot()
        self.setup_interface()

    def  setup_plot(self):
        """Member function to setup the graph window in the main UI.
        """

        #Create a PlotWidget object
        self.data_plot = plot_widget.PlotWidget()

        #Create a BoxLayout element to hold PlotWidget
        self.plot_layout = QtGui.QVBoxLayout()
        self.plot_layout.addWidget(self.data_plot)


    def setup_interface(self):
        """Member function to instantiate and build the composite elements of the 
        UI."""

        #Main widget houses layout elements (Layout cannot be placed directly in a QMainWindow).
        central_widget = QtGui.QWidget()
        test_splitter = QtGui.QSplitter(QtCore.Qt.Vertical)
        button_splitter = QtGui.QSplitter(QtCore.Qt.Horizontal)

        #UI BoxLayout elements
        central_layout = QtGui.QVBoxLayout()
        #button_layout = QtGui.QHBoxLayout()

        #UI PushButton elements
        exit_button = QtGui.QPushButton("Close")
        run_button = QtGui.QPushButton("Run...")

        #UI Text output
        self.editor = QtGui.QTextEdit()
        self.editor.setReadOnly(True)
        self.terminal = QtGui.QTextBrowser()
        self.terminal.setReadOnly(True)


        #UI PushButton signals
        run_button.clicked.connect(self.run_c_program)
        run_button.clicked.connect(self.data_plot.redraw_plot)
        exit_button.clicked.connect(QtCore.QCoreApplication.instance().quit)

        #Build the UI from composite elements
        central_layout.addLayout(self.plot_layout)
        central_layout.addWidget(self.editor)
        button_splitter.addWidget(run_button)
        button_splitter.addWidget(exit_button)
        test_splitter.addWidget(button_splitter)
        test_splitter.addWidget(self.terminal)
        test_splitter.setCollapsible(1, True)
        central_layout.addWidget(test_splitter)
        central_widget.setLayout(central_layout)
        self.setCentralWidget(central_widget)


        self.show()

class OutLog:
    def __init__(self, edit, out=None, color=None):
        """(edit, out=None, color=None) -> can write stdout, stderr to a
        QTextEdit.
        edit = QTextEdit
        out = alternate stream ( can be the original sys.stdout )
        color = alternate color (i.e. color stderr a different color)
        """
        self.edit = edit
        self.out = None
        self.color = color

    def write(self, m):
        if self.color:
            tc = self.edit.textColor()
            self.edit.setTextColor(self.color)

        self.edit.moveCursor(QtGui.QTextCursor.End)
        log.debug("this is m {}".format(m))
        self.edit.insertPlainText( m )

        if self.color:
            self.edit.setTextColor(tc)

        if self.out:
            self.out.write(m)




def main():


    app = QtGui.QApplication(sys.argv)

    log.debug("Window starting.")
    window = MainWindow()
    sys.stdout = OutLog(window.terminal, sys.stdout)
    sys.stderr = OutLog(window.terminal, sys.stderr, QtGui.QColor(255,0,0))
    window.show()

    sys.exit(app.exec_())
    log.info("System shutdown.")


if __name__ == '__main__':
    main()

"Help me Obi-Wan..."

Thanks in advance guys (and gals :-))

Answer 1

It seems that all you need to do is override sys.stderr and sys.stdout with a wrapper object that emits a signal whenever output is written.

Below is a demo script that should do more or less what you want. Note that the wrapper class does not restore sys.stdout/sys.stderr from sys.__stdout__/sys.__stderr__, because the latter objects may not be same as the ones that were orignally replaced.

import sys
from PyQt4 import QtGui, QtCore

class OutputWrapper(QtCore.QObject):
    outputWritten = QtCore.pyqtSignal(object, object)

    def __init__(self, parent, stdout=True):
        QtCore.QObject.__init__(self, parent)
        if stdout:
            self._stream = sys.stdout
            sys.stdout = self
        else:
            self._stream = sys.stderr
            sys.stderr = self
        self._stdout = stdout

    def write(self, text):
        self._stream.write(text)
        self.outputWritten.emit(text, self._stdout)

    def __getattr__(self, name):
        return getattr(self._stream, name)

    def __del__(self):
        try:
            if self._stdout:
                sys.stdout = self._stream
            else:
                sys.stderr = self._stream
        except AttributeError:
            pass

class Window(QtGui.QMainWindow):
    def __init__(self):
        QtGui.QMainWindow.__init__(self)
        widget = QtGui.QWidget(self)
        layout = QtGui.QVBoxLayout(widget)
        self.setCentralWidget(widget)
        self.terminal = QtGui.QTextBrowser(self)
        self._err_color = QtCore.Qt.red
        self.button = QtGui.QPushButton('Test', self)
        self.button.clicked.connect(self.handleButton)
        layout.addWidget(self.terminal)
        layout.addWidget(self.button)
        stdout = OutputWrapper(self, True)
        stdout.outputWritten.connect(self.handleOutput)
        stderr = OutputWrapper(self, False)
        stderr.outputWritten.connect(self.handleOutput)

    def handleOutput(self, text, stdout):
        color = self.terminal.textColor()
        self.terminal.setTextColor(color if stdout else self._err_color)
        self.terminal.moveCursor(QtGui.QTextCursor.End)
        self.terminal.insertPlainText(text)
        self.terminal.setTextColor(color)

    def handleButton(self):
        if QtCore.QTime.currentTime().second() % 2:
            print('Printing to stdout...')
        else:
            sys.stderr.write('Printing to stderr...\n')

if __name__ == '__main__':

    app = QtGui.QApplication(sys.argv)
    window = Window()
    window.setGeometry(500, 300, 300, 200)
    window.show()
    sys.exit(app.exec_())

NB:

Instances of the OutputWrapper should be created as early as possible, so as to ensure that other modules that need sys.stdout/sys.stderr (such as the logging module) use the wrapped versions wherever necessary.

Answer 2

self.out = None is probably a typo and should be self.out = out. That way, you can see anything that printed in the console as well. This is the first step to be sure that the code prints anything at all.

The next thing is that you need to realize which output you're redirecting. A subprocess get its own stdio, so no amount of redirection of the parent's stdout is going to have any effect.

Getting stdio correct with a subprocess isn't trivial. I suggest to start with subprocess.communicate() which gives you all the output as a single string. That's usually good enough.