http://flame-blaze.net/archives/3341 を少し改変したやつ

text_edit.py

# -*- coding: utf-8 -*-
import sys
import subprocess
import time

from PySide import QtCore, QtGui
from PySide.QtUiTools import QUiLoader


def load_ui(widget, ui_file):
    loader = QUiLoader()
    try:
        f = QtCore.QFile(ui_file)
        f.open(QtCore.QFile.ReadOnly)
        ui = loader.load(f, widget)
    finally:
        f.close()
    return ui


class TextEdit(QtGui.QWidget):
    ui_file = 'textEdit.ui'

    def __init__(self, parent=None):
        super(TextEdit, self).__init__(parent)
        self.ui = load_ui(self, self.ui_file)

        layout = QtGui.QVBoxLayout()
        layout.addWidget(self.ui)
        self.setLayout(layout)

        self.cursor = self.ui.textEdit.textCursor()

        self.ui.pushButton.clicked.connect(self.start)

    def start(self):
        p = subprocess.Popen(['HELP'])
        while not p.poll():
            time.sleep(1)
        self.cursor.movePosition(QtGui.QTextCursor.End)
        self.cursor.insertText(p.stdout())


def main():
    app = QtGui.QApplication(sys.argv)

    # 日本語使用時に入れておくもの
    QtCore.QTextCodec.setCodecForCStrings(QtCore.QTextCodec.codecForLocale())

    window = TextEdit()
    window.show()
    return app.exec_()


if __name__ == '__main__':
    sys.exit(main())