#!/usr/bin/env python2.6
# coding: utf-8
import sys
from PyQt4.QtCore import *
from PyQt4.QtGui import *

class Widget(QWidget):
    def __init__(self, parent=None):
        QWidget.__init__(self, parent)
        self.field = QLineEdit(self)
        self.button = QPushButton(self)
        layout = QHBoxLayout(self)
        layout.addWidget(self.field)
        layout.addWidget(self.button)

# ключевая часть начинается здесь
        self.button.clicked.connect(self.selectFile)

    def selectFile(self):
        path = QFileDialog.getOpenFileName()
        self.field.setText(path)
# и заканчивается здесь

if __name__ == '__main__':
    app = QApplication(sys.argv)
    w = Widget()
    w.show()
    sys.exit(app.exec_())

from PyQt4 import QtCore, QtGui

class Ui_Form(object):
    def setupUi(self, Widget):
        Widget.setObjectName("Widget")
        Widget.resize(259, 41)
        self.horizontalLayout = QtGui.QHBoxLayout(Widget)
        self.horizontalLayout.setObjectName("horizontalLayout")
        self.lineEdit = QtGui.QLineEdit(Widget)
        self.lineEdit.setObjectName("lineEdit")
        self.horizontalLayout.addWidget(self.lineEdit)
        self.pushButton = QtGui.QPushButton(Widget)
        self.pushButton.setObjectName("pushButton")
        self.horizontalLayout.addWidget(self.pushButton)

        self.retranslateUi(Widget)
        QtCore.QMetaObject.connectSlotsByName(Widget)

    def retranslateUi(self, Widget):
        Widget.setWindowTitle(QtGui.QApplication.translate("Widget", "Widget", None, QtGui.QApplication.UnicodeUTF8))
        self.pushButton.setText(QtGui.QApplication.translate("Widget", "...", None, QtGui.QApplication.UnicodeUTF8))

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


class UIclass(QWidget, Ui_Form):
    def __init__(self, parent=None):
       QWidget.__init__(self,parent)
       self.setupUi(self)
       
       #self.pushButton.clicked.connect(self.selectFile)
       self.connect(self.pushButton, SIGNAL("clicked()"), SLOT('self.selectFile'))

       def selectFile(self):
           path = QFileDialog.getOpenFileName()
           self.lineEdit.setText(path)

app = QApplication(sys.argv)
w = UIclass()
w.show()
app.exec_()

        self.connect(self.pushButton, SIGNAL("clicked()"), self, SLOT("selectFile()"))

    @pyqtSlot()
    def selectFile(self):
        path = QFileDialog.getOpenFileName()
        self.lineEdit.setText(path)

self.connect(self.pushButton, SIGNAL("clicked()"), self.selectFile)

 # coding: utf-8
import sys, subprocess
from PyQt4.QtCore import *
from PyQt4.QtGui import *


class MainWidget(QWidget):
    def __init__(self, parent=None):
       QWidget.__init__(self,parent)
       self.button = QPushButton("Start_new_process")
       self.textEdit=QTextEdit()

       layout = QVBoxLayout()
       layout.addWidget(self.button)
       layout.addWidget(self.textEdit)
       self.setLayout(layout)

       self.connect(self.button, SIGNAL("clicked()"), SLOT('new_process()'))

    @pyqtSignature('')        #   новое, вместо QtCore.pyqtSlot(),
    def new_process(self): #   принимает аргументом строку
        print 'in SLOT'
        text=self.textEdit.toPlainText().toUtf8()

        # запускаем процесс, stdin запущенного процесса - subprocess.PIPE
        p=subprocess.Popen("/home/mike/slot_py/widget.py", shell=True, stdin=subprocess.PIPE)
        # записываем text в stdin запущенного процесса
        p.communicate(text)

if __name__=='__main__':
    app = QApplication(sys.argv)
    w =MainWidget()
    w.show()
    sys.exit(app.exec_())

# coding: utf-8
import sys
from PyQt4.QtCore import *
from PyQt4.QtGui import *

class Widget(QWidget):
    def __init__(self, parent=None):
       QWidget.__init__(self,parent)
       self.textEdit = QTextEdit()

       text=sys.stdin.read() # читаем станд. ввод
       text=unicode(text, "utf-8") # преобразуем utf-8 в unicode
       self.textEdit.setText(QString(text)) #  text ---> self.textEdit

       layout = QVBoxLayout()
       layout.addWidget(self.textEdit)
       self.setLayout(layout)

if __name__=='__main__':
    app = QApplication(sys.argv)
    w = Widget()
    w.show()
    sys.exit(app.exec_())

p=subprocess.Popen("dd --list", shell=True, stdout=subprocess.PIPE)
    rez = p.communicate()
    print (rez)

>>>(b'', None)

Python 2.6 (r26:66714, Dec  3 2008, 06:05:48) 
[GCC 4.3.2 [gcc-4_3-branch revision 141291]] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys, subprocess                                            
>>> p=subprocess.Popen("dd --help", shell=True, stdout=subprocess.PIPE)
>>> result=p.communicate()
>>> print result[0]       
Использование: dd [ОПЕРАНД]...
       или:    dd КЛЮЧ        
Копирует файл, преобразует и форматирует в зависимости от операндов.
...