Develop Reference

PYQT5: Is it possible in python to send back a value from a QDialog without closing it?

Is it possible for a QDialog to send a value back to the application without also closing the dialog window? A common example would be the insert symbol dialog in Microsoft Word. (In fact, this is what I am trying to replicate.)
I have included some basic code for a main window with a label and a button. Pushing the button opens the dialog window. The dialog window contains a textLineEdit widget, an OK button, and a Cancel button. After pushing OK any text entered into the dialog's textLineEdit is set as the text in the main window's label. Would it be possible to return the entered text without closing the dialog? I know it would require some on_OK_clicked procedure in the Dialog class, but I cannot seem to return a value without also closing the dialog. Any ideas would be greatly appreciated.
import sys
from PyQt5 import QtCore, QtWidgets
class Ui_MainWindow(object):
def setupUi(self, MainWindow):
MainWindow.setObjectName("MainWindow")
MainWindow.resize(320, 86)
self.centralwidget = QtWidgets.QWidget(MainWindow)
self.centralwidget.setObjectName("centralwidget")
self.verticalLayout = QtWidgets.QVBoxLayout(self.centralwidget)
self.verticalLayout.setObjectName("verticalLayout")
self.label = QtWidgets.QLabel(self.centralwidget)
self.label.setObjectName("label")
self.verticalLayout.addWidget(self.label)
spacerItem = QtWidgets.QSpacerItem(20, 13, QtWidgets.QSizePolicy.Minimum, QtWidgets.QSizePolicy.Expanding)
self.verticalLayout.addItem(spacerItem)
self.pushButton = QtWidgets.QPushButton(self.centralwidget)
self.pushButton.setObjectName("pushButton")
self.verticalLayout.addWidget(self.pushButton)
MainWindow.setCentralWidget(self.centralwidget)
self.retranslateUi(MainWindow)
QtCore.QMetaObject.connectSlotsByName(MainWindow)
def retranslateUi(self, MainWindow):
_translate = QtCore.QCoreApplication.translate
MainWindow.setWindowTitle(_translate("MainWindow", "MainWindow"))
self.label.setText(_translate("MainWindow", "TextLabel"))
self.pushButton.setText(_translate("MainWindow", "Open Dialog"))
class Ui_Dialog(object):
def setupUi(self, Dialog):
Dialog.setObjectName("Dialog")
Dialog.resize(400, 94)
self.lineEdit = QtWidgets.QLineEdit(Dialog)
self.lineEdit.setGeometry(QtCore.QRect(20, 20, 361, 22))
self.lineEdit.setObjectName("lineEdit")
self.buttonBox = QtWidgets.QDialogButtonBox(Dialog)
self.buttonBox.setGeometry(QtCore.QRect(40, 50, 341, 32))
self.buttonBox.setOrientation(QtCore.Qt.Horizontal)
self.buttonBox.setStandardButtons(QtWidgets.QDialogButtonBox.Cancel|QtWidgets.QDialogButtonBox.Ok)
self.buttonBox.setObjectName("buttonBox")
self.retranslateUi(Dialog)
self.buttonBox.accepted.connect(Dialog.accept) # type: ignore
self.buttonBox.rejected.connect(Dialog.reject) # type: ignore
QtCore.QMetaObject.connectSlotsByName(Dialog)
def retranslateUi(self, Dialog):
_translate = QtCore.QCoreApplication.translate
Dialog.setWindowTitle(_translate("Dialog", "Dialog"))
class Dialog(QtWidgets.QDialog, Ui_Dialog):
def __init__(self, parent=None):
QtWidgets.QDialog.__init__(self, parent)
self.setupUi(self)
class MainWindow(QtWidgets.QMainWindow, Ui_MainWindow):
def __init__(self, parent=None):
QtWidgets.QMainWindow.__init__(self, parent)
self.setupUi(self)
self.pushButton.clicked.connect(self.onClicked)
def onClicked(self):
dlg = Dialog()
if dlg.exec_():
self.label.setText(dlg.lineEdit.text())
if __name__ == '__main__':
app = QtWidgets.QApplication(sys.argv)
w = MainWindow()
w.show()
sys.exit(app.exec_())

Thanks to ekhumoro for the help I needed. The solution was to use an Apply button in the QDialogButtonBox and to connect the click action to a slot in the main window. In this case, I found I had to use a lambda function as the slot. (This is the last piece I was missing.) dlg.buttonBox.button(QtWidgets.QDialogButtonBox.Apply).clicked.connect(lambda: self.label.setText(dlg.lineEdit.text()))
Here is the full working code.
import sys
from PyQt5 import QtCore, QtWidgets
class Ui_MainWindow(object):
def setupUi(self, MainWindow):
MainWindow.setObjectName("MainWindow")
MainWindow.resize(320, 86)
self.centralwidget = QtWidgets.QWidget(MainWindow)
self.centralwidget.setObjectName("centralwidget")
self.verticalLayout = QtWidgets.QVBoxLayout(self.centralwidget)
self.verticalLayout.setObjectName("verticalLayout")
self.label = QtWidgets.QLabel(self.centralwidget)
self.label.setObjectName("label")
self.verticalLayout.addWidget(self.label)
spacerItem = QtWidgets.QSpacerItem(20, 13, QtWidgets.QSizePolicy.Minimum, QtWidgets.QSizePolicy.Expanding)
self.verticalLayout.addItem(spacerItem)
self.pushButton = QtWidgets.QPushButton(self.centralwidget)
self.pushButton.setObjectName("pushButton")
self.verticalLayout.addWidget(self.pushButton)
MainWindow.setCentralWidget(self.centralwidget)
self.retranslateUi(MainWindow)
QtCore.QMetaObject.connectSlotsByName(MainWindow)
def retranslateUi(self, MainWindow):
_translate = QtCore.QCoreApplication.translate
MainWindow.setWindowTitle(_translate("MainWindow", "MainWindow"))
self.label.setText(_translate("MainWindow", "TextLabel"))
self.pushButton.setText(_translate("MainWindow", "Open Dialog"))
class Ui_Dialog(object):
def setupUi(self, Dialog):
Dialog.setObjectName("Dialog")
Dialog.resize(400, 94)
self.lineEdit = QtWidgets.QLineEdit(Dialog)
self.lineEdit.setGeometry(QtCore.QRect(20, 20, 361, 22))
self.lineEdit.setObjectName("lineEdit")
self.buttonBox = QtWidgets.QDialogButtonBox(Dialog)
self.buttonBox.setGeometry(QtCore.QRect(40, 50, 341, 32))
self.buttonBox.setOrientation(QtCore.Qt.Horizontal)
self.buttonBox.setStandardButtons(QtWidgets.QDialogButtonBox.Close|QtWidgets.QDialogButtonBox.Apply)
self.buttonBox.button(QtWidgets.QDialogButtonBox.Apply).setText("Insert")
self.buttonBox.setObjectName("buttonBox")
self.retranslateUi(Dialog)
self.buttonBox.rejected.connect(Dialog.reject) # type: ignore
QtCore.QMetaObject.connectSlotsByName(Dialog)
def retranslateUi(self, Dialog):
_translate = QtCore.QCoreApplication.translate
Dialog.setWindowTitle(_translate("Dialog", "Dialog"))
class Dialog(QtWidgets.QDialog, Ui_Dialog):
def __init__(self, parent=None):
QtWidgets.QDialog.__init__(self, parent)
self.setupUi(self)
class MainWindow(QtWidgets.QMainWindow, Ui_MainWindow):
def __init__(self, parent=None):
QtWidgets.QMainWindow.__init__(self, parent)
self.setupUi(self)
self.pushButton.clicked.connect(self.onClicked)
def onClicked(self):
dlg = Dialog()
dlg.buttonBox.button(QtWidgets.QDialogButtonBox.Apply).clicked.connect(lambda: self.label.setText(dlg.lineEdit.text()))
dlg.exec()
if __name__ == '__main__':
app = QtWidgets.QApplication(sys.argv)
w = MainWindow()
w.show()
sys.exit(app.exec_())

Start a QT Thread when button is clicked

This is a sample code with just a push button and a textedit, i need to write line x line using a loop in a textedit when button is clicked.
The problem is that i need to implement threads because without them the program crashes or it doesn't write line x line, but all together when the loop ends;
and i need to implement in my application a textedit that shows info while the program loads "something".
So my question is, how to rewrite this code using threads to write in textedit when button is clicked?
The method i should run when the button is clicked is "write"
it should work like a print inside a for loop , it would print not all at once
from PyQt5 import QtCore, QtGui, QtWidgets
class Ui_MainWindow(object):
def setupUi(self, MainWindow):
MainWindow.setObjectName("MainWindow")
MainWindow.resize(640, 480)
self.centralwidget = QtWidgets.QWidget(MainWindow)
self.centralwidget.setObjectName("centralwidget")
self.textEdit = QtWidgets.QTextEdit(self.centralwidget)
self.textEdit.setGeometry(QtCore.QRect(150, 220, 321, 71))
self.textEdit.setObjectName("textEdit")
self.pushButton = QtWidgets.QPushButton(self.centralwidget)
self.pushButton.setGeometry(QtCore.QRect(240, 100, 75, 23))
self.pushButton.setObjectName("pushButton")
MainWindow.setCentralWidget(self.centralwidget)
self.menubar = QtWidgets.QMenuBar(MainWindow)
self.menubar.setGeometry(QtCore.QRect(0, 0, 640, 21))
self.menubar.setObjectName("menubar")
MainWindow.setMenuBar(self.menubar)
self.statusbar = QtWidgets.QStatusBar(MainWindow)
self.statusbar.setObjectName("statusbar")
MainWindow.setStatusBar(self.statusbar)
self.retranslateUi(MainWindow)
QtCore.QMetaObject.connectSlotsByName(MainWindow)
def retranslateUi(self, MainWindow):
_translate = QtCore.QCoreApplication.translate
MainWindow.setWindowTitle(_translate("MainWindow", "MainWindow"))
self.pushButton.setText(_translate("MainWindow", "PushButton"))
self.pushButton.clicked.connect(self.write)
def write(self):
string=""
for i in range(1000):
string="\n"+str(i)
self.textEdit.insertPlainText(string)
import sys
app = QtWidgets.QApplication(sys.argv)
MainWindow = QtWidgets.QMainWindow()
ui = Ui_MainWindow()
ui.setupUi(MainWindow)
MainWindow.show()
sys.exit(app.exec_())

First of all it is recommended not to modify the class generated by Qt Designer so you must regenerate the file and assume that it is called mainwindow.py: pyuic5 your_file.ui -o mainwindow.py -x.
Considering the above, the logic is to generate the information in another thread and send it to the GUI to modify it:
main.py
import threading
from PyQt5 import QtCore, QtGui, QtWidgets
from mainwindow import Ui_MainWindow
class WriterWorker(QtCore.QObject):
textChanged = QtCore.pyqtSignal(str)
def start(self):
threading.Thread(target=self._execute, daemon=True).start()
def _execute(self):
string = ""
for i in range(1000):
string = "\n"+str(i)
self.textChanged.emit(string)
class MainWindow(QtWidgets.QMainWindow, Ui_MainWindow):
def __init__(self, parent=None):
super(MainWindow, self).__init__(parent)
self.setupUi(self)
self.writer_worker = WriterWorker()
self.writer_worker.textChanged.connect(self.on_text_changed)
self.pushButton.clicked.connect(self.writer_worker.start)
#QtCore.pyqtSlot(str)
def on_text_changed(self, text):
self.textEdit.insertPlainText(text)
if __name__ == "__main__":
import sys
app = QtWidgets.QApplication(sys.argv)
w = MainWindow()
w.show()
sys.exit(app.exec_())

How to passing value from main to dialog

I'm newbie for using Pyqt5.
I use qtdesigner for building GUI.
I have MainWindow for passing value to dialogwindow
I want to LineEdit( in dialogwindow) show value after user input and click button (in MainWindow)
I try
self.ui = Ui_Dialog(self,data)
but it doesn't work
My code mainpage
MainWindow.py
from PyQt5 import QtCore, QtGui, QtWidgets
from dialog import Ui_Dialog
class Ui_MainWindow(object):
def openDialog(self):
data = self.lineEdit.text()
self.window = QtWidgets.QDialog()
self.ui = Ui_Dialog(self,data)
self.ui.setupUi(self.window)
# MainWindow.hide()
self.window.show()
def setupUi(self, MainWindow):
MainWindow.setObjectName("MainWindow")
MainWindow.resize(505, 236)
self.centralwidget = QtWidgets.QWidget(MainWindow)
self.centralwidget.setObjectName("centralwidget")
self.label = QtWidgets.QLabel(self.centralwidget)
self.label.setGeometry(QtCore.QRect(120, 40, 91, 16))
self.label.setObjectName("label")
self.lineEdit = QtWidgets.QLineEdit(self.centralwidget)
self.lineEdit.setGeometry(QtCore.QRect(290, 40, 113, 22))
self.lineEdit.setObjectName("lineEdit")
self.pushButton = QtWidgets.QPushButton(self.centralwidget)
self.pushButton.setGeometry(QtCore.QRect(190, 110, 93, 28))
self.pushButton.clicked.connect(self.openDialog)
self.pushButton.setObjectName("pushButton")
MainWindow.setCentralWidget(self.centralwidget)
self.menubar = QtWidgets.QMenuBar(MainWindow)
self.menubar.setGeometry(QtCore.QRect(0, 0, 505, 26))
self.menubar.setObjectName("menubar")
MainWindow.setMenuBar(self.menubar)
self.statusbar = QtWidgets.QStatusBar(MainWindow)
self.statusbar.setObjectName("statusbar")
MainWindow.setStatusBar(self.statusbar)
self.retranslateUi(MainWindow)
QtCore.QMetaObject.connectSlotsByName(MainWindow)
def retranslateUi(self, MainWindow):
_translate = QtCore.QCoreApplication.translate
MainWindow.setWindowTitle(_translate("MainWindow", "MainWindow"))
self.label.setText(_translate("MainWindow", "Passing Value"))
self.pushButton.setText(_translate("MainWindow", "Send"))
if __name__ == "__main__":
import sys
app = QtWidgets.QApplication(sys.argv)
MainWindow = QtWidgets.QMainWindow()
ui = Ui_MainWindow()
ui.setupUi(MainWindow)
MainWindow.show()
sys.exit(app.exec_())
Dialog code.py
from PyQt5 import QtCore, QtGui, QtWidgets
class Ui_Dialog(object):
def setupUi(self, Dialog):
Dialog.setObjectName("Dialog")
Dialog.resize(577, 253)
self.lineEdit = QtWidgets.QLineEdit(Dialog)
self.lineEdit.setGeometry(QtCore.QRect(260, 100, 113, 22))
self.lineEdit.setObjectName("lineEdit")
self.label = QtWidgets.QLabel(Dialog)
self.label.setGeometry(QtCore.QRect(180, 100, 55, 16))
self.label.setObjectName("label")
self.retranslateUi(Dialog)
QtCore.QMetaObject.connectSlotsByName(Dialog)
def retranslateUi(self, Dialog):
_translate = QtCore.QCoreApplication.translate
Dialog.setWindowTitle(_translate("Dialog", "Dialog"))
self.label.setText(_translate("Dialog", "Value"))
if __name__ == "__main__":
import sys
app = QtWidgets.QApplication(sys.argv)
Dialog = QtWidgets.QDialog()
ui = Ui_Dialog()
ui.setupUi(Dialog)
Dialog.show()
sys.exit(app.exec_())
advice me plz
Thank you,
Mint

PyQt recommends not modifying the .py generated by pyuic and Qt Designer but creating another file that uses that class to fill a widget so I recommend regenerating the MainWindow.py and dialog.py files.
Now create a main.py where we inherit the appropriate class by setting a constructor with the requirement:
main.py
from PyQt5 import QtCore, QtGui, QtWidgets
from dialog import Ui_Dialog
from MainWindow import Ui_MainWindow
class Dialog(QtWidgets.QDialog, Ui_Dialog):
def __init__(self, text, parent=None):
super(Dialog, self).__init__(parent)
self.setupUi(self)
self.lineEdit.setText(text)
class MainWindow(QtWidgets.QMainWindow, Ui_MainWindow):
def __init__(self, parent=None):
super(MainWindow, self).__init__(parent)
self.setupUi(self)
self.pushButton.clicked.connect(self.openDialog)
def openDialog(self):
data = self.lineEdit.text()
w = Dialog(data)
w.exec_()
if __name__ == '__main__':
import sys
app = QtWidgets.QApplication(sys.argv)
w = MainWindow()
w.show()
sys.exit(app.exec_())

self.close and self.hide not working in pyqt5

I am new to python. I am using pyqt5 for GUI development. I have a main Window which should close and new dialog appears after clicking the pushButton. But it doesnot closes and neither show any error and opens the nextDialog. I also want to close nextDialog when OK button is clicked in nextDialog. Please help to check the issue. I am trying to develeop a new project but stuck on this issue. Codes are given below.
Main.py
from PyQt5 import QtCore, QtGui, QtWidgets
from nextDialog import Ui_Dialog
class Ui_MainWindow(QtWidgets.QMainWindow):
def setupUi(self, MainWindow):
MainWindow.setObjectName("MainWindow")
MainWindow.resize(370, 171)
self.centralwidget = QtWidgets.QWidget(MainWindow)
self.centralwidget.setObjectName("centralwidget")
self.pushButton = QtWidgets.QPushButton(self.centralwidget)
self.pushButton.setGeometry(QtCore.QRect(110, 50, 75, 23))
self.pushButton.setObjectName("pushButton")
MainWindow.setCentralWidget(self.centralwidget)
self.pushButton.clicked.connect(self.opennext)
self.retranslateUi(MainWindow)
QtCore.QMetaObject.connectSlotsByName(MainWindow)
def opennext(self):
self.Dialog = QtWidgets.QDialog()
self.ui = Ui_Dialog()
self.ui.setupUi(self.Dialog)
self.Dialog.show()
self.close() #Not working
def retranslateUi(self, MainWindow):
_translate = QtCore.QCoreApplication.translate
MainWindow.setWindowTitle(_translate("MainWindow", "MainWindow"))
self.pushButton.setText(_translate("MainWindow", "Open"))
if __name__ == "__main__":
import sys
app = QtWidgets.QApplication(sys.argv)
MainWindow = QtWidgets.QMainWindow()
ui = Ui_MainWindow()
ui.setupUi(MainWindow)
MainWindow.show()
sys.exit(app.exec_())
from PyQt5 import QtCore, QtGui, QtWidgets
import sys
nextDialog.py
class Ui_Dialog(QtWidgets.QMainWindow):
def setupUi(self, Dialog):
Dialog.setObjectName("Dialog")
Dialog.resize(346, 182)
self.pushButton = QtWidgets.QPushButton(Dialog)
self.pushButton.setGeometry(QtCore.QRect(110, 80, 75, 23))
self.pushButton.setObjectName("pushButton")
self.pushButton.clicked.connect(self.exit)
self.retranslateUi(Dialog)
QtCore.QMetaObject.connectSlotsByName(Dialog)
def exit(self):
self.hide() #This also not working, I want either of these two to
#work
def retranslateUi(self, Dialog):
_translate = QtCore.QCoreApplication.translate
Dialog.setWindowTitle(_translate("Dialog", "Dialog"))
self.pushButton.setText(_translate("Dialog", "OK"))

Try it:
main.py
from PyQt5 import QtCore, QtGui, QtWidgets
from nextDialog import Ui_Dialog
class MainWindow(QtWidgets.QMainWindow):
def __init__(self):
super().__init__()
self.centralwidget = QtWidgets.QWidget()
self.setCentralWidget(self.centralwidget)
self.pushButton = QtWidgets.QPushButton("Open", self.centralwidget)
self.pushButton.setGeometry(QtCore.QRect(110, 50, 75, 23))
self.pushButton.clicked.connect(self.opennext)
def opennext(self):
self.Dialog = QtWidgets.QDialog()
self.ui = Ui_Dialog()
self.ui.setupUi(self.Dialog)
self.Dialog.show()
self.close() #Not working
if __name__ == "__main__":
import sys
app = QtWidgets.QApplication(sys.argv)
window = MainWindow()
# ui = Ui_MainWindow()
# ui.setupUi(MainWindow)
window.show()
sys.exit(app.exec_())
nextDialog.py
import sys
from PyQt5 import QtCore, QtGui, QtWidgets
class Ui_Dialog(QtWidgets.QMainWindow):
def setupUi(self, Dialog):
Dialog.setObjectName("Dialog")
Dialog.resize(346, 182)
self.pushButton = QtWidgets.QPushButton(Dialog)
self.pushButton.setGeometry(QtCore.QRect(110, 80, 75, 23))
self.pushButton.setObjectName("pushButton")
self.pushButton.clicked.connect(Dialog.close) # <---
self.retranslateUi(Dialog)
QtCore.QMetaObject.connectSlotsByName(Dialog)
def retranslateUi(self, Dialog):
_translate = QtCore.QCoreApplication.translate
Dialog.setWindowTitle(_translate("Dialog", "Dialog"))
self.pushButton.setText(_translate("Dialog", "OK"))

Hi kind of difficult to understand your code, I am using just Designer to make my GUI,
in anycase I made the first part of the puzzle work for your main.py
line 4 should read class Ui_MainWindow(object):
and to close the first window about lane 25 use MainWindow.close()
added import sys too at the beginning, here the code
main.py:
#!/usr/bin/env python3
# -*- coding: utf-8 -*-
"""
Created on Sat Apr 18 17:43:19 2020
#author: Pietro
"""
import sys
from PyQt5 import QtCore, QtGui, QtWidgets
from nextDialog import Ui_Dialog
class Ui_MainWindow(object):
def setupUi(self, MainWindow):
MainWindow.setObjectName("MainWindow")
MainWindow.resize(370, 171)
self.centralwidget = QtWidgets.QWidget(MainWindow)
self.centralwidget.setObjectName("centralwidget")
self.pushButton = QtWidgets.QPushButton(self.centralwidget)
self.pushButton.setGeometry(QtCore.QRect(110, 50, 75, 23))
self.pushButton.setObjectName("pushButton")
MainWindow.setCentralWidget(self.centralwidget)
self.pushButton.clicked.connect(self.opennext)
self.retranslateUi(MainWindow)
QtCore.QMetaObject.connectSlotsByName(MainWindow)
def opennext(self):
self.Dialog = QtWidgets.QDialog()
self.ui = Ui_Dialog()
self.ui.setupUi(self.Dialog)
self.Dialog.show()
MainWindow.close()
def retranslateUi(self, MainWindow):
_translate = QtCore.QCoreApplication.translate
MainWindow.setWindowTitle(_translate("MainWindow", "MainWindow"))
self.pushButton.setText(_translate("MainWindow", "Open"))
if __name__ == "__main__":
app = QtWidgets.QApplication(sys.argv)
MainWindow = QtWidgets.QMainWindow()
ui = Ui_MainWindow()
ui.setupUi(MainWindow)
MainWindow.show()
sys.exit(app.exec_())
this open first window then close it when open buttton is pressed and new window appears, second part of the riddle is gonna take longer I'll try it as soon as I get some spare time
Dialog.py from answer below seems to work dont know why (dont know how main.py works too, but it is kind of more illogic to me the fact that def exit doesnt work).
nextDialog.py
#!/usr/bin/env python3
# -*- coding: utf-8 -*-
"""
Created on Sat Apr 18 17:43:19 2020
#author: Pietro
"""
import sys
from PyQt5 import QtCore, QtGui, QtWidgets
class Ui_Dialog(QtWidgets.QMainWindow):
def setupUi(self, Dialog):
Dialog.setObjectName("Dialog")
Dialog.resize(346, 182)
self.pushButton = QtWidgets.QPushButton(Dialog)
self.pushButton.setGeometry(QtCore.QRect(110, 80, 75, 23))
self.pushButton.setObjectName("pushButton")
self.pushButton.clicked.connect(Dialog.close) # as answer below dont know why def exit doesnt work
self.retranslateUi(Dialog)
QtCore.QMetaObject.connectSlotsByName(Dialog)
# def exit(self):
# print('exit' *5)
# Dialog.close()
def retranslateUi(self, Dialog):
_translate = QtCore.QCoreApplication.translate
Dialog.setWindowTitle(_translate("Dialog", "Dialog"))
self.pushButton.setText(_translate("Dialog", "OK"))

QDialog not opening from Main window (pyQt)

I'm trying to launch a dialog by clicking a button in the main window: Here's the (qtdesigner generated) code which I modified just to test it .. I've set the showDial function to show the dial when the button is clicked. But it doesn't work :
from PyQt4 import QtCore, QtGui
try:
_fromUtf8 = QtCore.QString.fromUtf8
except AttributeError:
_fromUtf8 = lambda s: s
class Ui_Dialog(object):
def setupUi(self, Dialog):
Dialog.setObjectName(_fromUtf8("Dialog"))
Dialog.setWindowModality(QtCore.Qt.WindowModal)
Dialog.resize(400, 300)
Dialog.setWindowTitle(QtGui.QApplication.translate("Dialog", "Dialog", None, QtGui.QApplication.UnicodeUTF8))
self.retranslateUi(Dialog)
QtCore.QMetaObject.connectSlotsByName(Dialog)
def retranslateUi(self, Dialog):
pass
class Ui_MainWindow(object):
def setupUi(self, MainWindow):
MainWindow.setObjectName(_fromUtf8("MainWindow"))
MainWindow.resize(309, 148)
MainWindow.setWindowTitle(QtGui.QApplication.translate("MainWindow", "MainWindow", None, QtGui.QApplication.UnicodeUTF8))
self.centralwidget = QtGui.QWidget(MainWindow)
self.centralwidget.setObjectName(_fromUtf8("centralwidget"))
self.pushButton = QtGui.QPushButton(self.centralwidget)
self.pushButton.setGeometry(QtCore.QRect(50, 30, 191, 71))
self.pushButton.setText(QtGui.QApplication.translate("MainWindow", "Open Dialog", None, QtGui.QApplication.UnicodeUTF8))
self.pushButton.setObjectName(_fromUtf8("pushButton"))
MainWindow.setCentralWidget(self.centralwidget)
self.retranslateUi(MainWindow)
QtCore.QObject.connect(self.pushButton, QtCore.SIGNAL(_fromUtf8("clicked(QAbstractButton*)")), self.showDial)
QtCore.QMetaObject.connectSlotsByName(MainWindow)
def retranslateUi(self, MainWindow):
pass
def showDial(self):
Dialog = QtGui.QDialog()
u = Ui_Dialog()
u.setupUi(Dialog)
Dialog.exec_()
if __name__ == "__main__":
import sys
app = QtGui.QApplication(sys.argv)
MainWindow = QtGui.QMainWindow()
ui = Ui_MainWindow()
ui.setupUi(MainWindow)
MainWindow.show()
sys.exit(app.exec_())

There is error in signal connection, it should be:
QtCore.QObject.connect(self.pushButton, QtCore.SIGNAL(_fromUtf8("clicked()")), self.showDial)
or in more pythonic New-style Signal and Slot syntax for PyQt 4.5+:
self.pushButton.clicked.connect(self.showDial)