http://nikolak.com/pyqt-qt-designer-getting-started/

design.py

# -*- coding: utf-8 -*-

# Form implementation generated from reading ui file 'design.ui'
#
# Created: Wed May 27 16:39:17 2015
#      by: PyQt4 UI code generator 4.11.3
#
# WARNING! All changes made in this file will be lost!

from PyQt4 import QtCore, QtGui

try:
    _fromUtf8 = QtCore.QString.fromUtf8
except AttributeError:
    def _fromUtf8(s):
        return s

try:
    _encoding = QtGui.QApplication.UnicodeUTF8
    def _translate(context, text, disambig):
        return QtGui.QApplication.translate(context, text, disambig, _encoding)
except AttributeError:
    def _translate(context, text, disambig):
        return QtGui.QApplication.translate(context, text, disambig)

class Ui_MainWindow(object):
    def setupUi(self, MainWindow):
        MainWindow.setObjectName(_fromUtf8("MainWindow"))
        MainWindow.resize(240, 345)
        self.centralwidget = QtGui.QWidget(MainWindow)
        self.centralwidget.setObjectName(_fromUtf8("centralwidget"))
        self.verticalLayout = QtGui.QVBoxLayout(self.centralwidget)
        self.verticalLayout.setObjectName(_fromUtf8("verticalLayout"))
        self.listWidget = QtGui.QListWidget(self.centralwidget)
        self.listWidget.setObjectName(_fromUtf8("listWidget"))
        self.verticalLayout.addWidget(self.listWidget)
        self.btnBrowse = QtGui.QPushButton(self.centralwidget)
        self.btnBrowse.setObjectName(_fromUtf8("btnBrowse"))
        self.verticalLayout.addWidget(self.btnBrowse)
        MainWindow.setCentralWidget(self.centralwidget)

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

    def retranslateUi(self, MainWindow):
        MainWindow.setWindowTitle(_translate("MainWindow", "MainWindow", None))
        self.btnBrowse.setText(_translate("MainWindow", "Pick a folder", None))

design.ui

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>MainWindow</class>
 <widget class="QMainWindow" name="MainWindow">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>240</width>
    <height>345</height>
   </rect>
  </property>
  <property name="windowTitle">
   <string>MainWindow</string>
  </property>
  <widget class="QWidget" name="centralwidget">
   <layout class="QVBoxLayout" name="verticalLayout">
    <item>
     <widget class="QListWidget" name="listWidget"/>
    </item>
    <item>
     <widget class="QPushButton" name="btnBrowse">
      <property name="text">
       <string>Pick a folder</string>
      </property>
     </widget>
    </item>
   </layout>
  </widget>
 </widget>
 <resources/>
 <connections/>
</ui>

main.py

from PyQt4 import QtGui  # Import the PyQt4 module we'll need
import sys  # We need sys so that we can pass argv to QApplication

import design  # This file holds our MainWindow and all design related things

# it also keeps events etc that we defined in Qt Designer
import os  # For listing directory methods


class ExampleApp(QtGui.QMainWindow, design.Ui_MainWindow):
    def __init__(self):
        # Explaining super is out of the scope of this article
        # So please google it if you're not familar with it
        # Simple reason why we use it here is that it allows us to
        # access variables, methods etc in the design.py file
        super(self.__class__, self).__init__()
        self.setupUi(self)  # This is defined in design.py file automatically
        # It sets up layout and widgets that are defined
        self.btnBrowse.clicked.connect(self.browse_folder)  # When the button is pressed
                                                            # Execute browse_folder function

    def browse_folder(self):
        self.listWidget.clear() # In case there are any existing elements in the list
        directory = QtGui.QFileDialog.getExistingDirectory(self,
                                                           "Pick a folder")
        # execute getExistingDirectory dialog and set the directory variable to be equal
        # to the user selected directory

        if directory: # if user didn't pick a directory don't continue
            for file_name in os.listdir(directory): # for all files, if any, in the directory
                self.listWidget.addItem(file_name)  # add file to the listWidget


def main():
    app = QtGui.QApplication(sys.argv)  # A new instance of QApplication
    form = ExampleApp()  # We set the form to be our ExampleApp (design)
    form.show()  # Show the form
    app.exec_()  # and execute the app


if __name__ == '__main__':  # if we're running file directly and not importing it
    main()  # run the main function