output_file.py

# -*- coding: utf-8 -*-
from PyQt4 import QtCore, QtGui
from PyQt4.QtGui import QFileDialog
from qgis.gui import QgsMessageBar
import output_file_dialog
from os import chdir

class OutputFile(QtGui.QDialog):

    def __init__(self):
        QtGui.QDialog.__init__(self)
        # Set up the user interface from Designer.
        # After setupUI you can access any designer object by doing:
        # self.<objectname>
        self.ui = output_file_dialog.Ui_Dialog()
        self.ui.setupUi(self)

        # #As in a QGIS plugin
        # self.ui.lineEdit.clear()
        # self.ui.pushButton.clicked.connect(self.select_output_file)

    def select_output_file(self):
        chdir('c:/Users')
        filename = QFileDialog.getSaveFileName(self, "Select output file ","", '*.txt')
        self.ui.lineEdit.setText(filename)
        self.save_file()

    def save_file(self):
        filename = self.ui.lineEdit.text()
        output_file = open(filename, 'w')
        output_file.write("My text")
        output_file.close()

# Create the dialog and keep reference
reload(output_file_dialog)
dlg = OutputFile() #create Dialog object
dlg.show() #show Dialog object

dlg.ui.lineEdit.clear() #clear text in lineEdit object
dlg.ui.pushButton.clicked.connect(dlg.select_output_file) #send signal if
                                                                                  #pushButton is clicked