Read and write csv file using drop down box in python

I want to build a GUI to read and write csv files. Based on the output file I am creating another GUI for plotting. I am using pyQt4. There a 6 files in the folder. I want to read all the files having same name write in "out.csv" file. Then the other GUI will plot from "out.csv".

My code is:

def update(self):

        cat1 = self.dropdown1.currentText()

        counter = 1

        for folder, sub_folders, files in os.walk(my_directory):
            for special_file in files:
                if cat1 == 'iono_tropo.csv':
                    file_path = os.path.join(folder, 'iono_tropo.csv')
                elif cat1 == 'PR_log.csv':
                    file_path = os.path.join(folder, 'PR_log.csv')
                elif cat1 == 'Satellite_info.csv':
                    file_path = os.path.join(folder, 'Satellite_info.csv')
                elif cat1 == 'Sol_1.csv':
                    file_path = os.path.join(folder, 'Sol_1.csv')
                elif cat1 == 'Sol_2.csv':
                    file_path = os.path.join(folder, 'Sol_2.csv')
                else:
                    file_path = os.path.join(folder, 'Sol_3.csv')

                    with open(file_path) as read_file:
                        print('Reading csv file' + str(counter))
                        if counter == 1:
                            lines = read_file.readlines()
                        else:
                            lines += read_file.readlines()[1:]

                            with open ("out.csv","w") as f:
                                for line in lines:
                                    f.write(line)

                        counter += 1

The script shows no error. But there is now "out.csv" file at destination. I am new in python program. When I run the code separately (not in GUI), it working. But here it shows nothing. Help please. Thanks in advance.