How to write into a cvs? : I want to write all the file names into a comma separated csv file.

I have tried this, but it separate all the characters, not only the whole file names.

    for filename in filenames:
        with open('C:\Users\igyulavics\Desktop\estfile.csv', 'w') as csvFile:
            a = (os.path.join(dirname, filename))
            print a
            writer = csv.writer(csvFile)
            writer.writerows(a)
        csvFile.close()

So it gives back:
c,Users,1, etc but I want this:
c:\Users\1,c:\Users\2