How to write datas into a file?

[Krszt]

I have program, which looks for every files starting with mli_value_IHAR_. I want these files' special date ID into a file (every date into a new line). Here is the code that I wrote. My problem is that the final txt file contains only the last element's date. Where do I make a mistake?

for path, subdirs, files in os.walk(r'./'):
    for filename in files:
        if filename.startswith("mli_value_IHAR_"):
            f = os.path.join(path, filename)
            print(f)
            date = (f[-12:-4])
            print(date)
            with open('test.txt', 'w') as data_file:
                data_file.write(str('%s\n' % date))