How to put together datas into a file?

[ichabod801]

Well, val_dB is a float. You explicitly made it one on the second line (although given that you made it a float and then made it a string on the previous line, I'm a little confused as to the process). Floats don't add to strings, you need to convert it back to string to do that:

myfile.write(date_ID + " " + val + str(val_dB) + "\n")

Better yet is to use string formatting:

myfile.write('{} {}{}\n'.format(date_ID, val, val_dB)

That will do the conversion for you, and allows for all sorts of ways to format the resulting string. You can see the full format method syntax here.