Python file naming question?

I'm having a user enter the name of the file they want to open. Then, I'm supposed to write the lines of that file into an output file that is named the same thing, but with "ln_" preceding it. So, if the user enters "scores.txt", then the outputfile should be named "ln_scores.txt". How do I go about opening the output file with the correct name if I don't know what the user will enter?