If the user does not pass the full path to the file (on Unix type systems this means a path that starts with a slash), the path is interpreted relatively to the current working directory. The current working directory usually is the directory in which you started the program. In your case, the file test.rtf must be in the same directory in which you execute the program.
You are obviously performing programming tasks in Python under Mac OS. There, I recommend to work in the terminal (on the command line), i.e. start the terminal, cd to the directory where your input file is located and start the Python script there using the command
$ python script.py
In order to make this work, the directory containing the python executable must be in the PATH, a so-called environment variable that contains directories that are automatically used for searching executables when you enter a command. You should make use of this, because it simplifies daily work greatly. That way, you can simply cd to the directory containing your Python script file and run it.
In any case, if your Python script file and your data input file are not in the same directory, you always have to specify either a relative path between them or you have to use an absolute path for one of them.