Exporting .txt from webpage to downloads directory

Im developing a web application (phonebook) using python and I want to be able to export my existing address books to my downloads (be able to specify download direction) the way i have it now, it exports the text file to my local webapp directory ./phone_book

What I want to accomplish is make the users able export/download the address book textfile to directory they desire on their machine.

thank you. Please ask if you need more details