Pop quiz : Suppose you are developing a Flask app. Flask uses port 5000 as its default port. Now suppose that you have your Flask app running on macOS on port 5000. You're testing in Safari 1 and you enter this URL into the address bar: http://localhost:5000 . Question : What will happen? Answer : You won't get your Flask app. Instead, you'll likely get a whole lot of nothing, which can be very confusing because of two overlapping factors. It turns out, 5000 is the default por...