If you've setup everything for the Google Sheets API Python Sample.
Some folks reported having to do an incognito browser window or needing to use Firefox. Your milage may vary.
When I ran the app, it opened safari where I clicked through to allow access but then it hung and finally chocked with a message from the browser about not being able to find a localhost server.
Turns out the solution was to copy the URL that shows up in the console and paste it into chrome.
Took me a bit to find that answer via this page. Pasting this here in hopes it helps folks find the answer faster.