Flask Session Not Persisting (Postman works, Javascript doesn't)

Question

I\'m developing a Flask server to communicate between some backend Python functionality and Javascript clients over the web. I\'m attempting to utilize Flask\'s session

Answer 1

After a few hours of testing, I managed to figure out the issue. Although I think @amanb's answer highlights the problem, I'm going to answer my own question because what I found is ultimately a simpler solution.

In order to make the POST request return the expected value, I simply needed to add a credentials: 'same-origin' line to the fetch body. This looks like the following:

var url = 'http://my_server_ip/update';
fetch(url, {
  method: 'POST',
  body: JSON.stringify('arbitrary_string'),
  credentials: 'same-origin',   // this line has been added
  headers: {
    'Content-Type': 'application/json'
  }
})
.then(response => response.json())
.then((data) => {
  console.log(data);
})

According to Mozilla's Fetch usage guide,

By default, fetch won't send or receive any cookies from the server, resulting in unauthenticated requests if the site relies on maintaining a user session.

So it seems I looked over this. Changing the credentials to allow communication of the cookie/session between client and server resolved the issue.