###JavaScript

$.get is asynchronous. It triggers an HTTP request and returns. The code meant to prevent submission runs after submission is processed, and therefore can't prevent it.

The preventDefault has to go into the submit handler, not the $.get callback, and would be called each time you try to submit, cancelling the submit attempt. Inside the get callback, depending on result, call the form's submit() method, that one submits the form without triggering the submit handlers again.

###Python

SELECT lets db.execute return a list of dicts, so [0] would pick first record, and [username] would pick the field of the name stored in variable username. Which is not what you want, you want to compare with values. Instead of [0][username], you could do something like

usernames = (user["username"] for user in db.execute("SELECT username FROM users"))

And you can jsonify a boolean expression:

return jsonify(username in usernames)

edit: username not in usernames (according to problem description)

You might like WHERE clause in your SELECT. Gets so much more readable, saving you much code.

JavaScript

$.get is asynchronous. It triggers an HTTP request and returns. The code meant to prevent submission runs after submission is processed, and therefore can't prevent it.

The preventDefault has to go into the submit handler, not the $.get callback, and would be called each time you try to submit, cancelling the submit attempt. Inside the get callback, depending on result, call the form's submit() method, that one submits the form without triggering the submit handlers again.

Python

SELECT lets db.execute return a list of dicts, so [0] would pick first record, and [username] would pick the field of the name stored in variable username. Which is not what you want, you want to compare with values. Instead of [0][username], you could do something like

usernames = (user["username"] for user in db.execute("SELECT username FROM users"))

And you can jsonify a boolean expression:

return jsonify(username in usernames)

edit: username not in usernames (according to problem description)

You might like WHERE clause in your SELECT. Gets so much more readable, saving you much code.

###JavaScript

$.get is asynchronous. It triggers an HTTP request and returns. The code meant to prevent submission runs after submission is processed, and therefore can't prevent it.

The preventDefault has to go into the submit handler, not the $.get callback, and would be called each time you try to submit, cancelling the submit attempt. Inside the get callback, depending on result, call the form's submit() method, that one submits the form without triggering the submit handlers again.

###Python

SELECT lets db.execute return a list of dicts, so [0] would pick first record, and [username] would pick the field of the name stored in variable username. Which is not what you want, you want to compare with values. Instead of [0][username], you could do something like

usernames = (user["username"] for user in db.execute("SELECT username FROM users"))

And you can jsonify a boolean expression:

return jsonify(username in usernames)

You might like WHERE clause in your SELECT. Gets so much more readable, saving you much code.

###JavaScript

$.get is asynchronous. It triggers an HTTP request and returns. The code meant to prevent submission runs after submission is processed, and therefore can't prevent it.

The preventDefault has to go into the submit handler, not the $.get callback, and would be called each time you try to submit, cancelling the submit attempt. Inside the get callback, depending on result, call the form's submit() method, that one submits the form without triggering the submit handlers again.

###Python

SELECT lets db.execute return a list of dicts, so [0] would pick first record, and [username] would pick the field of the name stored in variable username. Which is not what you want, you want to compare with values. Instead of [0][username], you could do something like

usernames = (user["username"] for user in db.execute("SELECT username FROM users"))

And you can jsonify a boolean expression:

return jsonify(username in usernames)

edit: username not in usernames (according to problem description)

You might like WHERE clause in your SELECT. Gets so much more readable, saving you much code.