(python socket) Is there a way to trigger a JS script on the front-end when a condition is met?

Merry Christmas. I am making a website and I want to change the view past-login with JavaScript.

The code responsible for detecting a valid login attempt looks like this:

def messages(status_OK, status_400, status_401, filtered_data):
    # change page after signup
    if filtered_data[0] == 'method=sign_up':
        if filtered_data[1] == 'username=' or filtered_data[2] == 'password=' or filtered_data[3] == 'email=':
            with open('status/error/signup_400.html', 'rb') as err: response = err.read()
            return status_400 + response
        with open('status/signup_success.html', 'rb') as success: response = success.read()
        return status_OK + response
        # change page after login
    elif filtered_data[0] == 'method=login_name':
        if postlogin.username():
            with open('status/username_success.html', 'rb') as success: response = success.read()
            return status_OK + response
        elif filtered_data[1] == 'username=' or filtered_data[2] == 'password=':
            with open('status/error/username_400.html', 'rb') as err: response = err.read()
            return status_400 + response
        else:
            with open('status/username_failed.html', 'rb') as fail: response = fail.read()
            return status_401 + response
    elif filtered_data[0] == 'method=login_email':
        if postlogin.username():
            with open('status/email_success.html', 'rb') as success: response = success.read()
            return status_OK + response
        elif filtered_data[1] == 'email=' or filtered_data[2] == 'password=':
            with open('status/error/email_400.html', 'rb') as err: response = err.read()
            return status_400 + response
        else:
            with open('status/email_failed.html', 'rb') as fail: response = fail.read()
            return status_401 + response

I want it to do more than just returning another webpage, specifically, I want it to execute JS on the front-end to set a flag that changes the webpage view ACCORDING TO THE MENTIONED CODE ^^^^^^^

So, how do I do it?

I know how to change the view, I just need a way to execute JS on the front-end without needing user actions.