This assumes you are already familiar with Flask Sessions. If not, check out my article FreeKB - Flask - Getting Started with Sessions.

By default, sessions are set as permanent.

• Permanent session: A cookie is stored in the browser and not deleted until it expires
• Non-permanent session: A cookie is stored in the browser and is deleted when the browser or tab is closed

PERMANENT_SESSION_LIFETIME can be set in __init__.py. The session will expire when PERMANENT_SESSION_LIFETIME has been reached. In this example, the session lifetime is 14400 seconds (4 hours).

from flask import Flask
from datetime import timedelta

def myapp():
    app = Flask(__name__)
    
    app.config['SECRET_KEY'] = "akDFJ34mdfsYMH567sdf" # this must be set in order to use sessions
    
    app.config['PERMANENT_SESSION_LIFETIME'] = 86400

    # Or using timedelta hours
    app.config['PERMANENT_SESSION_LIFETIME'] = timedelta(hours=24)

    # Or using timedelta days
    app.config['PERMANENT_SESSION_LIFETIME'] = timedelta(days=7)    
    
    return app

session.permanent can be used to set a specific session as permanent or non-permanent. This will take precedence (override) SESSION_PERMANENT in __init__.py.

from flask import Blueprint, render_template, session

views = Blueprint('routes_home', __name__)

@views.route('/')
def home():    
    session.permanent = False
    session["foo"] = "bar"
    return render_template('home.html')

Or you can use before_request to set session.permanent for all routes in your view.

from flask import Blueprint, render_template, session

views = Blueprint('routes_home', __name__)

@views.before_request
def make_session_permanent():
    session.permanent = True

@views.route('/')
def home():    
    session["foo"] = "bar"
    return render_template('home.html')

Be careful to not use session.clear() as this will remove the cookies from the users browser.

from flask import Blueprint, render_template, session

views = Blueprint('routes_home', __name__)

@views.route('/')
def home():    
    session.permanent = False
    session["foo"] = "bar"
    session.clear()
    return render_template('home.html')

To validate this is working as expected, you can view the cookie that gets created.

• If session.permanent is set to True then "Expires" should match PERMANENT_SESSION_LIFETIME
• If session.permanent is set to False then "Expires" should be "When the browsing session ends" (this is the default if session permanent has not been defined)