Sessions, Cookies & Messages

The web forgets — so we help it remember

HTTP is stateless: each request stands alone, and the server forgets you the instant it replies. To remember anything between pages — that you are logged in, what is in your cart — we use two tools that work together.

A cookie is a small piece of text the server asks the browser to store and send back on every future request. A session is server-side storage tied to a visitor; Django puts only a random session id in a cookie, and keeps the actual data safely on the server. This is exactly how login remembered the user in the auth lesson.

Using the session

Django gives every request a request.session object that behaves like a Python dictionary — but its contents survive across requests. A classic use is a simple "items in cart" counter:

def add_to_cart(request, product_id):
    cart = request.session.get("cart", [])   # read (default: empty list)
    cart.append(product_id)
    request.session["cart"] = cart            # write it back
    return redirect("cart")

def cart(request):
    cart = request.session.get("cart", [])
    return render(request, "cart.html", {"count": len(cart)})

Line by line: request.session.get("cart", []) reads the saved cart, defaulting to an empty list the first time. We append the new product id, then assign it back to request.session["cart"] — that assignment is what saves it. On the next page, the same key returns the stored list, so the cart "remembers" across requests.

The messages framework — flash messages

After an action you often want a one-time notice: "Post saved!", "Wrong password". The messages framework shows a message once on the next page, then forgets it (a "flash message"). You add a message in the view and render any pending ones in the template:

# views.py
from django.contrib import messages

def save_post(request):
    # ... save the post ...
    messages.success(request, "Post saved!")
    return redirect("post_list")

<!-- in the template (e.g. base.html) -->
{% if messages %}
  <ul class="messages">
    {% for message in messages %}
      <li class="{{ message.tags }}">{{ message }}</li>
    {% endfor %}
  </ul>
{% endif %}

The message levels match common notice types: messages.success, messages.error, messages.warning and messages.info. The {{ message.tags }} gives you a CSS class (like success or error) so you can colour each one.