Flask sessions

This example fails when two or more people use the site concurrently:

shopping cart fail.py

from flask import *
app = Flask(__name__)

cart = []

cartPage = '''
<form action="/buy" method="POST">
  <input type="radio" name="drink" value="coke"> coke <br>
  <input type="radio" name="drink" value="mdew"> mdew <br>
  <input type="submit" value="buy">
</form>
{% if myCart is defined %}
<h3>Drinks purchased:</h3>
  {% for each_item in myCart %}
    {{ each_item }}<br>
  {% endfor %}
{% endif %}'''

@app.route("/buy", methods=["POST"])
def bought():
    cart.append(request.form["drink"])
    return render_template_string(cartPage, myCart=cart)

@app.route("/")
def start():
    return render_template_string(cartPage)

app.run(debug=True)

To fix this, we need a specific variable for each user that can be accessed (for that user) via multiple pages. This type of variable is called a session variable.

The following examples above show session variables in use:

session variable.py

from flask import *
app = Flask(__name__)

app.secret_key = "sssshhhhhhhhh!"

loginPage = '''
{% if user is not defined %}
<form action="/login" method="POST">
  <input type="text" name="username"<br>
  <input type="submit" value="log in">
</form>
{% else %}
  <p>Welcome {{ user }}. <a href="/logout">logout</a></p>
{% endif %}'''

@app.route("/login", methods=["POST"])
def login():
    session["myName"] = request.form["username"]
    return render_template_string(loginPage, user=session["myName"])

@app.route("/")
def launch():
    return render_template_string(loginPage)

@app.route("/logout")
def logout():
    session.pop("myName", None)
    return redirect("/")

app.run(debug=True)

shopping cart better.py

from flask import *
app = Flask(__name__)

app.secret_key = "sssshhhhhhhhh!"

cartPage = '''
<form action="/buy" method="POST">
  <input type="radio" name="drink" value="coke"> coke <br>
  <input type="radio" name="drink" value="mdew"> mdew <br>
  <input type="submit" value="buy">
</form>
{% if myCart is defined %}
<h3>Drinks purchased:</h3>
  {% for each_item in myCart %}
    {{ each_item }}<br>
  {% endfor %}
{% endif %}'''

@app.route("/buy", methods=["POST"])
def bought():
    temp = session["cart"]
    temp.append(request.form["drink"])
    session["cart"] = temp
    return render_template_string(cartPage, myCart=session["cart"])

@app.route("/")
def start():
    if "cart" not in session:
        session["cart"] = []
    return render_template_string(cartPage)

app.run(debug=True)