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Using Flask Sessions

Flask exposes a session dict-like object:

python
from flask import session
python
from flask import session

Requirements

Sessions require:

  • app.config["SECRET_KEY"]app.config["SECRET_KEY"]

Setting and reading values

python
from flask import Flask, session
 
app = Flask(__name__)
app.config["SECRET_KEY"] = "dev-key"
 
 
@app.route("/set")
def set_value():
    session["favorite_color"] = "blue"
    return "ok"
 
 
@app.route("/get")
def get_value():
    return {"favorite_color": session.get("favorite_color")}
python
from flask import Flask, session
 
app = Flask(__name__)
app.config["SECRET_KEY"] = "dev-key"
 
 
@app.route("/set")
def set_value():
    session["favorite_color"] = "blue"
    return "ok"
 
 
@app.route("/get")
def get_value():
    return {"favorite_color": session.get("favorite_color")}

Removing values

python
session.pop("favorite_color", None)
python
session.pop("favorite_color", None)

Important constraints

Because Flask’s default session is stored in a cookie:

  • keep session data small
  • don’t store secrets in sessions (client can read)
  • sign integrity is provided, confidentiality is not

How Flask-Login uses session

Flask-Login stores:

  • the logged-in user id

in the session so it persists across requests.

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