---
tags:
- notes
- programming
- web
author: TrudeEH
draft: false
showToc: true
title: Flask
---
Flask is a web framework for Python. It facilitates the creation of web apps (dynamic web pages).
## Run Flask
Flask comes with its own server for debugging purposes, which can be started with:
```Bash
flask run
```
## Folder Structure
```Bash
app.py # main code
requirements.txt # required libraries
static/ # files that never change
templates/ # dynamic files
```
## "Hello, name" — Example App
**templates/index.html**
```HTML
Hello
hello, {{ name_placeholder }}
```
**app.py**
`http://website.domain/?name=Trude`
```Python
from flask import Flask, render_template, request
app = Flask(__name__)
# When user visits / (the website root), load index.html.
# If the key name exists, store the value in a variable. If not, store world.
@app.route("/")
def index():
name = request.args.get("name", "world")
return render_template("index.html", name_placeholder=name)
```
## HTML Form
### Sample App
**templates/index.html**
```HTML
Hello
```
**app.py**
`http://website.domain/?name=Trude`
```Python
from flask import Flask, render_template, request
app = Flask(__name__)
# When user visits / (the website root), load index.html.
# If the key name exists, store the value in a variable. If not, store world.
@app.route("/")
def index():
name = request.args.get("name", "world")
return render_template("index.html", name_placeholder=name)
```
### Custom Route
Both the form and /greet works.
**templates/index.html**
```HTML
Hello
```
**templates/greet.html**
```HTML
Hello
hello,{{name}}
```
**app.py**
`http://website.domain/?name=Trude`
```Python
from flask import Flask, render_template, request
app = Flask(__name__)
# When user visits / (the website root), load index.html.
# If the key name exists, store the value in a variable. If not, store world.
@app.route("/")
def index():
name = request.args.get("name", "world")
return render_template("index.html", name_placeholder=name)
@app.route("/greet")
def greet():
return render_template("greet.html", name=request.args.get("name", "world"))
```
### Avoid HTML Repetition
Use a layout instead of copying blocks.
**templates/index.html**
```HTML
{% extends "layout.html" %}
{% block body %}
{% endblock %}
```
**templates/greet.html**
```HTML
{% extends "layout.html" %}
{% block body %}
hello, {{name}}
{% endblock %}
```
**templates/layout.html**
```HTML
Hello
{% block body %}{% endblock %}
```
**app.py**
`http://website.domain/?name=Trude`
```Python
from flask import Flask, render_template, request
app = Flask(__name__)
# When user visits / (the website root), load index.html.
# If the key name exists, store the value in a variable. If not, store world.
@app.route("/")
def index():
name = request.args.get("name", "world")
return render_template("index.html", name_placeholder=name)
@app.route("/greet")
def greet():
return render_template("greet.html", name=request.args.get("name", "world"))
```
### Hide Sensitive Requests from the URL
Use post instead of get.
**templates/index.html**
```HTML
{% extends "layout.html" %}
{% block body %}
{% endblock %}
```
**templates/greet.html**
```HTML
{% extends "layout.html" %}
{% block body %}
hello, {{name}}
{% endblock %}
```
**templates/layout.html**
```HTML
Hello
{% block body %}{% endblock %}
```
**app.py**
`http://website.domain/greet`
The name doesn't appear in the URL anymore.
```Python
from flask import Flask, render_template, request
app = Flask(__name__)
# When user visits / (the website root), load index.html.
# If the key name exists, store the value in a variable. If not, store world.
@app.route("/")
def index():
name = request.args.get("name", "world")
return render_template("index.html", name_placeholder=name)
@app.route("/greet", methods=["POST"])
def greet():
return render_template("greet.html", name=request.form.get("name", "world"))
```
### Combine Routes to save Resources
GET is always the default.
POST is used to send information to the server.
**templates/index.html**
```HTML
{% extends "layout.html" %}
{% block body %}
{% endblock %}
```
**templates/greet.html**
```HTML
{% extends "layout.html" %}
{% block body %}
hello, {{name}}
{% endblock %}
```
**templates/layout.html**
```HTML
Hello
{% block body %}{% endblock %}
```
**app.py**
`http://website.domain/`
```Python
from flask import Flask, render_template, request
app = Flask(__name__)
# When user visits / (the website root), load index.html.
# If the key name exists, store the value in a variable. If not, store world.
@app.route("/", methods=["GET, POST"])
def index():
if request.method == "GET":
return render_template("index.html")
elif request.method == "POST":
return render_template("greet.html", name=request.form.get("name", "world"))
```
## Cookies - Session
Used to store data and to recognize the user.
- Server → Client
- `Set-Cookie session=value`
- Client → Server
- `Cookie session`
```Python
from flask import session
# Configure app
app = Flask(__name__)
# Configure session
app.config["SESSION_PERMANENT"] = False
app.config["SESSION_TYPE"] = "filesystem"
Session(app)
# Redirect to login if no cookie exists
@app.route("/")
def index():
if not session.get("name"):
return redirect("/login")
return render_template("index.html")
# EXAMPLES
# session is a dictionary
session["cart"] = []
books = db.execute("SELECT * FROM books WHERE id IN (?)", session["cart"])
```