Flask
Flask a microframework written in Python that makes it easy to get a simple web application up and running with some features that can be useful in the development process.
Install package Flask, create new application.py file with the code and set FLASK_APP=application.py
To start server: flask run
# Import the class `Flask` from the `flask` module
from flask import Flask
# Instantiate a new web application called `app`, with `__name__` representing the current file
app = Flask(__name__)
# A decorator; when the user goes to the route `/`, execute the function
@app.route("/")
def index():
return "Hello, world!"
A route is the part of the URL that determines which page is being requested. The route for the default page is simply “/”.
#when the user goes to the route `/Dmitry'
@app.route("/Dmitry")
def name():
return "Hello, Dmitry!"
#when the user goes to the route `/<any>`
@app.route("/<string:user_name>")
def name_variable(user_name):
user_name1 = 'Mr ' + user_name.capitalize()
return f"Hi, {user_name1}!"
HTML files
Separate HTML files can be rendered (render_template function from Flask module) with variables:
from flask import Flask, render_template
app = Flask(__name__)
@app.route("/<string:name>")
def index(name):
return render_template("index.html", var_name=name)
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
<style>
body {
text-align: center
}
</style>
</head>
<body>
<h1>Hello, {{ var_name }} </h1>
</body>
</html>
These templates are rendered using a separate templating language called Jinja2.
Jinja2
Jinja is a modern and designer-friendly templating language for Python.
- In .py: return render_template(“index.html”, var_name=name)
- In html: <h1>Hello, {{ var_name }} </h1>
Jinja2 allows conditional statements:
{% if var_name %}
<h2>Yes!</h2>
<h3>Hello, {{ var_name }}</h3>
<p></p>
<a href="{{ url_for('loop') }}">See more...</a>
{% else %}
<a href="{{ url_for('index') }}">Go back</a>
{% endif %}
- Loops:
<ul>
{% for each in list %}
<li>{{ each }}</li>
{% endfor %}
</ul>
If there are multiple routes on the Flask server, then one route can link to another as so:
<a href="{{ url_for('func_name') }}">See more...</a>
func_name is the name of a function associated with a route.
@app.route("/")
def index():
name = True
return render_template("index.html", var_name=name)
@app.route("/looplist")
def loop():
name = False
my_list = ['AA', 'BB', 'CC']
return render_template("index.html", var_name=name, list=my_list)
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
<style>
body {
text-align: center
}
</style>
</head>
<body>
<h1>This is constant!</h1>
<p></p>
<p></p>
{% if var_name %}
<h2>Yes!</h2>
<h3>Hello, {{ var_name }}</h3>
<p></p>
<a href="{{ url_for('loop') }}">See more...</a>
{% else %}
<ul>
{% for each in list %}
<li>{{ each }}</li>
{% endfor %}
</ul>
<a href="{{ url_for('index') }}">Go back</a>
{% endif %}
</body>
</html>
Jinja2 ‘template inheritance’
Create some kind of layout with variables in blocks:
<!DOCTYPE html>
<html>
<head>
<title>My Website!</title>
<style>
body { text-align: center}
</style>
</head>
<body>
<h1>{% block heading %}{% endblock %}</h1>
{% block body %}
{% endblock %}
</body>
</html>
In the different HTML files, just extend with the layout and fill blocks.
index.html:
{% extends "layout.html" %}
{% block heading %}
Index Page
{% endblock %}
{% block body %}
<h2>Hello, {{ var_name }}</h2>
<a href="{{ url_for('loop') }}">See more...</a>
{% endblock %}
loop.html:
{% extends "layout.html" %}
{% block heading %}
Loop page
{% endblock %}
{% block body %}
<ul>
{% for each in list %}
<li>{{ each }}</li>
{% endfor %}
</ul>
<a href="{{ url_for('index') }}">Go back</a>
{% endblock %}
application.py
@app.route("/")
def index():
name ='User'
return render_template("index.html", var_name=name)
@app.route("/loop")
def loop():
my_list = ['AA', 'BB', 'CC']
return render_template("loop.html", list=my_list)
Forms
the results from HTML forms can now be actually stored and used.
<form action="get-ip-address" method="post"> <input type="text" name="ip_address" placeholder="Enter IP Address"> <button>Add</button> <form>
- action attribute lists the route that should be ‘notified’ when the form is submitted
- method attribute is how the HTTP request to submit the form should be made
- name attribute of the input
The Python code to process this form: import request
from flask import Flask, render_template, request
app = Flask(__name__)
@app.route("/")
def index():
return render_template("index.html")
@app.route("/get-ip-address", methods=["POST"])
def print_ip_address():
# take user request, access form, get var nam and store in ip_address
ip_address = request.form.get("ip_address")
return render_template("ip_address.html", ip_address=ip_address)
If there are multiple request methods: list of methods and ifelse
@app.route("/get-ip-address", methods=["POST", "GET"])
def print_ip_address():
if request.method == "GET":
return "Please submit the form"
else:
ip_address = request.form.get("ip_address")
return render_template("ip_address.html", ip_address=ip_address)
Sessions
Sessions are how Flask can keep track of data that pertains to a particular user: could be a global variable (access to all users) or per user(cookies). Install Flask-Session
from flask import Flask, render_template, request, session
from flask_session import Session
app = Flask(__name__)
app.config["SESSION_PERMANENT"] = False
app.config["SESSION_TYPE"] = "filesystem"
Session(app)
@app.route("/", methods=["GET", "POST"])
def index():
if session.get("ip_address") is None:
session["ip_address"] = []
if request.method == "POST":
ip_address = request.form.get("ip_address")
session["ip_address"].append(ip_address)
return render_template("index.html", ip_addresses=session["ip_address"])
{% extends "layout.html" %}
{% block heading %}
Index Page
{% endblock %}
{% block body %}
<ul>
{% for each in ip_addresses %}
<li>{{ each }}</li>
{% endfor %}
</ul>
<form action="/" method="post">
<input type="text" name="ip_address" placeholder="Enter IP Address">
<button>Add</button>
<form>
{% endblock %}
Run app from the script
from flask import Flask, render_template, request, session
from flask_session import Session
app = Flask(__name__)
if __name__ == '__main__':
app.debug = True
app.run()
Debug mode
For every change in the files, the flask server must be restarted to apply those changes. With debug mode enabled – all changes are applied on the fly and no need to re-run flask
set FLASK_ENV=development
For more: Flask Documentation
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