Login authentication with Flask

The Flask Logo

In this tutorial you will learn how to build a login web app with Python using Flask.

Installing Flask

Install Flask using the command below:

$ pip install Flask

Create a file called hello.py

from flask import Flask
app = Flask(__name__)
 
@app.route("/")
def index():
    return "Hello World!"
 
if __name__ == "__main__":
    app.run(host='0.0.0.0', port=4000)

Finally run the web app using this command:

$ python hello.py

Open http://localhost:4000/ in your webbrowser, and “Hello World!” should appear.

Building a Flask login screen

Create this Python file and save it as app.py:

from flask import Flask
from flask import Flask, flash, redirect, render_template, request, session, abort
import os
 
app = Flask(__name__)
 
@app.route('/')
def home():
    if not session.get('logged_in'):
        return render_template('login.html')
    else:
        return "Hello Boss!"
 
@app.route('/login', methods=['POST'])
def do_admin_login():
    if request.form['password'] == 'password' and request.form['username'] == 'admin':
        session['logged_in'] = True
    else:
        flash('wrong password!')
    return home()
 
if __name__ == "__main__":
    app.secret_key = os.urandom(12)
    app.run(debug=True,host='0.0.0.0', port=4000)

There are two routes (paths you can see in your browser URL bar) created here:

@app.route('/')
@app.route('/login', methods=['POST'])

The first one displays the login screen or the home screen, based on the condition if you are logged in. The second route validates the login variables on login.

We create the directory /templates/. Create the file /templates/login.html with this code:

{% block body %}
{% if session['logged_in'] %}
<p>You're logged in already!</p>
{% else %}
<form action="/login" method="POST">
  <input type="username" name="username" placeholder="Username">
  <input type="password" name="password" placeholder="Password">
  <input type="submit" value="Log in">
</form>
{% endif %}
{% endblock %}

Run the web app using this command:

$ python hello.py

Open http://localhost:4000/ in your webbrowser, and the login screen should appear. The login credentials are shown in the do_admin_login() function.

Making it look amazing

While functional, the login screen looks like an early 90s User Interface (UI). We picked a random login template from codepen.io. We create the directory /static/ with the file style.css.

* {
box-sizing: border-box;
}
 
*:focus {
	outline: none;
}
body {
font-family: Arial;
background-color: #3498DB;
padding: 50px;
}
.login {
margin: 20px auto;
width: 300px;
}
.login-screen {
background-color: #FFF;
padding: 20px;
border-radius: 5px
}
 
.app-title {
text-align: center;
color: #777;
}
 
.login-form {
text-align: center;
}
.control-group {
margin-bottom: 10px;
}
 
input {
text-align: center;
background-color: #ECF0F1;
border: 2px solid transparent;
border-radius: 3px;
font-size: 16px;
font-weight: 200;
padding: 10px 0;
width: 250px;
transition: border .5s;
}
 
input:focus {
border: 2px solid #3498DB;
box-shadow: none;
}
 
.btn {
  border: 2px solid transparent;
  background: #3498DB;
  color: #ffffff;
  font-size: 16px;
  line-height: 25px;
  padding: 10px 0;
  text-decoration: none;
  text-shadow: none;
  border-radius: 3px;
  box-shadow: none;
  transition: 0.25s;
  display: block;
  width: 250px;
  margin: 0 auto;
}
 
.btn:hover {
  background-color: #2980B9;
}
 
.login-link {
  font-size: 12px;
  color: #444;
  display: block;
	margin-top: 12px;
}

Modify the login.html template as:

<link rel="stylesheet" href="/static/style.css" type="text/css">
{% block body %}
{% if session['logged_in'] %}
<p>You're logged in already!</p>
{% else %}
 
 
<form action="/login" method="POST">
	<div class="login">
		<div class="login-screen">
			<div class="app-title">
				<h1>Login</h1>
			</div>
 
			<div class="login-form">
				<div class="control-group">
				<input type="text" class="login-field" value="" placeholder="username" name="username">
				<label class="login-field-icon fui-user" for="login-name"></label>
				</div>
 
				<div class="control-group">
				<input type="password" class="login-field" value="" placeholder="password" name="password">
				<label class="login-field-icon fui-lock" for="login-pass"></label>
				</div>
 
                <input type="submit" value="Log in" class="btn btn-primary btn-large btn-block" >
			    <br>
			</div>
		</div>
	</div>
</form>
 
{% endif %}
{% endblock %}

Once you restart the application this screen should appear:

Python login screen Flask — Python login screen **Flask**

Awesome, ain’t it? 🙂

What about logout?

As you may have seen, there is no logout button or functionality. Creating that is very easy. The solution proposed below is only one of the many solutions. We create a new route /logout which directs to the function logout(). This function clears the session variable and returns to the login screen.

@app.route("/logout")
def logout():
    session['logged_in'] = False
    return home()

The full code:

from flask import Flask
from flask import Flask, flash, redirect, render_template, request, session, abort
import os
 
app = Flask(__name__)
 
@app.route('/')
def home():
    if not session.get('logged_in'):
        return render_template('login.html')
    else:
        return "Hello Boss!  <a href='/logout'>Logout</a>"
 
@app.route('/login', methods=['POST'])
def do_admin_login():
    if request.form['password'] == 'password' and request.form['username'] == 'admin':
        session['logged_in'] = True
    else:
        flash('wrong password!')
    return home()
 
@app.route("/logout")
def logout():
    session['logged_in'] = False
    return home()
 
if __name__ == "__main__":
    app.secret_key = os.urandom(12)
    app.run(debug=True,host='0.0.0.0', port=4000)

Connecting a database

If you want a multi-user login system, you should add a database layer to the application. Flask does not have out of the box database support. You have to use a third party library if you want database support. In this tutorial we will use SqlAlchemy. If you do not have that installed type:

$ sudo pip install Flask-SqlAlchemy

SQLAlchemy is an SQL toolkit and object-relational mapper (ORM) for the Python programming language. It has support for MySQL, Microsoft SQL Server and many more relational database management systems. If all of these terms are unfamiliar to you, just keep reading.

Create the file tabledef.py:

from sqlalchemy import *
from sqlalchemy import create_engine, ForeignKey
from sqlalchemy import Column, Date, Integer, String
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import relationship, backref
 
engine = create_engine('sqlite:///tutorial.db', echo=True)
Base = declarative_base()
 
########################################################################
class User(Base):
    """"""
    __tablename__ = "users"
 
    id = Column(Integer, primary_key=True)
    username = Column(String)
    password = Column(String)
 
    #----------------------------------------------------------------------
    def __init__(self, username, password):
        """"""
        self.username = username
        self.password = password
 
# create tables
Base.metadata.create_all(engine)

Execute it with:

python tabledef.py

This file will create the database structure. Inside the directory you will find a file called tutorial.db. Create a file called dummy.py which will contain this code:

import datetime
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker
from tabledef import *
 
engine = create_engine('sqlite:///tutorial.db', echo=True)
 
# create a Session
Session = sessionmaker(bind=engine)
session = Session()
 
user = User("admin","password")
session.add(user)
 
user = User("python","python")
session.add(user)
 
user = User("jumpiness","python")
session.add(user)
 
# commit the record the database
session.commit()
 
session.commit()

Execute:

$ python dummy.py

This will put dummy data into your database. Finally we update our app.py

Validating the login credentials with SqlAlchemy
The next step is to write the functionality that validates the user and password exist in the database. Using SqlAlchemy we can do this (dummy/pseudo code):

@app.route('/test')
def test():
 
    POST_USERNAME = "python"
    POST_PASSWORD = "python"
 
    Session = sessionmaker(bind=engine)
    s = Session()
    query = s.query(User).filter(User.username.in_([POST_USERNAME]), User.password.in_([POST_PASSWORD]) )
    result = query.first()
    if result:
        return "Object found"
    else:
        return "Object not found " + POST_USERNAME + " " + POST_PASSWORD

We use SqlAlchemys Oject Relational Mapping (ORM). We map the objects to relational database tables and vice versa. The definition (User) is given in tabledef.py. The s.query function is where the query is build. We have two conditions: the username and password must match. The query.first() returns true if the object exists, false if it does not. This gives us this total code:

from flask import Flask
from flask import Flask, flash, redirect, render_template, request, session, abort
import os
from sqlalchemy.orm import sessionmaker
from tabledef import *
engine = create_engine('sqlite:///tutorial.db', echo=True)
 
app = Flask(__name__)
 
@app.route('/')
def home():
    if not session.get('logged_in'):
        return render_template('login.html')
    else:
        return "Hello Boss!  <a href='/logout'>Logout</a>"
 
@app.route('/login', methods=['POST'])
def do_admin_login():
 
    POST_USERNAME = str(request.form['username'])
    POST_PASSWORD = str(request.form['password'])
 
    Session = sessionmaker(bind=engine)
    s = Session()
    query = s.query(User).filter(User.username.in_([POST_USERNAME]), User.password.in_([POST_PASSWORD]) )
    result = query.first()
    if result:
        session['logged_in'] = True
    else:
        flash('wrong password!')
    return home()
 
@app.route("/logout")
def logout():
    session['logged_in'] = False
    return home()
 
if __name__ == "__main__":
    app.secret_key = os.urandom(12)
    app.run(debug=True,host='0.0.0.0', port=4000)

You can now login with any user defined in the database table.

What about security?

We have demonstrated a simple login app above. However, it is your job to properly secure it. There are many guys out there that are going to try to break into your app.

Download Flask Examples

Best practices:

Hash your database passwords. Don’t store them in plain text.
Secure the connection, use HTTPS.
Log the failed login attempts.
Use captcha to prevent brute force of logins.
Others? write them in comments.

6 thoughts on “Login authentication with Flask”

Log in to Reply
James Riley - June 3, 2017

Thanks!

Two questions about your routes/endpoints:

Confused because you are only allowing a POST request to the ‘/login’ endpoint, but your screenshot shows the login screen with the ‘/login’ URL. Shouldn’t the user be browsing to the ‘/’ endpoint to get to the login page? How could they browse to the ‘/login’ endpoint if only POST requests are allowed?

Second, with this code, when I successfully issue a POST request to the ‘/login’ endpoint and the home() function is invoked, the content that I returned by home() displays, but my URL bar indicates the ‘/login’ URL. This doesn’t really make sense, right? Once we get through the login page, shouldn’t we be at the ‘/’ address?
1. Log in to Reply
  Frank - June 4, 2017
  
  The screenshot is off, it should show the ‘/’ URL endpoint.
  
  Yes, home() should not be called there. Instead to login there we have two options, the first : a reload to the URL ‘/’. An alternative method, and perhaps better, is to turn it into an API method that you call using an AJAX request. That way there is no hard reload and you save bandwidth when the user base scales.
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Catalin Simion - February 12, 2016

Very, very useful, thanks ! 🙂
Log in to Reply
Ghis - April 8, 2015

Nice tutorial. These two variable names could cause confusion / bugs though : session & Session
Log in to Reply
Thorstens - April 2, 2015

many thanks for this usefull and newby friendly post!

/thorsten
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Anthony - March 30, 2015

Nice tutorial.