Google Choice: Flask – Message Flashing

Welcome to GoogleDG – your one-stop destination for free learning resources, guides, and digital tools.

At GoogleDG, we believe that knowledge should be accessible to everyone. Our mission is to provide readers with valuable ebooks, tutorials, and tech-related content that makes learning easier, faster, and more enjoyable.

What We Offer:

📘 Free & Helpful Ebooks – covering education, technology, self-development, and more.
💻 Step-by-Step Tutorials – practical guides on digital tools, apps, and software.
🌐 Tech Updates & Tips – simplified information to keep you informed in the fast-changing digital world.
🎯 Learning Support – resources designed to support students, professionals, and lifelong learners.
✔ Latest world News

Our Vision

To create a digital knowledge hub where anyone, from beginners to advanced learners, can find trustworthy resources and grow their skills.

Why Choose Us?

✔ Simple explanations of complex topics
✔ 100% free access to resources
✔ Regularly updated content
✔ A community that values knowledge sharing

We are continuously working to expand our content library and provide readers with the most useful and relevant digital learning materials.

📩 If you’d like to connect, share feedback, or suggest topics, feel free to reach us through the Contact page.

Friday, February 17, 2017

Flask – Message Flashing

A good GUI based application provides feedback to a user about the interaction. For example, the desktop applications use dialog or message box and JavaScript uses alerts for similar purpose.
Generating such informative messages is easy in Flask web application. Flashing system of Flask framework makes it possible to create a message in one view and render it in a view function called next.

A Flask module contains flash() method. It passes a message to the next request, which generally is a template.

flash(message, category)

Here,

message parameter is the actual message to be flashed.
category parameter is optional. It can be either ‘error’, ‘info’ or ‘warning’.

In order to remove message from session, template calls get_flashed_messages().

get_flashed_messages(with_categories, category_filter)

Both parameters are optional. The first parameter is a tuple if received messages are having category. The second parameter is useful to display only specific messages.
The following flashes received messages in a template.

{% with messages = get_flashed_messages() %}
   {% if messages %}
      {% for message in messages %}
         {{ message }}
      {% endfor %}
   {% endif %}
{% endwith %}

Let us now see a simple example, demonstrating the flashing mechanism in Flask. In the following code, a ‘/’ URL displays link to the login page, with no message to flash.

@app.route('/')
def index():
   return render_template('index.html')

The link leads a user to ‘/login’ URL which displays a login form. When submitted, the login() view function verifies a username and password and accordingly flashes a ‘success’ message or creates ‘error’ variable.

@app.route('/login', methods = ['GET', 'POST'])
def login():
   error = None
   
   if request.method == 'POST':
      if request.form['username'] != 'admin' or \
         request.form['password'] != 'admin':
         error = 'Invalid username or password. Please try again!'
      else:
         flash('You were successfully logged in')
         return redirect(url_for('index'))
   return render_template('login.html', error = error)

In case of error, the login template is redisplayed with error message.

Login.html

<!doctype html>
<html>
   <body>
   
      <h1>Login</h1>
      
      {% if error %}
      <p><strong>Error:</strong> {{ error }}
      {% endif %}
      
      <form action = "" method = post>
         <dl>
            <dt>Username:</dt>
            
            <dd>
               <input type = text name = username 
                  value = "{{request.form.username }}">
            </dd>
            
            <dt>Password:</dt>
            <dd><input type = password name = password></dd>
         </dl>
         <p><input type = submit value = Login></p>
      </form>
      
   </body>
</html>

On the other hand, if login is successful, a success message is flashed on the index template.

Index.html

<!doctype html>
<html>
   
   <head>
      <title>Flask Message flashing</title>
   </head>
   <body>
   
      {% with messages = get_flashed_messages() %}
         {% if messages %}
            <ul>
               {% for message in messages %}
               <li<{{ message }}</li>
               {% endfor %}
            </ul>
         {% endif %}
      {% endwith %}
  
      <h1>Flask Message Flashing Example</h1>
      <p>Do you want to <a href = "{{ url_for('login') }}">
         <b>log in?</b></a></p>
      
   </body>
</html>

A complete code for Flask message flashing example is given below −

Flash.py

from flask import Flask, flash, redirect, render_template, request, url_for
app = Flask(__name__)
app.secret_key = 'random string'

@app.route('/')
def index():
   return render_template('index.html')

@app.route('/login', methods = ['GET', 'POST'])
def login():
   error = None
   
   if request.method == 'POST':
      if request.form['username'] != 'admin' or \
         request.form['password'] != 'admin':
         error = 'Invalid username or password. Please try again!'
      else:
         flash('You were successfully logged in')
         return redirect(url_for('index'))
   
   return render_template('login.html', error = error)

if __name__ == "__main__":
   app.run(debug = True)