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How to Flash Messages on Frontend using Flask


The Flask flash() function is an efficient way to display temporary messages to the user. This can be used to display a variety of messages, including error, notification, warning, and status messages.

By the end of this article, you’ll be able to learn:

  • How to use the flash() function
  • Flashing messages on the frontend
  • Flashing messages with categories
  • Filtering flash messages based on categories
  • Best practices for effectively using flashed messages


Before you dive into this tutorial on flashing messages with Flask, make sure you have a fundamental understanding of the following concepts:

  • Familiarity with Flask, including route handling, templates, and basic application structure.
  • A basic understanding of HTML markup is necessary, as you’ll be working with HTML templates to render the flashed messages on the frontend.
  • Understanding of Jinja2 templating engine, which is integrated with Flask for rendering dynamic content within HTML templates.

How to Use flash() Method

As previously mentioned, the flash() function is used to display messages to the users after the specific request is made. Its primary goal is to offer relevant feedback, which enhances the user experience.

The flash() function accepts two parameters:

  • message: The message to display to the user.
  • category: Specifies the message category. This is an optional parameter.

Using flash() within Flask App

This section describes how to integrate the message flashing functionality into the Flask application and display it on the frontend.

Importing flash and other required modules

Create an file and import the flash from flask, along with other necessary modules, into the file.

You could also import these modules in a single line as well.

Flask App Setup

An instance of Flask is created and stored within the app variable to initialize the Flask application.

The Flask application’s secret key is assigned as "Sachin" using the secret_key attribute of the app object. This key will serve for session management.

Note: It’s essential to store the secret key in a secure environment, rather than hardcoding it directly within the application.

Implementing flash() Function within View Functions

The route "/info" is created to manage both GET and POST requests, and it is linked to the add_info() view function.

Within the add_info() view function, it checks whether the request is a POST request, and then retrieves the values from the "Name" and "Profession" form fields.

Following that, it examines if any of the fields are empty. If they are, a message "Invalid: Every field is required." is displayed using the flash() function, and the route remains the same. Conversely, if none of the fields are empty, a message "Success: Info added successfully." is flashed using the flash() function, and the route continues for further information input.

However, this won’t immediately display the message on the frontend. To achieve that, you must use the get_flashed_messages() function within the template.

Implementing get_flashed_messages() Function within Message Template

Create a file named message.html within the templates directory in your project.

The get_flashed_messages() function retrieves the flashed messages and preserves them in the session. The function is invoked using the with statement and its output is stored in the msg variable.

The {% if msg %} statement evaluates whether any flashed messages are stored in the msg variable. If such messages exist, the {% for message in msg %} block iterates through each message and renders them using {{ message }} within the Bootstrap alert component.

Ultimately, the loop terminated using {% endfor %}, the conditional block is terminated using {% endif %}, and the with statement block is terminated with {% endwith %}.

Now you can include the message template in any of your templates to display the messages.

Preparing the Frontend

Create two HTML files in the templates folder with the names base.html and info.html.


This file will contain the basic HTML layout with Bootstrap CSS and JavaScript.


This file will hold a form for inputting information and includes the message.html template to display flashed messages.

Running the Flask Application

When you execute the file and evaluate the frontend, you’ll observe flashed messages upon making requests.

Here’s a preview of the application hosted on the server using the "/info" route.

Flask Webpage preview
Flask Webpage preview

When submitting the form without completing the “Profession” field, the application displays the relevant flashed message.

Invalid message displayed
Invalid message displayed

When the form is submitted with all fields completed, the relevant message is displayed.

Success message displayed
Success message displayed

Flashing Messages with Categories

Each message serves a unique purpose. Some aim to caution users, while others indicate successful task completion. Assigning categories to the messages makes them distinct from one another and provides a better user experience.

The get_flashed_messages() function offers a with_categories parameter that can be set to True. Doing so enables you to utilize the category assigned to the message in the flash() function.

Assigning Categories to the Messages

To assign categories, make use of the flash() function. Adjust your add_info() view function in the file as demonstrated in the following code:

The categories danger and success are assigned to both messages. These categories correspond to Bootstrap CSS classes. Specifically, “danger” signifies errors, while “success” signifies successful completion.

Retrieving Categories and Applying Styles

To utilize categories from flashed messages in templates, retrieve them using get_flashed_messages(with_category=True) within the template. Then, iterate through them in a similar manner as you did with regular messages.

Adjust your message.html template by implementing the provided code as provided below:

The categories are retrieved and utilized to apply specific CSS properties to the Bootstrap alert component, tailored to the respective request.

With these changes in place, run the application again. You’ll observe messages being highlighted in varying colors that correspond to the specific request.

The illustration below depicts how the message’s appearance differs between invalid and successful form submissions, respectively.

Invalid message highlighted with red
Invalid message highlighted with red
Success message highlighted with green
Success message highlighted with green

Another Use Case of Categories

Categories could also serve as prefixes to messages, offering another practical application. Insert the provided attribute inside the <div> block of the Bootstrap alert component.

The provided code will add the corresponding category as a prefix to the actual message. This will result in an appearance similar to what’s displayed in the image below.

Used category as a prefix
Used category as a prefix

Filtering Flash Messages Based on Categories

To filter messages based on categories, you can utilize the category_filter parameter within the get_flashed_messages() function in the template. This parameter accepts a list of message categories.

Create a new file named filtered_message.html within the templates folder. Then, insert the provided code snippet into this file.

The code snippet mentioned above retrieves and filters messages based on the “danger” category using get_flashed_messages(category_filter=["danger"]). If there are flashed messages matching this category, the code iterates through them and displays them when the relevant request is made.

To display only messages with the “danger” category on the frontend, include the filtered_message.html template within info.html.

Best Practices for Using Flashed Messages in Flask

Flashed messages are highly valuable for enhancing the user experience through prompt feedback. To maximize their impact, it’s important to implement the following practices.

  • Provide clear and concise messages, and avoid displaying messages on every request to prevent overwhelming users.
  • When presenting error messages, include instructions on how users can address the issue.
  • Assign suitable categories to messages that accurately reflect their content and purpose.
  • As flashed messages are temporary, consider adding a dismissal button to allow users to remove them once they are no longer needed.
  • Refrain from revealing any sensitive details in messages, as this could potentially compromise the security of your application.


Flashing messages on an application helps to notify users about some action performed, be it task completion, error, or warning. It enhances the user experience but only if it is used keeping in mind the best practices.

In this article, the secret key is hardcoded in the application, which is not a good practice. Store sensitive details like secret keys within the protected environment and refrain from revealing them in messages.

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That’s all for now

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