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How to handle Exceptions and Message Flashing using Flask

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by vsasikalabe dot icon Updated: Mar 24, 2023

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Feedback provides good applications and user interfaces. With the flashing system, Flask provides a straightforward way to give feedback to a user. The flashing system records a message at the end of a request. It causes message flashing to fail silently. Note that browsers and sometimes web servers limit cookie sizes. Flashing messages are too large for session cookies. While handling a request, Flask catches an exception. It is first looked up by code. If there is no registered handler for the code, Flask looks up the error by its class; the chosen is the most specific handler.  


It accepts the following parameters. 

  • Message: The message is flashed to the user. 
  • Category: It is an optional parameter. This may represent any error, information, or warning. 

Using the flask module's flash() method, the messages are generated in the flask script. The method get_flashed_messages() is called in the HTML template for extracting the messages in the template from the session. 

The syntax: get_flashed_messages(with_categories, category_filter)   

This method flashed all the messages from the session. When with_categories are True, the return value will be a list of tuples in the form (category, message) instead. By default, just the messages are returned. Exception handling can be done by registering error handlers.  

In Flask, the following points are vital to Handle Exceptions: 

  • Enables the smooth flow of programs- Handling abnormalities permit your code to run smoothly regardless of errors. 
  • Security purposes- Handling exceptions help to secure your programs since you are unlikely to leak or expose your code's configurations to random people. 


Here is an example of how to Handling Exceptions and Message Flashing using Flask: 

Preview of the output that you will get on running this code from your IDE.

Code

In this solution we used flask library of python.

Flask Exception Handling and Message Flashing

PythondotLines of Code : 53dotLicense : Strong Copyleft (CC BY-SA 4.0)

Dependent Libraries : 
from flask import Flask, flash, redirect, render_template, \
     request, url_for

server = Flask(__name__)
server.secret_key = b'_5#y2L"F4Q8z\n\xec]/'


def add_stuff(x,y):
    if isinstance(x, int) and isinstance(y, int):
        z = x + y
        return z
    else:
        raise ValueError("Not integers")       


@server.route('/math')
def foo():
    a = 1
    b = "some string"
    out = None
    try:
        out = add_stuff(a,b)
    except Exception as e:
        flash(str(e))
    if out is not None:
        return render_template('math.html', out=out)
    else:
        return render_template('math.html')

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
</head>

<body>
    {% with messages = get_flashed_messages() %}
      {% if messages %}
        <ul>
        {% for message in messages %}
          <li>{{ message }}</li>
        {% endfor %}
        </ul>
      {% endif %}
    {% endwith %}

{% if out %}
    {{out}}
{% endif %}
</body>
</html>

Instructions

Follow the steps carefully to get the output easily.

  1. Download and Install the PyCharm Community Edition on your desktop.
  2. Install flask on your IDE from python interpreter in setting options.
  3. Create new python file on your IDE.
  4. Copy the snippet using the 'copy' button and paste(till line no 28) it in your python file.
  5. By clicking on the topic we get html code ,copy and paste this code into html file(math.html).
  6. Add app.run statement to the end.
  7. Run the current file to generate the output.


I hope you found this useful. I have added the link to dependent library, version information in the following sections.


I found this code snippet by searching for ' Flask Exception Handling and Message Flashing ' in kandi. You can try any such use case!

Environment Tested

I tested this solution in the following versions. Be mindful of changes when working with other versions.

  1. PyCharm Community Edition 2022.3.1
  2. The solution is created in Python 3.11.1 Version
  3. flask 2.2.2 Version


Using this solution, we can do Flask Exception Handling and Message Flashing.This process also facilities an easy to use, hassle free method to create a hands-on working version of code in python which would help us to do Flask Exception Handling and Message Flashing.

Dependent Libraries

flaskby pallets

Python doticonstar image 63300 doticonVersion:2.2.5doticon
License: Permissive (BSD-3-Clause)

The Python micro framework for building web applications.

Support
    Quality
      Security
        License
          Reuse

            flaskby pallets

            Python doticon star image 63300 doticonVersion:2.2.5doticon License: Permissive (BSD-3-Clause)

            The Python micro framework for building web applications.
            Support
              Quality
                Security
                  License
                    Reuse

                      If you do not have flask library that is required to run this code, you can install it by clicking on the above link.

                      You can search for any dependent library on kandi like flask.

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