Custom Error Pages in python Flask application

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flask_error_pages

Skill - Custom Error Pages in python Flask application

Table of Contents

Skills Required

Please make sure to have all the skills mentioned above to understand and execute the code mentioned below. Go through the above skills if necessary for reference or revision

  • In this post we will learn how to create custom error pages in a flask application and simulate errors in a flask application

flask_custom_error_poster

Use cases

We can serve custom error pages for

  • Providing additional content in the error pages like links to other pages etc
  • Custom styling of error pages

Sample Server setup

The following python code serves a page home.html from the templates folder at the root URL. Also the theme.css is linked from the static/styles folder

# server.py file
from flask import Flask, render_template

app = Flask(__name__)

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

app.run(host="0.0.0.0", port=50100, debug=True)

templates/home.html file

<!-- templates/home.html file -->
<!DOCTYPE html>
<html lang="en">

<head>
    <link rel="stylesheet" href="{{url_for('static', filename='styles/theme.css')}}">
    <title>Home</title>
</head>

<body>
    <h2>Hello World!!!</h2>
</body>

</html>

static/styles/theme.css file

/* static/styles/theme.css file */
body {
    color: rgb(201, 209, 217);
    background-color: rgb(13, 17, 23);
}

a {
    color: rgb(83, 189, 235);
}

‘register_error_handler’ method or ‘errorhandler’ decorator for custom error pages

# server.py file
from flask import Flask, render_template

app = Flask(__name__)

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

def genericErrorHandler(error):
    return render_template("message.html", title=error.name, message=error.description), error.code

for errCode in [400, 401, 403, 404]:
    app.register_error_handler(errCode, genericErrorHandler)


@app.errorhandler(500)
@app.errorhandler(501)
def serverError(error):
    return render_template("message.html", title="Internal Server Error", message="Some Internal Error occured..."), error.code


app.run(host="0.0.0.0", port=50100, debug=True)
  • As shown in the above code, using the app.register_error_handler method or errorhandler decorator, we can render error page for a specific HTTP error codes
  • The error handler function takes in an error object. Each error object contains code, name, description properties
  • A template located at templates/message.html is rendered while handling the error

templates/message.html file

<!-- templates/message.html file -->
<!DOCTYPE html>
<html lang="en">

<head>
    <link rel="stylesheet" href="{{url_for('static', filename='styles/theme.css')}}">
    <title>{{ title }}</title>
</head>

<body>
    <h2>{{ title }}</h2>
    <h4>{{ message }}</h4>
    <a href="{{ url_for('index') }}">Click here</a> to go to Home page
</body>

</html>
  • This is the template for our custom error page
  • We are able to control the styling
  • Link to home page is also added

Default error handler for all HTTP errors

  • Instead of explicitly specifying the error handler for each HTTP error code, we can define a default error handler for all HTTP exceptions as shown below
# server.py file
from flask import Flask, render_template
from werkzeug.exceptions import HTTPException

app = Flask(__name__)

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


@app.errorhandler(HTTPException)
def handleException(error):
    return render_template("message.html", title=error.name, message=error.description), error.code


@app.errorhandler(500)
def serverError(error):
    return render_template("message.html", title="Server Error", message="Oops, some error occured..."), error.code


app.run(host="0.0.0.0", port=50100, debug=True)
  • In the above example, using the @app.errorhandler(HTTPException) decorator, all the HTTP errors are handled by the handleException method by default
  • However for error code 500, since we have specifically mentioned a method with decorator @app.errorhandler(500), the serverError method will be called for handling HTTP error with status code 500

Simulate HTTP errors using ‘abort’ method

  • To check the rendering of error pages, we can throw HTTP errors using the abort function from the flask module as shown below
from flask import Flask, abort, render_template
from werkzeug.exceptions import HTTPException

app = Flask(__name__)

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


@app.route('/simulate500')
def simulate500():
    return abort(500)


@app.errorhandler(HTTPException)
def handleException(error):
    return render_template("message.html", title=error.name, message=error.description), error.code

app.run(host="0.0.0.0", port=50100, debug=True)
  • In the above python server code, visiting the URL /simulate500 will call the abort(500) method which throws the HTTP exception with status code 500

Video

The video for this post can be seen here

References

Location: Mumbai, Maharashtra, India

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