Error handling within a flask application

Any website should somehow be able to deal with unexpected errors. In this blog post, I will describe how I handle errors within my Flask application, which follows this great tutorial.

In Flask, it is quite easy to register error handlers which re-direct to a custom error page like this

from flask import render_template

@app.errorhandler(404)
def page_not_found(e):
    return render_template('404.html'), 404

However, rather than going through the list of possible errors and creating a route and page for each, one can also create a general error handler which handles all errors. To do that you have to register a general error handler in your app like this

from werkzeug.exceptions import default_exceptions

def init_app(app):
    ''' Function to register error_handler in app '''
    for exception in default_exceptions:
        app.register_error_handler(exception, error_handler)

    app.register_error_handler(Exception, error_handler) 

Calling the init_app(app) function will register the error_handler() function for all exceptions in the default_exceptions list of the werkzeug package, which should cover all errors except for some exotic ones. Now we just have to write the error_handler() function.

def error_handler(error):
    ''' Catches all errors in the default_exceptions list '''
    msg = "Request resulted in {}".format(error)
    if current_user.is_authenticated:
        current_app.logger.error(msg, exc_info=error)
    else:
        current_app.logger.warning(msg, exc_info=error)

    if isinstance(error, HTTPException):
        description = error.get_description(request.environ)
        code = error.code
        name = error.name
    else:
        description = ("We encountered an error "
                       "while trying to fulfill your request")
        code = 500
        name = 'Internal Server Error'

    templates_to_try = ['errors/error{}.html'.format(code), 'errors/generic_error.html']
    return render_template(templates_to_try,
                           code=code,
                           name=Markup(name),
                           description=Markup(description),
                           error=error), code

This function first checks whether the error has been caused by an authenticated user, and if so it writes an error message to the logfile. If the user was not authenticated, it only writes a warning message. The reason for this distinction is that robots cause all sorts of 404 errors in my app, which I don't care much about, but if a user causes an exception, I definitely want to know about it.

In the next step, the handler extracts all information it can get from the error message and then renders a template. The render_template function will go through the templates_to_try list until it finds an existing template, so this way you can register custom pages for certain errors but if a custom error does not exist, it will just render the general error page, which in my case looks like this

{% extends "master.html" %}

{% block title %}Error{% endblock %}

{% block description %}
<meta name="description" content="Error message.">
{% endblock %}

{% block body %}

    <div id="navbar_wrapper">
        <div id="site_content">
            <div class="container">
                <div class="col-xs-12 col-sm-9 col-md-10 col-lg-10">
                    <br>
                    <h1>{{ code }}:{{ name }}</h1>
                    <p>{{description}}</p>
                    <p>The administrator has been notified. Sorry for the inconvenience!</p>

                    <button class="btn btn-primary" onclick="history.back(-1)">Go Back</button> 

                </div>
            </div>
        </div>
    </div>

{% endblock %}

The advantage of such a custom error page is that you can add a back button, to get the users back on your page if you haven't alienated them yet with the error.

I put the entire example on Github. Let me know if you have any comments or questions.
cheers
Florian