If someone access a url that you have not implemented, Flask will return a 404 error, but it won't look nice. We would like to create customized 404 page.

Sometime in a route that exists we might want to return a 404 error. For example we have a route that has a variable part for some content. (e.g. the ISBN number of a book.) Like this: /book/ISBN If the user accesses an ISBN number that does not exist our route will still be executed but now we would like to return a 404 error.

Create a customized 404 page in Flask

Add this code snippet to your Flask application and create the template file "404.html" as you wish. 

@app.errorhandler(404)
def page_not_found(error):
   return render_template('404.html', title = '404'), 404

Return 404 in the middle of a route

@app.route("/book/<isbn>")
def book(isbn)
    ...
    if isbn not in all_the_isbns:
        abort(404)
    ...
    return render_template('page.html')