Flask Framework Cookbook - Second Edition

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It is also possible to create custom errors as per the application requirements and bind them to error codes and custom error screens. This can be done as follows:

class MyCustom404(Exception): 
    pass 
 
@app.errorhandler(MyCustom404) 
def special_page_not_found(error): 
    return render_template("errors/custom_404.html"), 404