Flask register_blueprint error (Python)

Question

I have a project on Flask. And when I include Blueprint on my Flask app I have errors. The code is from a book, Miguel Grinberg about Flask.

Project tree:

.
├── app
│   ├── __init__.py
│   ├── main
│   │   ├── errors.py
│   │   ├── __init__.py
│   │   └── views.py
│   ├── static
│   │   ├── static_files
│   └── templates
│       └── html_files
├── config.py
├── manage.py

create_app() in app/__init__.py

def create_app(config_name):
    app = Flask(__name__)
    app.config.from_object(config[config_name])
    config[config_name].init_app(app)
    bootstrap.init_app(app)
    db.init_app(app)

    from .main import main as main_blueprint
    app.register_blueprint(main_blueprint)   

    return app

listing app/main/__init__.py

from flask import Blueprint

main = Blueprint('main', __name__)

from . import views, errors

Passenger row process output

Traceback (most recent call last):
  File "/opt/passenger/passenger-4.0.57/helper-scripts/wsgi-loader.py", line 320, in <module>
    app_module = load_app()
  File "/opt/passenger/passenger-4.0.57/helper-scripts/wsgi-loader.py", line 61, in load_app
    return imp.load_source('passenger_wsgi', startup_file)
  File "/home/m/mallts/dev.wget-studio.ru/myenv/lib/python3.4/imp.py", line 171, in load_source
    module = methods.load()
  File "<frozen importlib._bootstrap>", line 1220, in load
  File "<frozen importlib._bootstrap>", line 1200, in _load_unlocked
  File "<frozen importlib._bootstrap>", line 1129, in _exec
  File "<frozen importlib._bootstrap>", line 1471, in exec_module
  File "<frozen importlib._bootstrap>", line 321, in _call_with_frames_removed
  File "passenger_wsgi.py", line 16, in <module>
    from manage import app as application
  File "/home/m/mallts/dev.wget-studio.ru/fikls/manage.py", line 7, in <module>
    app = create_app('default') #(os.getenv('FLASK_CONFIG') or 'default')
  File "/home/m/mallts/dev.wget-studio.ru/fikls/app/__init__.py", line 19, in create_app
    app.register_blueprint(main_blueprint)   
  File "/home/m/mallts/dev.wget-studio.ru/myenv/lib/python3.4/site-packages/flask/app.py", line 62, in wrapper_func
    return f(self, *args, **kwargs)
  File "/home/m/mallts/dev.wget-studio.ru/myenv/lib/python3.4/site-packages/flask/app.py", line 889, in register_blueprint
    blueprint.register(self, options, first_registration)
  File "/home/m/mallts/dev.wget-studio.ru/myenv/lib/python3.4/site-packages/flask/blueprints.py", line 153, in register
    deferred(state)
  File "/home/m/mallts/dev.wget-studio.ru/myenv/lib/python3.4/site-packages/flask/blueprints.py", line 128, in wrapper
    func(state)
  File "/home/m/mallts/dev.wget-studio.ru/myenv/lib/python3.4/site-packages/flask/blueprints.py", line 399, in <lambda>
    self.name, code_or_exception, f))
  File "/home/m/mallts/dev.wget-studio.ru/myenv/lib/python3.4/site-packages/flask/app.py", line 62, in wrapper_func
    return f(self, *args, **kwargs)
  File "/home/m/mallts/dev.wget-studio.ru/myenv/lib/python3.4/site-packages/flask/app.py", line 1090, in _register_error_handler
    'It is currently not possible to register a 500 internal ' \
AssertionError: It is currently not possible to register a 500 internal server error on a per-blueprint level.

My app/main/errors.py

from flask import render_template
from . import main 

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

@main.errorhandler(500)
def internal_server_error(e):
    return render_template('500.html'), 500

My app/main/views.py

from flask import render_template, url_for, session, redirect
from . import main 

@main.route('/')
def index():
    return render_template('index.html')

Answer 1

I just ran into this same issue with another application that I was refactoring to use blueprints. There's no need for a workaround, since Flask offers a decorator for just such a case: app_errorhandler. It works exactly like errorhandler in that it registers an error route for the entire app, but it works with blueprints. Like so:

from flask import render_template
from . import main 

@main.app_errorhandler(404)
def page_not_found(e):
    return render_template('404.html'), 404

@main.app_errorhandler(500)
def internal_server_error(e):
    return render_template('500.html'), 500

Grinberg's book, Flask Web Development -- an excellent read -- uses this decorator for error pages registered on the main blueprint. You can review the companion code here. I missed it at first, too. :P