Flask's built-in server always 404 with SERVER_NAME set

Question

Here is a minimal example:

from flask import Flask

app = Flask(__name__)
app.config['DEBUG'] = True
app.config['SERVER_NAME'] = 'myapp.dev:5000'


@app.route('/')
def hello_world():
    return 'Hello World!'

@app.errorhandler(404)
def not_found(error):
    print(str(error))
    return '404', 404


if __name__ == '__main__':
    app.run(debug=True)

If I set SERVER_NAME, Flask would response every URL with a 404 error, and when I comment out that line, it functions correctly again.

/Users/sunqingyao/Envs/flask/bin/python3.6 /Users/sunqingyao/Projects/play-ground/python-playground/foo/foo.py
 * Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)
 * Restarting with stat
 * Debugger is active!
 * Debugger PIN: 422-505-438
127.0.0.1 - - [30/Oct/2017 07:19:55] "GET / HTTP/1.1" 404 -
404 Not Found: The requested URL was not found on the server.  If you entered the URL manually please check your spelling and try again.

Please note that this is not a duplicate of Flask 404 when using SERVER_NAME, since I'm not using Apache or any production web server. I'm just dealing with Flask's built-in development server.

I'm using Python 3.6.2, Flask 0.12.2, Werkzeug 0.12.2, PyCharm 2017.2.3 on macOS High Sierra, if it's relevant.

Answer 1

Sometimes I find Flask's docs to be confusing (see the quotes above by @dm295 - the meaning of the implications surrounding 'SERVER_NAME' is hard to parse). But an alternative setup to (and inspired by) @Dancer Phd's answer is to specify the 'HOST' and 'PORT' parameters in a config file instead of 'SERVER_NAME'.

For example, let's say you use this config strategy proposed in the Flask docs, add the host & port number like so:

class Config(object):
    DEBUG = False
    TESTING = False
    DATABASE_URI = 'sqlite://:memory:'
    HOST = 'http://localhost' #
    PORT = '5000'

class ProductionConfig(Config):
    DATABASE_URI = 'mysql://user@localhost/foo'

class DevelopmentConfig(Config):
    DEBUG = True

class TestingConfig(Config):
    TESTING = True