flask server_name issue

demo.py

from flask import Flask, url_for
import pytest


@pytest.fixture
def app():
    app = Flask(__name__)

    @app.route('/')
    def view():
        return url_for('view', _external=True)

    return app


@pytest.fixture()
def client(app):
    return app.test_client()


def root_url(app: Flask, req_url=None):
    if req_url is None:
        with app.app_context():
            return url_for('view')

    resp = app.test_client().get(req_url)
    if resp.status_code == 200:
        return resp.text

    return resp.status_code


def test_default(app: Flask):
    assert root_url(app, '/') == 'http://localhost/'
    assert root_url(app, 'http://example.com/') == 'http://example.com/'
    assert root_url(app, 'http://www.example.com/') == 'http://www.example.com/'
    assert root_url(app, 'http://bar.example.com/') == 'http://bar.example.com/'
    assert root_url(app, 'http://flaskr.com/') == 'http://flaskr.com/'

    with pytest.raises(RuntimeError) as excinfo:
        root_url(app)

    assert excinfo.match(
        "Unable to build URLs outside an active request without 'SERVER_NAME' configured",
    )


def test_servername(app: Flask):
    app.config['SERVER_NAME'] = 'www.example.com'

    assert root_url(app, '/') == 'http://www.example.com/'
    assert root_url(app, 'http://example.com/') == 404
    assert root_url(app, 'http://www.example.com/') == 'http://www.example.com/'
    assert root_url(app, 'http://bar.example.com/') == 404
    assert root_url(app, 'http://flask.com/') == 404

    assert root_url(app) == 'http://www.example.com/'


def test_currently_impossible(app: Flask):
    # Primary canonical app domain
    assert root_url(app, 'http://www.example.com/') == 'http://www.example.com/'

    # Acceptable alternate domain
    assert root_url(app, 'http://www.demo.com/') == 'http://www.demo.com/'

    # Servable legacy domain, but use canonical domain for external URLs
    assert root_url(app, 'http://www.demo.com/') == 'http://www.example.com/'

    # Use canonical domain
    assert root_url(app) == 'http://www.example.com/'