__init__.py

import os
from flask import ( Flask,
Blueprint, flash, g, redirect, render_template, request, session, url_for)

def create_app(test_config=None):
    app = Flask(__name__, instance_relative_config=True)
    app.config.from_mapping(SECRET_KEY='dev',  TESTING=True)

    if test_config is None:
        app.config.from_pyfile('config.py', silent=True)
    else:
        app.config.from_mapping(test_config)

    try:
        os.makedirs(app.instance_path)
    except OSError:
        pass

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

    return app