A minimal Flask application looks something like this:

from flask import Flask
app = Flask(__name__)

@app.route("/")
def index():
    return "Hello World!"

---

A large Flask application can separate one file into multiple files by blueprints.

Purpose

Make it easier for others to maintain the application.

Folder Structure of Large Application

/app
    /templates
    /static
    /views
        __init__.py
        index.py
    app.py

views/index.py

from flask import Blueprint, render_template

index_blueprint = Blueprint('index', __name__)

@index_blueprint.route("/")
def index():
    return "Hello World!"

app.py

from flask import Flask
from views.index import index_blueprint

application = Flask(__name__)
application.register_blueprint(index_blueprint)

Run application

$ export FLASK_APP=app.py
$ flask run