how do I run a python flask app code example

Example 1: simple flask app

# Extremely simple flask application, will display 'Hello World!' on the screen when you run it
# Access it by running it, then going to whatever port its running on (It'll say which port it's running on).
from flask import Flask
app = Flask(__name__)

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

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

Example 2: run flask app from command line

# For bash do this:
set FLASK_APP=<app_name>

# For powershell do this:
$env FLASK_APP=<app_name>

# Run it like this
flask run