pip install -r requirements.txt
For Linux and Mac:
export FLASK_APP=flaskr
export FLASK_ENV=development
For Windows cmd, use set instead of export:
set FLASK_APP=flaskr
set FLASK_ENV=development
For Windows PowerShell, use $env: instead of export:
$env:FLASK_APP = "flaskr"
$env:FLASK_ENV = "development"
flask init-db
flask run
Then visit http://127.0.0.1:5000/