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export FLASK_APP="mainfilename.py"
export FLASK_DEBUG=1
python -m flask run --host=0.0.0.0
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export FLASK_ENV=development
# To launch the app (Needed only once)
flask run
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Windows CMD
set FLASK_ENV=development
Windows PowerShell
$env:FLASK_ENV = "development"
Linux/MacOS
export FLASK_ENV=development