Never checked? They're statically checked. Also, tooling like https://pydantic-docs.helpmanual.io/ https://pydantic-docs.helpmanual.io/ can do runtime checking

Never checked? They're statically checked. Also, tooling like https://pydantic-docs.helpmanual.io/ https://pydantic-docs.helpmanual.io/ can do runtime checking for important parts of your app or you can use this https://github.com/agronholm/typeguard https://github.com/agronholm/typeguard to enforce all types at runtime (although I haven't measured the performance impact, probably something to do in a separate environment than production?).