Why --host 0.0.0.0 in Python servers is not always the right choice

A common Uvicorn startup flag, --host 0.0.0.0, binds a server to all available network interfaces, but its appropriateness depends entirely on the deployment environment. On a local virtual machine with no proxy in front, the wildcard bind is necessary to make the service reachable from outside the VM. On a production server with a public IP, binding to 0.0.0.0 exposes the application directly to the internet, making loopback binding behind a reverse proxy like Caddy the safer architectural choice. Inside a Docker container, however, loopback binding fails because each container operates in its own isolated network namespace, making 0.0.0.0 necessary again — with actual external exposure controlled separately via the runtime's port mapping. The key insight is that the bind address governs reachability, not security in isolation, and the correct value shifts based on what sits in front of the process.