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This uses UDS (Unix Domain Sockets) to communicate, which are file-like and can be mounted from the host filesystem inside a container.

As long as the socket is writable, the rest of the filesystem(s) don't have to be. Same goes for the networking, which can be very isolated and restricted.

It should be possible to do what you want.

You could use starlark but I don't have context for what you're using Python for specifically. Starlark has stronger hermetic guarantee.

chroot ? :)

Pretty sure chroot is not an isolation method, it's way too easy to bypass when used as such