How is this advantageous over str.format?

> IIUC it is just a locally bound object invalid outside of its scope, like you can't pass it around generically and have the vars substituted without wrapping in a function call which then leads to same thing.

You can indeed pass t-strings around arbitrarily, it's just that the vars are eagerly evaluated at construction time. Which is fine IMHO, because a lazy t-string would just be way too much magic for one bit of syntax (and you can always use a lambda if that's really what you wanted).