I’m having trouble understanding this - Can someone please help out with an example use case for this? It seems like before with an f string we had instant evaluation, now with a t string we control the evaluation, why would we further delay evaluation - Is it just to utilise running a function on a string first (i.e. save a foo = process(bar) line?)
You don't completely control the evaluation.
From the PEP:
> Template strings are evaluated eagerly from left to right, just like f-strings. This means that interpolations are evaluated immediately when the template string is processed, not deferred or wrapped in lambdas.
If one of the things you are interpolating is, as a silly example, an invocation of a slow recursive fibonacci function, the template string expression itself (resulting in a Template object) will take a long while to evaluate.
Are you saying that calling:
template = t”{fib_slow()}”
What t-strings offer over f-strings is the ability to control how the final string is put together from the calculated results. And the t-string doesn't have `__str__` - you have to explicitly pass it to a named formatting function to get a string. (There is `__repr__, but that's for debugging, of course.) So you can potentially reuse the same Template instance (created from the t-string) multiple times in different contexts to format the same information different ways.