Am I missing something, or is this a fancier string.Template [0]? Don't get me wrong, it looks very useful, especially the literals.

[0]: https://docs.python.org/3/library/string.html#template-strin...

string.Template does fundamentally the same sort of thing as str.format, or str.__mod__ (i.e. the % operator on strings). It was in a sense a prototype for str.format, introduced way back in Python 2.4 (https://peps.python.org/pep-0292/). It does create an instance of a separate type, but it neither pre-parses the string (the substitution is implemented using regexes that reprocess the original string each time) nor eagerly evaluates the values to substitute. Further, in order to change how the formatting is done, you'd have to subclass (with the new system, you just write a function that accepts the Template as a parameter).