I like the implementation, but it looks like nobody has pointed out that Python already has a built-in string template system [1]. Granted, it's not quite as simple to use, but I have been using it for a while.

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

edit: this was mentioned by milesrout in https://news.ycombinator.com/item?id=43649607