[chrisshroba]

I agree that the language should lend itself to being as bug-free as possible, but you and I disagree on which case is the common, less-likely-to-cause-bugs case. I think it's normal strings, you think it's f-strings. Fair enough.

I do want to point out that only interpolating f-strings containing variables that exist is actually more likely to cause bugs because a string "{foo}" may be fine at one point, but then down the road another programmer happens to declare a global variable foo somewhere completely different, thus causing this string to change. The more important issue, though, is that f-strings contain expressions, not just variables, so you can do things like f'{1+1}' or f'There are {len(os.environ)} env vars defined.'. It's not clear what special cases for expressions inside the braces would magically become f-strings and which would not.