way to convert string to f-string

[zcutlip]

I waste so many keystrokes by starting a string, realizing I want to format an object in the middle of it, skipping back to the beginning to prepend f, then returning to my typing position.

It'd be awesome if there was a keyboard shortcut for this, or, even better, if the need for an f-string could be automatically detected (e.g., the presence of curly braces and an in-scope object is referenced).

[rgov]

I like the idea of just typing a curly bracket inside a string to convert it to an f-string, as long as this is pushed to the undo stack and I can Cmd-Z quickly to undo it. I type literal curly brackets in my strings infrequently enough it wouldn't bug me much (no more than other kinds of quote/bracket auto-complete).

I think the best way to do this would be to trigger on } and apply a few rules:

1. Scan backwards for a matching { in the same string. If none is found, do not convert to an f-string. This avoids a number of false positives like print("int main() { return ", code, "}").

2. Scan forwards and backwards for another } in the same string. If one is found, this probably means the string is not meant to be an f-string, and it guarantees that conversion will be attempted at most once per string.

Plus two heuristics for avoiding conversion of strings that are formatted with str.format():

3. Do not convert if the sequence {} appears, because this is an invalid f-string specified and may indicate the use of str.format().

4. Try to detect if str.format() is being called on the string. This is imperfect because if the user is typing a new line of code, it may not have been written yet. For existing code the user is editing, one could just see if .format( comes directly after the closing quote -- by far the most common pattern.

   But note that, as long as you are targeting a version of Python with f-strings, the user probably won't be writing new code that uses str.format().

[brettcannon]

Moved to https://github.com/microsoft/vscode-python/issues/16989