Option to avoid converting overly complex logic into f-strings

[eli-schwartz]

See e.g. asottile/pyupgrade#526

It would be nice to have a configurable tool that allows me to choose what threshold of changes I feel comfortable making -- I want to be able to convert simple, obvious code to f-strings, but not code that I subjectively feel is harder to read, and if in doubt I'd prefer to do nothing and leave that to a manual touch-up. Or even perform and git commit objectively beneficial changes, then run the reformatting tool in a more aggressive mode and eyeball the results for subjective taste.

AFAICT this isn't currently possible with flynt but also hasn't previously come up (as opposed to other projects where it's come up and been rapidly shot down. 😢)

[ikamensh]

Thanks for the suggestion. As scope of flynt is just f-strings, I think this idea is appropriate for the project.

I wonder what universal metric you want to use for this. I could imagine some metric like AST graph depth / node count of a f-string, feel free to suggest other complexity metrics. If you have interest to make a PR for a new CLI flag that wouldn't change default behavior, I would be in favor or such a feature.