Summary

Currently, none of the F50x rules fire when the left-hand side of % is an f-string, because the check only matches Expr::StringLiteral. This means code like:

f"{banana}" % banana
f"hello %s %s" % (1, 2)

produces no warnings, even though mixing f-string interpolation with %-formatting is almost certainly a mistake — both serve the same purpose.

Proposal

Add a new rule (RUF072) that flags any use of % on an f-string. I can't think of a valid use case where using % over f-string interpolation is desired.

Related

• Mismatching number of %-placeholders in string formatting #24031 — original report
• F507: Fix false negative for non-tuple RHS in %-formatting #24142 — F507 fix for non-tuple RHS