f-string expressions can be formatted in four ways: with or without a conversion and with or without a format specifier Rather than have one bytecode that parses the opcode argument at runtime it would be more efficient and produce a cleaner interpreter for the compiler to produce one or two bytecode as required. The bytecodes should be: CONVERT_VALUE convert_fn FORMAT_SIMPLE FORMAT_WITH_SPEC For simple format expressions with no conversion or format specifier, which make up about 3/4 of all format expressions in the standard library, just the bytecode FORMAT_SIMPLE need be executed.