RustPython · youknowone · Dec 2, 2025 · Dec 2, 2025 · Dec 2, 2025 · Dec 2, 2025
diff --git a/crates/codegen/src/compile.rs b/crates/codegen/src/compile.rs
@@ -36,7 +36,8 @@ use rustpython_compiler_core::{
     Mode, OneIndexed, PositionEncoding, SourceFile, SourceLocation,
     bytecode::{
         self, Arg as OpArgMarker, BinaryOperator, BuildSliceArgCount, CodeObject,
-        ComparisonOperator, ConstantData, Instruction, Invert, OpArg, OpArgType, UnpackExArgs,
+        ComparisonOperator, ConstantData, ConvertValueOparg, Instruction, Invert, OpArg, OpArgType,
+        UnpackExArgs,
     },
 };
 use rustpython_wtf8::Wtf8Buf;
@@ -5636,7 +5637,12 @@ impl Compiler {
                     }
                 }
                 InterpolatedStringElement::Interpolation(fstring_expr) => {
-                    let mut conversion = fstring_expr.conversion;
+                    let mut conversion = match fstring_expr.conversion {
+                        ConversionFlag::None => ConvertValueOparg::None,
+                        ConversionFlag::Str => ConvertValueOparg::Str,
+                        ConversionFlag::Repr => ConvertValueOparg::Repr,
+                        ConversionFlag::Ascii => ConvertValueOparg::Ascii,
+                    };
 
                     if let Some(DebugText { leading, trailing }) = &fstring_expr.debug_text {
                         let range = fstring_expr.expression.range();
@@ -5645,35 +5651,39 @@ impl Compiler {
 
                         self.emit_load_const(ConstantData::Str { value: text.into() });
                         element_count += 1;
+
+                        // Match CPython behavior: If debug text is present, apply repr conversion.
+                        // if no `format_spec` specified.
+                        // See: https://github.com/python/cpython/blob/f61afca262d3a0aa6a8a501db0b1936c60858e35/Parser/action_helpers.c#L1456
+                        if matches!(
+                            (conversion, &fstring_expr.format_spec),
+                            (ConvertValueOparg::None, None)
+                        ) {
+                            conversion = ConvertValueOparg::Repr;
+                        }
                     }
 
-                    match &fstring_expr.format_spec {
-                        None => {
-                            self.emit_load_const(ConstantData::Str {
-                                value: Wtf8Buf::new(),
-                            });
-                            // Match CPython behavior: If debug text is present, apply repr conversion.
-                            // See: https://github.com/python/cpython/blob/f61afca262d3a0aa6a8a501db0b1936c60858e35/Parser/action_helpers.c#L1456
-                            if conversion == ConversionFlag::None
-                                && fstring_expr.debug_text.is_some()
-                            {
-                                conversion = ConversionFlag::Repr;
-                            }
+                    self.compile_expression(&fstring_expr.expression)?;
+
+                    match conversion {
+                        ConvertValueOparg::None => {}
+                        ConvertValueOparg::Str
+                        | ConvertValueOparg::Repr
+                        | ConvertValueOparg::Ascii => {
+                            emit!(self, Instruction::ConvertValue { oparg: conversion })
                         }
+                    }
+
+                    match &fstring_expr.format_spec {
                         Some(format_spec) => {
                             self.compile_fstring_elements(flags, &format_spec.elements)?;
+
+                            emit!(self, Instruction::FormatWithSpec);
+                        }
+                        None => {
+                            emit!(self, Instruction::FormatSimple);
                         }
                     }
-
-                    self.compile_expression(&fstring_expr.expression)?;
-
-                    let conversion = match conversion {
-                        ConversionFlag::None => bytecode::ConversionFlag::None,
-                        ConversionFlag::Str => bytecode::ConversionFlag::Str,
-                        ConversionFlag::Ascii => bytecode::ConversionFlag::Ascii,
-                        ConversionFlag::Repr => bytecode::ConversionFlag::Repr,
-                    };
-                    emit!(self, Instruction::FormatValue { conversion });
                 }
             }
         }

diff --git a/...s/codegen/src/snapshots/rustpython_codegen__compile__tests__nested_double_async_with.snap b/...s/codegen/src/snapshots/rustpython_codegen__compile__tests__nested_double_async_with.snap
diff --git a/crates/compiler-core/src/bytecode.rs b/crates/compiler-core/src/bytecode.rs
@@ -12,18 +12,56 @@ use num_complex::Complex64;
 use rustpython_wtf8::{Wtf8, Wtf8Buf};
 use std::{collections::BTreeSet, fmt, hash, marker::PhantomData, mem, num::NonZeroU8, ops::Deref};
 
+/// Oparg values for [`Instruction::ConvertValue`].
+///
+/// ## See also
+///
+/// - [CPython FVC_* flags](https://github.com/python/cpython/blob/8183fa5e3f78ca6ab862de7fb8b14f3d929421e0/Include/ceval.h#L129-L132)
+#[repr(u8)]
 #[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)]
-#[repr(i8)]
-#[allow(clippy::cast_possible_wrap)]
-pub enum ConversionFlag {
-    /// No conversion
-    None = -1, // CPython uses -1
+pub enum ConvertValueOparg {
+    /// No conversion.
+    ///
+    /// ```python
+    /// f"{x}"
+    /// f"{x:4}"
+    /// ```
+    None = 0,
     /// Converts by calling `str(<value>)`.
-    Str = b's' as i8,
-    /// Converts by calling `ascii(<value>)`.
-    Ascii = b'a' as i8,
+    ///
+    /// ```python
+    /// f"{x!s}"
+    /// f"{x!s:2}"
+    /// ```
+    Str = 1,
     /// Converts by calling `repr(<value>)`.
-    Repr = b'r' as i8,
+    ///
+    /// ```python
+    /// f"{x!r}"
+    /// f"{x!r:2}"
+    /// ```
+    Repr = 2,
+    /// Converts by calling `ascii(<value>)`.
+    ///
+    /// ```python
+    /// f"{x!a}"
+    /// f"{x!a:2}"
+    /// ```
+    Ascii = 3,
+}
+
+impl fmt::Display for ConvertValueOparg {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let out = match self {
+            Self::Str => "1 (str)",
+            Self::Repr => "2 (repr)",
+            Self::Ascii => "3 (ascii)",
+            // We should never reach this. `FVC_NONE` are being handled by `Instruction::FormatSimple`
+            Self::None => "",
+        };
+
+        write!(f, "{out}")
+    }
 }
 
 /// Resume type for the RESUME instruction
@@ -476,21 +514,21 @@ impl fmt::Display for Label {
     }
 }
 
-impl OpArgType for ConversionFlag {
+impl OpArgType for ConvertValueOparg {
     #[inline]
     fn from_op_arg(x: u32) -> Option<Self> {
-        match x as u8 {
-            b's' => Some(Self::Str),
-            b'a' => Some(Self::Ascii),
-            b'r' => Some(Self::Repr),
-            std::u8::MAX => Some(Self::None),
-            _ => None,
-        }
+        Some(match x {
+            0 => Self::None,
+            1 => Self::Str,
+            2 => Self::Repr,
+            3 => Self::Ascii,
+            _ => return None,
+        })
     }
 
     #[inline]
     fn to_op_arg(self) -> u32 {
-        self as i8 as u8 as u32
+        self as u32
     }
 }
 
@@ -777,9 +815,39 @@ pub enum Instruction {
     UnpackEx {
         args: Arg<UnpackExArgs>,
     },
-    FormatValue {
-        conversion: Arg<ConversionFlag>,
+    /// Convert value to a string, depending on `oparg`:
+    ///
+    /// ```python
+    /// value = STACK.pop()
+    /// result = func(value)
+    /// STACK.append(result)
+    /// ```
+    ///
+    /// Used for implementing formatted string literals (f-strings).
+    ConvertValue {
+        oparg: Arg<ConvertValueOparg>,
     },
+    /// Formats the value on top of stack:
+    ///
+    /// ```python
+    /// value = STACK.pop()
+    /// result = value.__format__("")
+    /// STACK.append(result)
+    /// ```
+    ///
+    /// Used for implementing formatted string literals (f-strings).
+    FormatSimple,
+    /// Formats the given value with the given format spec:
+    ///
+    /// ```python
+    /// spec = STACK.pop()
+    /// value = STACK.pop()
+    /// result = value.__format__(spec)
+    /// STACK.append(result)
+    /// ```
+    ///
+    /// Used for implementing formatted string literals (f-strings).
+    FormatWithSpec,
     PopException,
     Reverse {
         amount: Arg<u32>,
@@ -1656,6 +1724,9 @@ impl Instruction {
             CallMethodKeyword { nargs } => -1 - (nargs.get(arg) as i32) - 3 + 1,
             CallFunctionEx { has_kwargs } => -1 - (has_kwargs.get(arg) as i32) - 1 + 1,
             CallMethodEx { has_kwargs } => -1 - (has_kwargs.get(arg) as i32) - 3 + 1,
+            ConvertValue { .. } => 0,
+            FormatSimple => 0,
+            FormatWithSpec => -1,
             LoadMethod { .. } => -1 + 3,
             ForIter { .. } => {
                 if jump {
@@ -1709,7 +1780,6 @@ impl Instruction {
                 let UnpackExArgs { before, after } = args.get(arg);
                 -1 + before as i32 + 1 + after as i32
             }
-            FormatValue { .. } => -1,
             PopException => 0,
             Reverse { .. } => 0,
             GetAwaitable => 0,
@@ -1891,10 +1961,12 @@ impl Instruction {
             SetAdd { i } => w!(SetAdd, i),
             MapAdd { i } => w!(MapAdd, i),
             PrintExpr => w!(PrintExpr),
+            ConvertValue { oparg } => write!(f, "{:pad$}{}", "CONVERT_VALUE", oparg.get(arg)),
+            FormatSimple => w!(FORMAT_SIMPLE),
+            FormatWithSpec => w!(FORMAT_WITH_SPEC),
             LoadBuildClass => w!(LoadBuildClass),
             UnpackSequence { size } => w!(UnpackSequence, size),
             UnpackEx { args } => w!(UnpackEx, args),
-            FormatValue { conversion } => w!(FormatValue, ?conversion),
             PopException => w!(PopException),
             Reverse { amount } => w!(Reverse, amount),
             GetAwaitable => w!(GetAwaitable),

diff --git a/crates/vm/src/frame.rs b/crates/vm/src/frame.rs
@@ -1308,8 +1308,23 @@ impl ExecutingFrame<'_> {
                 let args = args.get(arg);
                 self.execute_unpack_ex(vm, args.before, args.after)
             }
-            bytecode::Instruction::FormatValue { conversion } => {
-                self.format_value(conversion.get(arg), vm)
+            bytecode::Instruction::ConvertValue { oparg: conversion } => {
+                self.convert_value(conversion.get(arg), vm)
+            }
+            bytecode::Instruction::FormatSimple => {
+                let value = self.pop_value();
+                let formatted = vm.format(&value, vm.ctx.new_str(""))?;
+                self.push_value(formatted.into());
+
+                Ok(None)
+            }
+            bytecode::Instruction::FormatWithSpec => {
+                let spec = self.pop_value();
+                let value = self.pop_value();
+                let formatted = vm.format(&value, spec.downcast::<PyStr>().unwrap())?;
+                self.push_value(formatted.into());
+
+                Ok(None)
             }
             bytecode::Instruction::PopException => {
                 let block = self.pop_block();
@@ -2237,23 +2252,21 @@ impl ExecutingFrame<'_> {
         Err(vm.new_value_error(msg))
     }
 
-    fn format_value(
+    fn convert_value(
         &mut self,
-        conversion: bytecode::ConversionFlag,
+        conversion: bytecode::ConvertValueOparg,
         vm: &VirtualMachine,
     ) -> FrameResult {
-        use bytecode::ConversionFlag;
+        use bytecode::ConvertValueOparg;
         let value = self.pop_value();
         let value = match conversion {
-            ConversionFlag::Str => value.str(vm)?.into(),
-            ConversionFlag::Repr => value.repr(vm)?.into(),
-            ConversionFlag::Ascii => vm.ctx.new_str(builtins::ascii(value, vm)?).into(),
-            ConversionFlag::None => value,
+            ConvertValueOparg::Str => value.str(vm)?.into(),
+            ConvertValueOparg::Repr => value.repr(vm)?.into(),
+            ConvertValueOparg::Ascii => vm.ctx.new_str(builtins::ascii(value, vm)?).into(),
+            ConvertValueOparg::None => value,
         };
 
-        let spec = self.pop_value();
-        let formatted = vm.format(&value, spec.downcast::<PyStr>().unwrap())?;
-        self.push_value(formatted.into());
+        self.push_value(value);
         Ok(None)
     }
 

diff --git a/crates/vm/src/stdlib/ast/other.rs b/crates/vm/src/stdlib/ast/other.rs
@@ -14,12 +14,12 @@ impl Node for ruff::ConversionFlag {
     ) -> PyResult<Self> {
         i32::try_from_object(vm, object)?
             .to_u32()
-            .and_then(bytecode::ConversionFlag::from_op_arg)
+            .and_then(bytecode::ConvertValueOparg::from_op_arg)
             .map(|flag| match flag {
-                bytecode::ConversionFlag::None => Self::None,
-                bytecode::ConversionFlag::Str => Self::Str,
-                bytecode::ConversionFlag::Ascii => Self::Ascii,
-                bytecode::ConversionFlag::Repr => Self::Repr,
+                bytecode::ConvertValueOparg::None => Self::None,
+                bytecode::ConvertValueOparg::Str => Self::Str,
+                bytecode::ConvertValueOparg::Repr => Self::Repr,
+                bytecode::ConvertValueOparg::Ascii => Self::Ascii,
             })
             .ok_or_else(|| vm.new_value_error("invalid conversion flag"))
     }