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Add support for except* handling and starred subscripts #6522

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3dea814
Initial plan
Copilot Dec 25, 2025
c37fc4f
Implement except* handling and starred subscript support
Copilot Dec 25, 2025
62bb015
Move except* and starred subscript tests into builtin_exceptions
Copilot Dec 25, 2025
fbcf0b8
Fix CPython-compatible assert in builtin_exceptions test
Copilot Dec 25, 2025
9b94189
Refine except* codegen using named constant
Copilot Dec 25, 2025
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159 changes: 153 additions & 6 deletions crates/codegen/src/compile.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -471,7 +471,16 @@ impl Compiler {
}
} else {
// VISIT(c, expr, e->v.Subscript.slice)
self.compile_expression(slice)?;
match slice {

@ShaharNaveh ShaharNaveh Dec 25, 2025

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I don't see this logic in CPython:
https://github.com/python/cpython/blob/627894459a84be3488a1789919679c997056a03c/Python/compile.c#L6669-L6678

Assuming that this works, I'd like to have a comment explaining why we are deviating from the CPython logic

Expr::Starred(starred) => {
self.starunpack_helper(
&[Expr::Starred(starred.clone())],
0,
CollectionType::Tuple,
)?;
}
other => self.compile_expression(other)?,
}

// Emit appropriate instruction based on context
match ctx {
Expand Down Expand Up @@ -2121,12 +2130,150 @@ impl Compiler {

fn compile_try_star_statement(

@ShaharNaveh ShaharNaveh Dec 25, 2025

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Suggested change
fn compile_try_star_statement(
fn compile_try_star_except(

&mut self,
_body: &[Stmt],
_handlers: &[ExceptHandler],
_orelse: &[Stmt],
_finalbody: &[Stmt],
body: &[Stmt],
handlers: &[ExceptHandler],
orelse: &[Stmt],
finalbody: &[Stmt],
) -> CompileResult<()> {
Err(self.error(CodegenErrorType::NotImplementedYet))
let handler_block = self.new_block();
let finally_block = self.new_block();

if !finalbody.is_empty() {

@ShaharNaveh ShaharNaveh Dec 25, 2025

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I'd rather have it split like CPython does it

https://github.com/python/cpython/blob/627894459a84be3488a1789919679c997056a03c/Python/compile.c#L3836-L3845

This makes it easier to maintain when looking for the diff between our impl and theirs

emit!(
self,
Instruction::SetupFinally {
handler: finally_block,
}
);
}

let else_block = self.new_block();

emit!(
self,
Instruction::SetupExcept {
handler: handler_block,
}
);
self.compile_statements(body)?;
emit!(self, Instruction::PopBlock);
emit!(self, Instruction::Jump { target: else_block });

self.switch_to_block(handler_block);
// incoming stack: [exc]
emit!(
self,
Instruction::CallIntrinsic1 {
func: bytecode::IntrinsicFunction1::EnsureExceptionGroup
}
);
// stack: [remainder]

for handler in handlers {
let ExceptHandler::ExceptHandler(ExceptHandlerExceptHandler {
type_, name, body, ..
}) = handler;

let skip_block = self.new_block();
let next_block = self.new_block();

// Split current remainder for this handler
if let Some(exc_type) = type_ {
self.compile_expression(exc_type)?;
} else {
return Err(self.error(CodegenErrorType::SyntaxError(
"except* must specify an exception type".to_owned(),
)));
}

emit!(
self,
Instruction::CallIntrinsic2 {
func: bytecode::IntrinsicFunction2::ExceptStarMatch
}
);
emit!(self, Instruction::UnpackSequence { size: 2 }); // stack: [rest, match]
emit!(self, Instruction::CopyItem { index: 1_u32 }); // duplicate match for truthiness test
emit!(self, Instruction::ToBool);
emit!(self, Instruction::PopJumpIfFalse { target: skip_block });

// Handler selected, stack: [rest, match]
if let Some(alias) = name {
self.store_name(alias.as_str())?;
} else {
emit!(self, Instruction::Pop);
}

self.compile_statements(body)?;

if let Some(alias) = name {
self.emit_load_const(ConstantData::None);
self.store_name(alias.as_str())?;
self.compile_name(alias.as_str(), NameUsage::Delete)?;
}

emit!(self, Instruction::Jump { target: next_block });

// Skip handler when no match (stack currently [rest, match])
self.switch_to_block(skip_block);
emit!(self, Instruction::Pop); // drop match
emit!(self, Instruction::Jump { target: next_block });

// Continue with remaining exceptions
self.switch_to_block(next_block);
}

let handled_block = self.new_block();

// If remainder is truthy, re-raise it
emit!(self, Instruction::CopyItem { index: 1_u32 });
emit!(self, Instruction::ToBool);
emit!(
self,
Instruction::PopJumpIfFalse {
target: handled_block
}
);
emit!(
self,
Instruction::Raise {
kind: bytecode::RaiseKind::Raise
}
);

// All exceptions handled
self.switch_to_block(handled_block);
emit!(self, Instruction::Pop); // drop remainder (None)
emit!(self, Instruction::PopException);

if !finalbody.is_empty() {
emit!(self, Instruction::PopBlock);
emit!(self, Instruction::EnterFinally);
}

emit!(
self,
Instruction::Jump {
target: finally_block,
}
);

// try-else path
self.switch_to_block(else_block);
self.compile_statements(orelse)?;

if !finalbody.is_empty() {
emit!(self, Instruction::PopBlock);
emit!(self, Instruction::EnterFinally);
}

self.switch_to_block(finally_block);
if !finalbody.is_empty() {
self.compile_statements(finalbody)?;
emit!(self, Instruction::EndFinally);
}

Ok(())
}

fn is_forbidden_arg_name(name: &str) -> bool {
Expand Down
4 changes: 4 additions & 0 deletions crates/compiler-core/src/bytecode.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -566,6 +566,8 @@ op_arg_enum!(
/// Generic subscript for PEP 695
SubscriptGeneric = 10,
TypeAlias = 11,
/// Ensure an exception is normalized to BaseExceptionGroup
EnsureExceptionGroup = 12,
}
);

Expand All @@ -580,6 +582,8 @@ op_arg_enum!(
SetFunctionTypeParams = 4,
/// Set default value for type parameter (PEP 695)
SetTypeparamDefault = 5,
/// Split an ExceptionGroup according to except* handler condition
ExceptStarMatch = 6,
}
);

Expand Down
24 changes: 23 additions & 1 deletion crates/vm/src/frame.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -8,7 +8,7 @@ use crate::{
tuple::{PyTuple, PyTupleRef},
},
bytecode,
convert::{IntoObject, ToPyResult},
convert::{IntoObject, ToPyObject, ToPyResult},
coroutine::Coro,
exceptions::ExceptionCtor,
function::{ArgMapping, Either, FuncArgs},
Expand Down Expand Up @@ -2460,6 +2460,18 @@ impl ExecutingFrame<'_> {
.map_err(|_| vm.new_type_error("LIST_TO_TUPLE expects a list"))?;
Ok(vm.ctx.new_tuple(list.borrow_vec().to_vec()).into())
}
bytecode::IntrinsicFunction1::EnsureExceptionGroup => {
if arg.fast_isinstance(vm.ctx.exceptions.base_exception_group) {
Ok(arg)
} else {
let exception_group_type = crate::exception_group::exception_group();
let wrapped = exception_group_type
.to_owned()
.to_pyobject(vm)
.call((vm.ctx.new_str(""), vm.ctx.new_tuple(vec![arg])), vm)?;
Ok(wrapped)
}
}
}
}

Expand Down Expand Up @@ -2494,6 +2506,16 @@ impl ExecutingFrame<'_> {
.into();
Ok(type_var)
}
bytecode::IntrinsicFunction2::ExceptStarMatch => {
let result = vm.call_method(&arg1, "split", (arg2,))?;
let result_tuple: PyTupleRef = result.try_into_value(vm)?;
if result_tuple.len() != 2 {
return Err(vm.new_type_error("ExceptionGroup.split must return 2-tuple"));
}
let matched = result_tuple[0].clone();
let rest = result_tuple[1].clone();
Ok(vm.ctx.new_tuple(vec![matched, rest]).into())
}
}
}

Expand Down
19 changes: 19 additions & 0 deletions extra_tests/snippets/except_star_and_starred_index.py
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@youknowone youknowone Dec 25, 2025

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Tests are good. Move it to append to builtin_exceptions.py

Original file line number Diff line number Diff line change
@@ -0,0 +1,19 @@
def test_except_star_with_plain_exception():
try:
raise ValueError("x")
except* ValueError as err:
assert isinstance(err, ExceptionGroup)
assert err.exceptions == (ValueError("x"),)
else:
raise AssertionError("except* handler did not run")


def test_starred_index_builds_tuple():
>
target[*"ab"] = 1
assert list(target.items()) == [(("a", "b"), 1)]


if __name__ == "__main__":
test_except_star_with_plain_exception()
test_starred_index_builds_tuple()
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