Add __replace__ and fix __reduce__ for structseq

- Add __replace__ method to PyStructSequence trait
- Move __reduce__ from #[pymethod] to extend_pyclass with
  contains_key guard, allowing per-type overrides
- Fix repr: remove trailing comma for single-field sequences

Author: youknowone

crates/vm/src/types/structseq.rs

@@ -2,7 +2,7 @@ use crate::{
     AsObject, Py, PyObject, PyObjectRef, PyPayload, PyRef, PyResult, VirtualMachine, atomic_func,
     builtins::{PyBaseExceptionRef, PyStrRef, PyTuple, PyTupleRef, PyType, PyTypeRef},
     class::{PyClassImpl, StaticType},
-    function::{Either, PyComparisonValue},
+    function::{Either, FuncArgs, PyComparisonValue, PyMethodDef, PyMethodFlags},
     iter::PyExactSizeIterator,
     protocol::{PyMappingMethods, PySequenceMethods},
     sliceable::{SequenceIndex, SliceableSequenceOp},
@@ -11,6 +11,15 @@ use crate::{
 };
 use std::sync::LazyLock;
 
+const DEFAULT_STRUCTSEQ_REDUCE: PyMethodDef = PyMethodDef::new_const(
+    "__reduce__",
+    |zelf: PyRef<PyTuple>, vm: &VirtualMachine| -> PyTupleRef {
+        vm.new_tuple((zelf.class().to_owned(), (vm.ctx.new_tuple(zelf.to_vec()),)))
+    },
+    PyMethodFlags::METHOD,
+    None,
+);
+
 /// Create a new struct sequence instance from a sequence.
 ///
 /// The class must have `n_sequence_fields` and `n_fields` attributes set
@@ -206,19 +215,13 @@ pub trait PyStructSequence: StaticType + PyClassImpl + Sized + 'static {
         };
         let (body, suffix) =
             if let Some(_guard) = rustpython_vm::recursion::ReprGuard::enter(vm, zelf.as_ref()) {
-                if field_names.len() == 1 {
-                    let value = zelf.first().unwrap();
-                    let formatted = format_field((value, field_names[0]))?;
-                    (formatted, ",")
-                } else {
-                    let fields: PyResult<Vec<_>> = zelf
-                        .iter()
-                        .map(|value| value.as_ref())
-                        .zip(field_names.iter().copied())
-                        .map(format_field)
-                        .collect();
-                    (fields?.join(", "), "")
-                }
+                let fields: PyResult<Vec<_>> = zelf
+                    .iter()
+                    .map(|value| value.as_ref())
+                    .zip(field_names.iter().copied())
+                    .map(format_field)
+                    .collect();
+                (fields?.join(", "), "")
             } else {
                 (String::new(), "...")
             };
@@ -232,8 +235,48 @@ pub trait PyStructSequence: StaticType + PyClassImpl + Sized + 'static {
     }
 
     #[pymethod]
-    fn __reduce__(zelf: PyRef<PyTuple>, vm: &VirtualMachine) -> PyTupleRef {
-        vm.new_tuple((zelf.class().to_owned(), (vm.ctx.new_tuple(zelf.to_vec()),)))
+    fn __replace__(zelf: PyRef<PyTuple>, args: FuncArgs, vm: &VirtualMachine) -> PyResult {
+        if !args.args.is_empty() {
+            return Err(vm.new_type_error("__replace__() takes no positional arguments".to_owned()));
+        }
+
+        if Self::Data::UNNAMED_FIELDS_LEN > 0 {
+            return Err(vm.new_type_error(format!(
+                "__replace__() is not supported for {} because it has unnamed field(s)",
+                zelf.class().slot_name()
+            )));
+        }
+
+        let n_fields = Self::Data::REQUIRED_FIELD_NAMES.len()
+            + Self::Data::OPTIONAL_FIELD_NAMES.len();
+        let mut items: Vec<PyObjectRef> = zelf.as_slice()[..n_fields].to_vec();
+
+        let mut kwargs = args.kwargs.clone();
+
+        // Replace fields from kwargs
+        let all_field_names: Vec<&str> = Self::Data::REQUIRED_FIELD_NAMES
+            .iter()
+            .chain(Self::Data::OPTIONAL_FIELD_NAMES.iter())
+            .copied()
+            .collect();
+        for (i, &name) in all_field_names.iter().enumerate() {
+            if let Some(val) = kwargs.shift_remove(name) {
+                items[i] = val;
+            }
+        }
+
+        // Check for unexpected keyword arguments
+        if !kwargs.is_empty() {
+            let names: Vec<&str> = kwargs.keys().map(|k| k.as_str()).collect();
+            return Err(vm.new_type_error(format!(
+                "Got unexpected field name(s): {:?}",
+                names
+            )));
+        }
+
+        PyTuple::new_unchecked(items.into_boxed_slice())
+            .into_ref_with_type(vm, zelf.class().to_owned())
+            .map(Into::into)
     }
 
     #[pymethod]
@@ -327,6 +370,20 @@ pub trait PyStructSequence: StaticType + PyClassImpl + Sized + 'static {
             .slots
             .richcompare
             .store(Some(struct_sequence_richcompare));
+
+        // Default __reduce__: only set if not already overridden by the impl's extend_class.
+        // This allows struct sequences like sched_param to provide a custom __reduce__
+        // (equivalent to METH_COEXIST in structseq.c).
+        if !class
+            .attributes
+            .read()
+            .contains_key(ctx.intern_str("__reduce__"))
+        {
+            class.set_attr(
+                ctx.intern_str("__reduce__"),
+                DEFAULT_STRUCTSEQ_REDUCE.to_proper_method(class, ctx),
+            );
+        }
     }
 }