Accept the format keyword in Struct initialization (#9064)

Keep the existing conversion and initialization path while binding format as positional-or-keyword, matching Python 3.14.

Assisted-by: OpenAI Codex:model identifier unavailable

crates/stdlib/src/pystruct.rs

@@ -289,10 +289,17 @@ pub(crate) mod _struct {
         }
     }
 
-    impl Initializer for PyStruct {
-        type Args = IntoStructFormatBytes;
+    #[derive(FromArgs)]
+    struct StructArgs {
+        #[pyarg(any)]
+        format: IntoStructFormatBytes,
+    }
 
-        fn init(zelf: &Py<Self>, fmt: Self::Args, vm: &VirtualMachine) -> PyResult<()> {
+    impl Initializer for PyStruct {
+        type Args = StructArgs;
+
+        fn init(zelf: &Py<Self>, args: Self::Args, vm: &VirtualMachine) -> PyResult<()> {
+            let fmt = args.format;
             // The format is read before anything is replaced, so a format that
             // cannot be read leaves the object as it was.
             let spec = fmt.format_spec(vm)?;

extra_tests/snippets/stdlib_struct.py

@@ -184,3 +184,21 @@ assert struct.pack("<H", ClearFormats()) == b"\x2a\x00"
 buffer = bytearray(2)
 struct.pack_into("<H", buffer, 0, ClearFormats())
 assert buffer == b"\x2a\x00"
+
+
+# Struct's format is positional-or-keyword, including inherited initialization.
+class KeywordStruct(struct.Struct):
+    pass
+
+
+for cls in (struct.Struct, KeywordStruct):
+    for fmt in (">h", b">h"):
+        s = cls(format=fmt)
+        assert s.format == ">h"
+        assert s.pack(7) == b"\x00\x07"
+        s.__init__(format=">hh")
+        assert s.unpack(b"\x00\x01\x00\x02") == (1, 2)
+        with assert_raises(struct.error):
+            s.__init__(format="$")
+        assert s.format == ">hh"
+        assert s.pack(1, 2) == b"\x00\x01\x00\x02"