RustPython · bschoenmaeckers · May 22, 2026 · May 26, 2026 · May 26, 2026 · May 26, 2026
diff --git a/Cargo.lock b/Cargo.lock
diff --git a/crates/capi/Cargo.toml b/crates/capi/Cargo.toml
@@ -12,6 +12,7 @@ license.workspace = true
 crate-type = ["cdylib", "rlib"]
 
 [dependencies]
+bitflags = { workspace = true }
 num-complex = { workspace = true }
 rustpython-vm = { workspace = true, features = ["threading", "compiler"] }
 rustpython-stdlib = {workspace = true, features = ["threading"] }

diff --git a/crates/capi/src/lib.rs b/crates/capi/src/lib.rs
@@ -18,6 +18,7 @@ pub mod floatobject;
 pub mod import;
 pub mod listobject;
 pub mod longobject;
+pub mod methodobject;
 pub mod object;
 pub mod pycapsule;
 pub mod pyerrors;

diff --git a/crates/capi/src/methodobject.rs b/crates/capi/src/methodobject.rs
@@ -0,0 +1,369 @@
+use crate::PyObject;
+use crate::object::PyTypeObject;
+use crate::object::define_py_check;
+use crate::pystate::with_vm;
+use core::ffi::{CStr, c_char, c_int};
+use core::ptr::NonNull;
+use rustpython_vm::function::{FuncArgs, HeapMethodDef, PosArgs, PyMethodFlags};
+use rustpython_vm::{AsObject, PyObjectRef, PyRef, PyResult, VirtualMachine};
+
+define_py_check!(fn PyCFunction_Check, types.builtin_function_or_method_type);
+define_py_check!(exact fn PyCFunction_CheckExact, types.builtin_function_or_method_type);
+
+#[repr(C)]
+pub struct PyMethodDef {
+    pub ml_name: *const c_char,
+    pub ml_meth: PyMethodPointer,
+    pub ml_flags: c_int,
+    pub ml_doc: *const c_char,
+}
+
+#[repr(C)]
+#[derive(Copy, Clone)]
+#[allow(non_snake_case)]
+pub union PyMethodPointer {
+    pub PyCFunction: unsafe extern "C" fn(slf: *mut PyObject, args: *mut PyObject) -> *mut PyObject,
+    pub PyCFunctionWithKeywords: unsafe extern "C" fn(
+        slf: *mut PyObject,
+        args: *mut PyObject,
+        kwargs: *mut PyObject,
+    ) -> *mut PyObject,
+    pub PyCFunctionFast: unsafe extern "C" fn(
+        slf: *mut PyObject,
+        args: *const *mut PyObject,
+        nargs: isize,
+    ) -> *mut PyObject,
+    pub PyCFunctionFastWithKeywords: unsafe extern "C" fn(
+        slf: *mut PyObject,
+        args: *const *mut PyObject,
+        nargs: isize,
+        kwnames: *mut PyObject,
+    ) -> *mut PyObject,
+}
+
+pub(crate) fn build_method_def(
+    vm: &VirtualMachine,
+    ml: &PyMethodDef,
+    has_self: bool,
+) -> PyResult<PyRef<HeapMethodDef>> {
+    let name = unsafe { CStr::from_ptr(ml.ml_name) }
+        .to_str()
+        .map_err(|_| vm.new_system_error("Method name was not valid UTF-8"))?;
+
+    let doc = NonNull::new(ml.ml_doc.cast_mut())
+        .map(|doc| {
+            unsafe { CStr::from_ptr(doc.as_ptr()) }
+                .to_str()
+                .map_err(|_| vm.new_system_error("Method doc was not valid UTF-8"))
+        })
+        .transpose()?;
+
+    let flags = PyMethodFlags::from_bits(ml.ml_flags as u32)
+        .ok_or_else(|| vm.new_system_error("PyMethodDef contains unknown flags"))?;
+
+    let method = ml.ml_meth;
+
+    if flags.contains(PyMethodFlags::METHOD) {
+        return Err(vm.new_system_error("METH_METHOD is not supported on abi3"));
+    }
+
+    let call_flags = flags
+        & (PyMethodFlags::VARARGS
+            | PyMethodFlags::KEYWORDS
+            | PyMethodFlags::NOARGS
+            | PyMethodFlags::O
+            | PyMethodFlags::FASTCALL);
+
+    bitflags::bitflags_match!(call_flags, {
+        PyMethodFlags::NOARGS => {
+            if has_self {
+                let callable = move |zelf: PyObjectRef, vm: &VirtualMachine| unsafe {
+                    let f = method.PyCFunction;
+                    let ret_ptr = f(zelf.as_raw().cast_mut(), core::ptr::null_mut());
+                    ret_ptr_to_pyresult(vm, ret_ptr)
+                };
+                Ok(vm.ctx.new_method_def(name, callable, flags, doc))
+            } else {
+                let callable = move |vm: &VirtualMachine| unsafe {
+                    let f = method.PyCFunction;
+                    let ret_ptr = f(core::ptr::null_mut(), core::ptr::null_mut());
+                    ret_ptr_to_pyresult(vm, ret_ptr)
+                };
+                Ok(vm.ctx.new_method_def(name, callable, flags, doc))
+            }
+        },
+        PyMethodFlags::VARARGS => {
+            let callable = move |args: PosArgs, vm: &VirtualMachine| unsafe {
+                call_function(vm, method, flags, Some(args))
+            };
+            Ok(vm.ctx.new_method_def(name, callable, flags, doc))
+        },
+        PyMethodFlags::VARARGS | PyMethodFlags::KEYWORDS => {
+            let callable = move | args: FuncArgs, vm: &VirtualMachine| unsafe {
+                call_function_with_keywords(vm, method, flags, args)
+            };
+            Ok(vm.ctx.new_method_def(name, callable, flags, doc))
+        },
+        PyMethodFlags::FASTCALL | PyMethodFlags::KEYWORDS => {
+            let callable = move |args: FuncArgs, vm: &VirtualMachine| unsafe {
+                call_fast_function_with_keywords(vm, method, flags, args)
+            };
+            Ok(vm.ctx.new_method_def(name, callable, flags, doc))
+        },
+        PyMethodFlags::FASTCALL => {
+            let callable = move |args: PosArgs, vm: &VirtualMachine| unsafe {
+                call_fast_function(vm, method, flags, args)
+            };
+            Ok(vm.ctx.new_method_def(name, callable, flags, doc))
+        },
+        PyMethodFlags::O => {
+            let f = unsafe { method.PyCFunction };
+            if has_self {
+                let callable = move |zelf: PyObjectRef, arg: PyObjectRef, vm: &VirtualMachine| -> PyResult {
+                    let ret_ptr = unsafe { f(zelf.as_raw().cast_mut(), arg.as_raw().cast_mut()) };
+                    ret_ptr_to_pyresult(vm, ret_ptr)
+                };
+                Ok(vm.ctx.new_method_def(name, callable, flags, doc))
+            } else {
+                let callable = move |arg: PyObjectRef, vm: &VirtualMachine| -> PyResult {
+                    let ret_ptr = unsafe { f(core::ptr::null_mut(), arg.as_raw().cast_mut()) };
+                    ret_ptr_to_pyresult(vm, ret_ptr)
+                };
+                Ok(vm.ctx.new_method_def(name, callable, flags, doc))
+            }
+        },
+        _ => {
+            Err(vm.new_system_error(format!(
+                "function {name} has unsupported or invalid calling-convention flags: {flags:?}"
+            )))
+        },
+    })
+}
+
+unsafe fn call_function<A: Into<FuncArgs>>(
+    vm: &VirtualMachine,
+    method: PyMethodPointer,
+    flags: PyMethodFlags,
+    args: Option<A>,
+) -> PyResult {
+    let f = unsafe { method.PyCFunction };
+    let (slf, arg_tuple) = if let Some(mut args) = args.map(Into::into) {
+        let slf = take_self_arg(&mut args, flags);
+        let arg_tuple = vm.ctx.new_tuple(args.args);
+        (slf, Some(arg_tuple))
+    } else {
+        (None, None)
+    };
+
+    let slf_ptr = slf
+        .as_ref()
+        .map(|obj| obj.as_object().as_raw().cast_mut())
+        .unwrap_or_default();
+
+    let arg_ptr = arg_tuple
+        .as_ref()
+        .map(|tuple| tuple.as_object().as_raw().cast_mut())
+        .unwrap_or_default();
+
+    let ret_ptr = unsafe { f(slf_ptr, arg_ptr) };
+    ret_ptr_to_pyresult(vm, ret_ptr)
+}
+
+unsafe fn call_function_with_keywords(
+    vm: &VirtualMachine,
+    method: PyMethodPointer,
+    flags: PyMethodFlags,
+    mut args: FuncArgs,
+) -> PyResult {
+    let f = unsafe { method.PyCFunctionWithKeywords };
+    let slf = take_self_arg(&mut args, flags);
+    let slf_ptr = slf
+        .as_ref()
+        .map(|obj| obj.as_object().as_raw().cast_mut())
+        .unwrap_or_default();
+    let arg_tuple = vm.ctx.new_tuple(args.args);
+    let kwargs = vm.ctx.new_dict();
+    for (k, v) in args.kwargs {
+        kwargs.set_item(&*k, v, vm)?;
+    }
+    let ret_ptr = unsafe {
+        f(
+            slf_ptr,
+            arg_tuple.as_object().as_raw().cast_mut(),
+            kwargs.as_object().as_raw().cast_mut(),
+        )
+    };
+    ret_ptr_to_pyresult(vm, ret_ptr)
+}
+
+unsafe fn call_fast_function_with_keywords(
+    vm: &VirtualMachine,
+    method: PyMethodPointer,
+    flags: PyMethodFlags,
+    mut args: FuncArgs,
+) -> PyResult {
+    let f = unsafe { method.PyCFunctionFastWithKeywords };
+    let slf = take_self_arg(&mut args, flags);
+    let slf_ptr = slf
+        .as_ref()
+        .map(|obj| obj.as_object().as_raw().cast_mut())
+        .unwrap_or_default();
+    let nargs = args.args.len();
+    let mut fastcall_args = args.args;
+    let kwnames_tuple = if !args.kwargs.is_empty() {
+        let mut kwnames = Vec::with_capacity(args.kwargs.len());
+        for (k, v) in args.kwargs {
+            kwnames.push(vm.ctx.new_str(k).into());
+            fastcall_args.push(v);
+        }
+        Some(vm.ctx.new_tuple(kwnames))
+    } else {
+        None
+    };
+    let kwnames_ptr = kwnames_tuple
+        .as_ref()
+        .map(|tuple| tuple.as_object().as_raw().cast_mut())
+        .unwrap_or_default();
+    // SAFETY: PyObjectRef is repr(transparent) over a pointer to PyObject, so a
+    // Vec<PyObjectRef> has a layout-compatible contiguous backing buffer. The
+    // vector is kept alive for the duration of the call.
+    let fastcall_arg_ptrs = fastcall_args.as_ptr().cast::<*mut PyObject>();
+    let ret_ptr = unsafe { f(slf_ptr, fastcall_arg_ptrs, nargs as isize, kwnames_ptr) };
+    ret_ptr_to_pyresult(vm, ret_ptr)
+}
+
+unsafe fn call_fast_function(
+    vm: &VirtualMachine,
+    method: PyMethodPointer,
+    flags: PyMethodFlags,
+    args: PosArgs,
+) -> PyResult {
+    let f = unsafe { method.PyCFunctionFast };
+    let mut args: FuncArgs = args.into();
+    let slf = take_self_arg(&mut args, flags);
+    let slf_ptr = slf
+        .as_ref()
+        .map(|obj| obj.as_object().as_raw().cast_mut())
+        .unwrap_or_default();
+    // SAFETY: PyObjectRef is repr(transparent) over a pointer to PyObject, so a
+    // Vec<PyObjectRef> has a layout-compatible contiguous backing buffer. The
+    // vector is kept alive for the duration of the call.
+    let fastcall_arg_ptrs = args.args.as_mut_ptr().cast::<*mut PyObject>();
+    let ret_ptr = unsafe { f(slf_ptr, fastcall_arg_ptrs, args.args.len() as isize) };
+    ret_ptr_to_pyresult(vm, ret_ptr)
+}
+
+fn ret_ptr_to_pyresult(vm: &VirtualMachine, ret_ptr: *mut PyObject) -> PyResult {
+    let ret_ptr = NonNull::new(ret_ptr).ok_or_else(|| {
+        vm.take_raised_exception()
+            .expect("Native function returned NULL, but there was no exception set")
+    })?;
+    Ok(unsafe { PyObjectRef::from_raw(ret_ptr) })
+}
+
+fn take_self_arg(args: &mut FuncArgs, flags: PyMethodFlags) -> Option<PyObjectRef> {
+    if flags.contains(PyMethodFlags::STATIC) {
+        None
+    } else {
+        args.take_positional()
+    }
+}
+
+#[unsafe(no_mangle)]
+pub unsafe extern "C" fn PyCMethod_New(
+    ml: *mut PyMethodDef,
+    slf: *mut PyObject,
+    _module: *mut PyObject,
+    _cls: *mut PyTypeObject,
+) -> *mut PyObject {
+    with_vm(|vm| -> PyResult {
+        assert!(
+            _cls.is_null(),
+            "PyCMethod_New does not support METH_METHOD on abi3"
+        );
+        let ml = unsafe { &*ml };
+        let zelf = unsafe { slf.as_ref().map(|obj| obj.to_owned()) };
+        Ok(build_method_def(vm, ml, zelf.is_some())?
+            .build_function(vm, zelf)
+            .into())
+    })
+}
+
+#[unsafe(no_mangle)]
+pub unsafe extern "C" fn PyCFunction_New(
+    ml: *mut PyMethodDef,
+    slf: *mut PyObject,
+) -> *mut PyObject {
+    unsafe { PyCMethod_New(ml, slf, core::ptr::null_mut(), core::ptr::null_mut()) }
+}
+
+#[unsafe(no_mangle)]
+pub unsafe extern "C" fn PyCFunction_NewEx(
+    ml: *mut PyMethodDef,
+    slf: *mut PyObject,
+    module: *mut PyObject,
+) -> *mut PyObject {
+    unsafe { PyCMethod_New(ml, slf, module, core::ptr::null_mut()) }
+}
+
+#[cfg(false)]
+mod tests {
+    use pyo3::exceptions::PyException;
+    use pyo3::ffi::{PyLong_FromLong, PyObject};
+    use pyo3::prelude::*;
+    use pyo3::types::{PyCFunction, PyInt, PyString};
+
+    #[test]
+    fn test_closure_function() {
+        Python::attach(|py| {
+            let f = PyCFunction::new_closure(py, None, None, |_args, _kwargs| "Hello from Rust!")
+                .unwrap();
+
+            assert_eq!(
+                f.call0().unwrap().cast::<PyString>().unwrap(),
+                "Hello from Rust!"
+            );
+        })
+    }
+
+    #[test]
+    fn test_function_no_args() {
+        Python::attach(|py| {
+            unsafe extern "C" fn c_fn(_self: *mut PyObject, _args: *mut PyObject) -> *mut PyObject {
+                assert!(_self.is_null());
+                assert!(_args.is_null());
+                unsafe { PyLong_FromLong(4200) }
+            }
+
+            let py_fn = PyCFunction::new(py, c_fn, c"py_fn", c"", None).unwrap();
+
+            let result = py_fn
+                .call0()
+                .unwrap()
+                .cast::<PyInt>()
+                .unwrap()
+                .extract::<u32>()
+                .unwrap();
+            assert_eq!(result, 4200);
+
+            assert!(py_fn.call((1,), None).is_err());
+            assert!(py_fn.call((1, 2), None).is_err());
+        })
+    }
+
+    #[test]
+    fn test_closure_function_error() {
+        Python::attach(|py| {
+            let f = PyCFunction::new_closure(py, None, None, |_args, _kwargs| {
+                Err::<(), _>(PyException::new_err("Something went wrong"))
+            })
+            .unwrap();
+
+            let err = f.call0().unwrap_err();
+            assert_eq!(
+                err.value(py).repr().unwrap(),
+                "Exception('Something went wrong')"
+            );
+        })
+    }
+}