Passing `mlx::core::array` from C++ to Python — TypeError: Unable to convert function return value to a Python type! #2793

codeSamuraii · 2025-11-18T22:33:34Z

codeSamuraii
Nov 18, 2025

Hello,

I'm trying to pass C++ mlx::core::array objects to Python via nanobind bindings, but I get the following error:

TypeError: Unable to convert function return value to a Python type! The signature was
    create_zeros(shape: collections.abc.Sequence[int]) -> mlx::core::array

The documentation suggests that I can return mlx::core::array objects directly from the bindings, but I haven't been able to pass C++ MLX arrays without explicitly using nb::ndarray or memviews.

I made a basic example to illustrate the issue.

Project Structure

mlx_minimal_example/
├── CMakeLists.txt
├── mlx_example/
│   └── __init__.py
├── pyproject.toml
├── README.md
├── src/
│   ├── bindings.cpp
│   ├── example.cpp
│   └── example.h
└── test_example.py

Code

src/bindings.cpp - The bindings import mlx.core first thing during module initialization

#include <nanobind/nanobind.h>
#include <nanobind/stl/vector.h>
#include "example.h"

namespace nb = nanobind;
using namespace nb::literals;

NB_MODULE(_ext, m) {
    std::cout << "bindings.cpp — Initializing nanobind module" << std::endl;

    nb::module_ mx = nb::module_::import_("mlx.core");
    std::cout << "bindings.cpp — Imported `mlx.core` module" << std::endl;

    m.doc() = "Minimal MLX nanobind extension example";
    m.def("create_zeros", &mlx_example::create_zeros, "shape"_a);
    m.def("create_ones", &mlx_example::create_ones, "shape"_a);
    m.def("create_range", &mlx_example::create_range, "n"_a);

    std::cout << "bindings.cpp — Module initialization complete" << std::endl;
}

src/example.cpp - Functions returning mlx::core::array

#include <mlx/mlx.h>
#include <iostream>

namespace mx = mlx::core;
namespace mlx_example {

mx::array create_zeros(const std::vector<int>& shape) {
    mx::Shape s(shape.begin(), shape.end());
    std::cout << "Creating zeros with shape: " << s << std::endl;
    auto arr = mx::zeros(s, mx::uint8);
    mx::eval(arr);
    return arr;
}

// ... other functions ...

}

mlx_example/__init__.py

import mlx.core as mx
print(f"__init__.py — `mlx.core` version: {mx.__version__}")

from _ext import create_zeros, create_ones, create_range
print(f"__init__.py — functions:\n{create_zeros}\n{create_ones}\n{create_range}")

Build Configuration

CMakeLists.txt

cmake_minimum_required(VERSION 3.22)
project(mlx_minimal_example LANGUAGES CXX)

set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

set(Python_FIND_VIRTUALENV ONLY)
find_package(Python REQUIRED COMPONENTS Interpreter Development.Module)

# ---- Nanobind ----
execute_process(
    COMMAND "${Python_EXECUTABLE}" -m nanobind --cmake_dir
    OUTPUT_STRIP_TRAILING_WHITESPACE
    OUTPUT_VARIABLE NANOBIND_CMAKE_DIR
)
list(PREPEND CMAKE_PREFIX_PATH "${NANOBIND_CMAKE_DIR}")
find_package(nanobind CONFIG REQUIRED)

# ---- MLX ----
execute_process(
    COMMAND "${Python_EXECUTABLE}" -m mlx --cmake-dir
    OUTPUT_STRIP_TRAILING_WHITESPACE
    OUTPUT_VARIABLE MLX_CMAKE_DIR
)
list(PREPEND CMAKE_PREFIX_PATH "${MLX_CMAKE_DIR}")
find_package(MLX CONFIG REQUIRED)

# ---- Extension build ----
nanobind_add_module(
  _ext
  NB_STATIC STABLE_ABI LTO NOMINSIZE
  NB_DOMAIN mlx
  src/bindings.cpp
  src/example.cpp
)

target_include_directories(_ext PRIVATE
  "src/"
  "${MLX_INCLUDE_DIRS}"
  "${NANOBIND_CMAKE_DIR}/../include"
)
target_link_directories(_ext PRIVATE
  "${MLX_LIBRARY_DIRS}"
)
target_link_libraries(_ext PRIVATE mlx)

install(TARGETS _ext LIBRARY DESTINATION .)

pyproject.toml:

[build-system]
requires = [
    "mlx",
    "scikit-build-core>=0.8",
    "nanobind @ git+https://github.com/wjakob/nanobind.git@master",
]
build-backend = "scikit_build_core.build"

[project]
name = "mlx_minimal_example"
version = "0.0.1"
description = "Minimal nanobind extension example for MLX arrays"
requires-python = ">=3.8"
dependencies = ["mlx"]

Build output

Click to view build logs

    *** Configuring CMake...
    loading initial cache file /var/folders/mv/23hl6nm967qb1rg2rxpgvgfc0000gn/T/tmpunaqj5gq/build/CMakeInit.txt
    CMake Warning (dev) at /opt/homebrew/share/cmake/Modules/GNUInstallDirs.cmake:433 (message):
      Unable to determine default CMAKE_INSTALL_LIBDIR directory because no
      target architecture is known.  Please enable at least one language before
      including GNUInstallDirs.
    Call Stack (most recent call first):
      /opt/homebrew/share/cmake/Modules/GNUInstallDirs.cmake:276 (_GNUInstallDirs_LIBDIR_get_default)
      /opt/homebrew/share/cmake/Modules/GNUInstallDirs.cmake:276 (cmake_language)
      /opt/homebrew/share/cmake/Modules/GNUInstallDirs.cmake:525 (_GNUInstallDirs_cache_path)
      CMakeLists.txt:69 (include)
    This warning is for project developers.  Use -Wno-dev to suppress it.

    -- Configuring done (0.1s)
    -- Generating done (0.0s)
    -- Build files have been written to: /var/folders/mv/23hl6nm967qb1rg2rxpgvgfc0000gn/T/tmpunaqj5gq/build
    *** Building project with Ninja...
    ninja: no work to do.
    *** Installing project into wheel...
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Using cached mlx_metal-0.29.4-py3-none-macosx_15_0_arm64.whl (36.7 MB)
Building wheels for collected packages: mlx_minimal_example
  Running command Building wheel for mlx_minimal_example (pyproject.toml)
  *** scikit-build-core 0.11.6 using CMake 4.1.2 (wheel)
  *** Configuring CMake...
  loading initial cache file /var/folders/mv/23hl6nm967qb1rg2rxpgvgfc0000gn/T/tmpti2wo613/build/CMakeInit.txt
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Successfully built mlx_minimal_example

Script output

__init__.py — `mlx.core` version: 0.29.4
bindings.cpp — Initializing nanobind module
bindings.cpp — Imported `mlx.core` module
bindings.cpp — Module initialization complete
__init__.py — functions:
<nanobind.nb_func object at 0x100e3f9a0>
<nanobind.nb_func object at 0x100e3fab0>
<nanobind.nb_func object at 0x100e3fbc0>

1. Testing create_zeros([2, 3]):
Creating zeros with shape: (2,3)
Traceback (most recent call last):
  File "/Users/remi/Programs/mlx_minimal_example/test_example.py", line 7, in <module>
    zeros = mle.create_zeros([2, 3])
TypeError: Unable to convert function return value to a Python type! The signature was
    create_zeros(shape: collections.abc.Sequence[int]) -> mlx::core::array

The documentation mentions importing mlx.core before the extension, which I do.

Is there an error in my configuration? Any help would be appreciated.

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Passing `mlx::core::array` from C++ to Python — TypeError: Unable to convert function return value to a Python type! #2793

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Passing mlx::core::array from C++ to Python — TypeError: Unable to convert function return value to a Python type! #2793

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codeSamuraii Nov 18, 2025

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Passing `mlx::core::array` from C++ to Python — TypeError: Unable to convert function return value to a Python type! #2793

codeSamuraii
Nov 18, 2025