nanobind-bazel: Bazel build rules for C++ Python bindings with nanobind

This repo contains Bazel build defs for Python bindings created with nanobind.

Here's the full list of exported rules:

• nanobind_extension, building a Python extension containing the bindings as a *.so file. These extensions can be used e.g. as a data dependency for a py_library target.
• nanobind_stubgen, a rule pointing to a py_binary to create a Python stub file from a previously built nanobind_extension. (Available only with nanobind>=v2.0.0.)
• nanobind_library, a C++ library target that can be used as a dependency of a nanobind_extension. Directly forwards its arguments to the cc_library rule.
• nanobind_shared_library, a C++ shared library target that can be used to produce smaller objects in scenarios with multiple independent bindings extensions. Directly forwards its arguments to the cc_shared_library rule.
• nanobind_test, a C++ test for a nanobind_library. Forwards its argument to a cc_test.

Each target is given nanobind's specific build flags, optimizations and dependencies.

Usage with bzlmod

nanobind-bazel is published to the Bazel Central Registry (BCR). To use it, specify it as a bazel_dep:

# the major version of nanobind-bazel is equal to the major version of the internally used nanobind.
# In this case, we are building bindings with nanobind@v2.
bazel_dep(name = "nanobind_bazel", version = "2.0.0")

To instead use a development version, you can declare a git_override() dependency in your MODULE.bazel:

bazel_dep(name = "nanobind_bazel", version = "")
git_override(
    module_name = "nanobind_bazel",
    commit = "COMMIT_SHA", # replace this with the actual commit SHA you want.
    remote = "https://github.com/nicholasjng/nanobind-bazel",
)

In local development scenarios, you can clone nanobind-bazel to your machine and then declare it as a local_path_override() like so:

bazel_dep(name = "nanobind_bazel", version = "")
local_path_override(
    module_name = "nanobind_bazel",
    path = "path/to/nanobind-bazel/", # replace this with the actual path.
)

Bazel versions

This library relies on the ability to pass inputs to the linker in cc_library targets, which became available starting in Bazel 6.4.0. For this reason, the minimum Bazel version compatible with this project is Bazel 6.4.0.

In general, since Bazel 7 enabled bzlmod by default, no more intentional development efforts are made to support the workspace system.

Licenses and acknowledgements

This library is heavily inspired by the pybind11-bazel project, which does the same thing for pybind11. As I have used some of the code from that repository, its license is included here, too.

In contrast to that project, though, nanobind does not support Python interpreter embedding, and endorses a few more size-related optimizations which I have included here.

Roadmap

• First successful test, e.g. on wjakob's nanobind example.
• A BCR release.
• A nanobind_shared_library target for a cc_shared_library using (lib)nanobind.
• Supporting custom nanobind build targets instead of the internal one.

Contributing

I welcome all contributions. If you encounter problems using these rules in your Bazel setup, please open an issue. If you'd like to help maintain the project, write me a message.