Flake & Rules_nixpkgs: Linking Python Toolchain In Bazel

Oct 11, 2025 by ADMIN 57 views

Python Toolchain Not Properly Linked to Flake Packages

Hey guys! Let's dive into a common issue when trying to wrangle Python toolchains with Nix flakes and Bazel. Specifically, we're going to tackle the problem of Bazel picking up the wrong Python version when you're trying to enforce a specific one using rules_nixpkgs. It's a bit of a head-scratcher, but we'll break it down.

The Problem: Wrong Python Version in Bazel Builds

So, the main issue? You're aiming to create a portable dev environment using Nix, complete with particular versions of Clang and Python. You're using rules_nixpkgs to make these available for your Bazel build jobs. Everything seems to work fine with Clang, but Python is being a pain. Bazel stubbornly grabs a different Python version (like 3.11) from your Nix store, instead of the intended version (say, 3.10) that you've specified in your flake.nix.

Why This Happens

Understanding why this happens requires a bit of digging into how Bazel and rules_nixpkgs interact with Nix. rules_nixpkgs is designed to bridge the gap between Bazel and Nix, allowing you to leverage Nix's reproducible build environment within Bazel. However, the configuration needs to be spot-on to ensure Bazel uses the exact toolchain you intend. When the Python toolchain isn't correctly linked, Bazel might fall back to a default Python installation available in your environment, which could be a different version than what you specified.

Key Configuration Files

To diagnose and fix this, let's look at the critical configuration files involved:

flake.nix: This file defines your Nix environment, specifying the exact versions of Python and other dependencies.
WORKSPACE.bazel: This file configures Bazel, telling it where to find the toolchains provided by Nix.

By meticulously setting up these files, you instruct Bazel on which tools to use during the build process. Now, let's see how these files should be structured to avoid the Python version mishap.

Diving into `flake.nix`

Your flake.nix file should look something like this:

{
  description = "Bazel Nix Demo Development Environment";
  
  # specify other flakes that this flake depends on
  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-25.05"; # main nix pkg collection on stable release channel
    flake-utils.url = "github:numtide/flake-utils"; # helper func for writing flakes
  };
  
  # this is the heart of the flake - a function that produces an
  # attribute set; inputs are 'function args' to this
  outputs = { self, nixpkgs, flake-utils }:
    flake-utils.lib.eachDefaultSystem (system:
      let
        pkgs = nixpkgs.legacyPackages.${system};
      in
      {
        devShells.default = pkgs.mkShell {
          buildInputs = with pkgs; [
            bazel
            bazel-buildtools
            python310
            clang_20
          ];

          shellHook = ''
            echo "🎯 Bazel + Nix packages ready"
          '';
        };

        # Expose packages for Bazel to use here
        packages = {
          python = pkgs.python310;
        };
      }
    );
}

In this file, you're using nixpkgs.legacyPackages.${system} to access packages and creating a development shell with Bazel, bazel-buildtools, python310, and clang_20 as build inputs. The critical part is exposing python310 in the packages attribute set. This makes it available for Bazel to consume via rules_nixpkgs.

Ensuring Correct Package Exposure

Make sure that the packages attribute set correctly exposes the desired Python version. This is where you explicitly tell Nix to make python310 available under the alias python. Incorrectly specifying this can lead Bazel to pick up the wrong version.

Configuring `WORKSPACE.bazel`

Now, let's look at your WORKSPACE.bazel file:

# NOTE must utilize older bazel WORKSPACE config for pointing build system
# to flakes nix packages using rules_nixpkgs as it does not see setup yet
# to work with MODULE.bazel

load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")

# Import the rules_nixpkgs repository from pinned remove version
http_archive(
    name = "io_tweag_rules_nixpkgs",
    sha256 = "b709567b161a3d4636537de2019724941d8d5cbc49f55144fe7ce3a7c9af4955",
    strip_prefix = "rules_nixpkgs-0.12.0",
    urls = ["https://github.com/tweag/rules_nixpkgs/archive/v0.12.0.tar.gz"],
)

load("@io_tweag_rules_nixpkgs//nixpkgs:repositories.bzl", "rules_nixpkgs_dependencies")
rules_nixpkgs_dependencies()

# Import nixpkgs from your flake
load("@io_tweag_rules_nixpkgs//nixpkgs:nixpkgs.bzl", "nixpkgs_local_repository")
nixpkgs_local_repository(
    name = "nixpkgs",
    nix_flake_lock_file = "//:flake.lock",
    nix_file_deps = ["//:flake.lock"],
)

# Configure C++ toolchain using the flake's nixpkgs
load("@io_tweag_rules_nixpkgs//nixpkgs:nixpkgs.bzl", "nixpkgs_cc_configure")
nixpkgs_cc_configure(
    name = "nixpkgs_config_cc",
    repository = "@nixpkgs",
    attribute_path = "clang_20",
)

# Configure python toolchain using the flake's nixpkgs
load("@io_tweag_rules_nixpkgs//nixpkgs:nixpkgs.bzl", "nixpkgs_python_configure")
nixpkgs_python_configure(
    name = "nixpkgs_config_python",
    repository = "@nixpkgs",
    python3_attribute_path = "python",
)

register_toolchains(
    "@nixpkgs_config_python//:toolchain",
)

Here, you're loading rules_nixpkgs and setting up the C++ and Python toolchains. Let's zoom in on the Python configuration:

nixpkgs_python_configure(
    name = "nixpkgs_config_python",
    repository = "@nixpkgs",
    python3_attribute_path = "python",
)

The python3_attribute_path is set to python. This tells rules_nixpkgs to look for the package named python in your Nix packages. Ensure this name matches the one you exposed in your flake.nix. If there's a mismatch, Bazel won't find the correct Python version.

Troubleshooting Steps

If you're still facing issues, here are some troubleshooting steps:

Verify flake.lock: Ensure your flake.lock file is up-to-date. Run nix flake update to update it.
Check Attribute Paths: Double-check that the attribute paths in WORKSPACE.bazel match the names in flake.nix.
Bazel Clean: Sometimes, Bazel caches old configurations. Try running bazel clean --expunge to clear the cache.
Verbose Output: Add --verbose to your Bazel build command to see more detailed output and identify where the wrong Python version is being picked up.

Debugging with Verbose Output

Using verbose output can be a lifesaver. When you run Bazel with the --verbose flag, you get a torrent of information about the build process. Sift through this to find out exactly which Python executable Bazel is using. Look for paths or version numbers that might indicate the wrong Python is being invoked.

Example Scenario and Solution

Let's say you've followed all the steps, but Bazel still picks up Python 3.11 instead of 3.10. Here's a possible scenario and solution:

Scenario: You have Python 3.11 installed system-wide, and Bazel is defaulting to it, despite your Nix configuration.
Solution: Isolate the Bazel build environment as much as possible. Ensure that the devShells.default in flake.nix includes only the necessary tools and that your system environment isn't bleeding into the Bazel build.

Final Thoughts

Getting Bazel, Nix, and rules_nixpkgs to play nice can be a bit tricky, but with careful configuration and a bit of debugging, you can enforce the exact toolchain you need. Always double-check your attribute paths, keep your flake.lock updated, and don't underestimate the power of verbose output. By following these steps, you'll be well on your way to a reproducible and reliable build environment. Happy building, folks!

Staying Updated with Toolchain Configurations

In the ever-evolving landscape of Bazel and Nix, it's crucial to stay updated with the latest best practices and configurations. Keep an eye on the rules_nixpkgs repository for updates and community discussions. Regularly reviewing your setup ensures that you're leveraging the most efficient and correct methods for linking your Python toolchain.

Community Resources and Support

Don't hesitate to tap into community resources for support. Forums, mailing lists, and online communities dedicated to Bazel and Nix are invaluable sources of information and assistance. Sharing your experiences and learning from others can significantly streamline your development process and help you overcome any hurdles you encounter.

Automating Toolchain Management

Consider automating your toolchain management to further enhance reproducibility and consistency. Tools and scripts can help streamline the process of updating dependencies, validating configurations, and ensuring that your build environment remains stable over time. This automation not only saves time but also reduces the risk of human error.

The Importance of Reproducible Builds

Ultimately, the goal is to achieve reproducible builds. By carefully configuring your Bazel and Nix environments, you can ensure that your builds are consistent across different machines and over time. This reproducibility is essential for maintaining the integrity of your projects and facilitating collaboration among developers.

Conclusion

Mastering the integration of Python toolchains with Flake packages using rules_nixpkgs in Bazel requires a deep understanding of the underlying systems and careful attention to configuration details. By following the guidelines and troubleshooting steps outlined in this article, you can create a reliable and reproducible build environment that meets your specific needs. Keep exploring, experimenting, and refining your setup to unlock the full potential of Bazel and Nix in your development workflow.