bazel-mypy-integration

Integrate MyPy type-checking into your Python Bazel builds.

⚠️ This software is now in 'production' use, but still in the PRE-RELEASE PHASE and under active development. Please give it a try, and ⭐️ or watch the repo to follow progress ⚠️

mypy is an incremental type system for Python. Incremental type systems arguably become essential when Python codebases grow to be very large [1, 2, 3, 4]. As Bazel is a build system designed for very large codebases, it makes sense that they should work together to help a Python codebase scale.

With bazel-mypy-integration, type errors are flagged as bazel build errors, so that teams can be sure their type-checking is being respected.

Usage

Aspect-based

It's recommended that you register this integration's Aspect to run everytime you build Python code. To do that you can add the following to your .bazelrc:

build --aspects @mypy_integration//:mypy.bzl%mypy_aspect
build --output_groups=+mypy

But if you want to invoke the integration directly, you can do so with:

bazel build --aspects @mypy_integration//:mypy.bzl%mypy_aspect --output_groups=mypy  //my/python/code/...

If there's a typing error in your Python code, then your build will fail. You'll see something like:

ERROR: /Users/jonathonbelotti/Code/thundergolfer/bazel-mypy-integration/examples/hangman/BUILD:1:1: 
MyPy hangman/hangman_dummy_out failed (Exit 1) hangman_mypy_exe failed: error executing command bazel-out/darwin-fastbuild/bin/hangman/hangman_mypy_exe

Use --sandbox_debug to see verbose messages from the sandbox
hangman/hangman.py:52: error: Syntax error in type annotation
hangman/hangman.py:52: note: Suggestion: Use Tuple[T1, ..., Tn] instead of (T1, ..., Tn)
Found 1 error in 1 file (checked 1 source file)
INFO: Elapsed time: 2.026s, Critical Path: 1.85s
INFO: 1 process: 1 darwin-sandbox.
FAILED: Build did NOT complete successfully

mypy_test rule-based

An alternative to registering the Aspect is to use the mypy_test rule which will run MyPy when the target run with bazel test.

load("@mypy_integration//:mypy.bzl", "mypy_test")

mypy_test(
    name = "foo_mypy_test",
    deps = [
        ":foo", # py_[library, binary, test] target
    ],
)

If there's a typing error in your Python code, then the test will fail. Using --test_output=all will ensure the MyPy error is visible in the console.

Installation

1. Create a file that will specify the version of mypy to use. You will pass the Bazel label for this file to the deps() function in @mypy_integration//repositories:deps.bzl, which below is named mypy_integration_deps(...):

mypy==0.790

❣️ Ensure that your selected MyPy version is compatible with your Python version. Incompatibilities can produce obscure looking errors.

(In the examples/ Bazel workspace this file is specified in tools/typing/)

2. Next, copy the WORKSPACE snippet

This can be found in the releases page for the release you use.

Note that by default mypy_integration_deps will default to passing "python3" as the interpreter used at install, but this can be overridden by setting python_interpreter or python_interpreter_target (but not both).

3. Finally, if using the Bazel Aspect, add the following to your .bazelrc so that MyPy checking is run whenever Python code is built:

build --aspects @mypy_integration//:mypy.bzl%mypy_aspect
build --output_groups=+mypy

3b. If using the Bazel rule, you'll add to a BUILD file something like:

load("@mypy_integration//:mypy.bzl", "mypy_test")

py_binary(
    name = "foo",
    srcs = glob(["foopy"]),
    main = "foo.py",
    python_version = "PY3",
    deps = [],
)

mypy_test(
    name = "foo_mypy",
    deps = [
        ":foo",
    ],
)

Configuration

To support the MyPy configuration file you can add the following to your .bazelrc:

build --@mypy_integration//:mypy_config=//:mypy.ini

where //:mypy.ini is a valid MyPy config file within your projects workspace.

🛠 Development

Testing

./test.sh runs some basic integration tests. Right now, running the integration in the Bazel workspace in examples/ tests a lot more functionality but can't automatically test failure cases.