bazel-mypy-integration

Integrate MyPy type-checking into your Python Bazel builds.

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⚠️ 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 ⚠️

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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

It's highly 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

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==0.750

⚠️ Note: Currently only 0.750 (latest) is being supported, but the idea is to support multiple versions in future using the above mechanism.

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

2. Next, add the following to your WORKSPACE:

mypy_integration_version = "0.0.10" # latest @ July 21st 2020

http_archive(
    name = "mypy_integration",
    sha256 = "cc1cb372140d6a29ba0dcf5e6ebc961c6d509a7296fb101e70317dfc61d108e7",
    strip_prefix = "bazel-mypy-integration-{version}".format(version = mypy_integration_version),
    url = "https://github.com/thundergolfer/bazel-mypy-integration/archive/{version}.tar.gz".format(
        version = mypy_integration_version,
    ),
)

load(
    "@mypy_integration//repositories:repositories.bzl",
    mypy_integration_repositories = "repositories",
)
mypy_integration_repositories()

load("@mypy_integration//:config.bzl", "mypy_configuration")
# Optionally pass a MyPy config file, otherwise pass no argument.
mypy_configuration("//tools/typing:mypy.ini")

load("@mypy_integration//repositories:deps.bzl", mypy_integration_deps = "deps")

mypy_integration_deps("//tools/typing:mypy_version.txt")

3. Finally, 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

Configuration

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

load("@mypy_integration//:config.bzl", "mypy_configuration")

mypy_configuration("//tools/typing:mypy.ini")

where //tools/typing:mypy.ini is a valid MyPy config file.

🛠 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.

Adopters

Here's a (non-exhaustive) list of companies that use bazel-mypy-integration in production. Don't see yours? You can add it in a PR!

• Canva