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Version: 2.22

check

How to use MyPy.

Activating MyPy​

To opt-in, add pants.backend.python.typecheck.mypy to backend_packages in your config file.

pants.toml
[GLOBAL]
backend_packages.add = [
  "pants.backend.python",
  "pants.backend.python.typecheck.mypy",
]

This will register a new check goal:

$ pants check helloworld/util/lang.py
$ pants check ::
Benefit of Pants: typecheck Python 2-only and Python 3-only code at the same time

MyPy determines which Python version to use based on its python_version option. If that's undefined, MyPy uses the interpreter the tool is run with. Because you can only use one config file at a time with MyPy, you cannot normally say to use 2.7 for part of your codebase but 3.6 for the rest; you must choose a single version.

Instead, Pants will group your targets based on their interpreter constraints, and run all the Python 2 targets together and all the Python 3 targets together. It will automatically set python_version to the minimum compatible interpreter, such as a constraint like ["==2.7.*", ">3.6"] using 2.7.

To turn this off, you can still set python_version in mypy.ini or --python-version/--py2 in --mypy-args; Pants will respect the value you set.

Hook up a MyPy config file​

Pants will automatically include your config file if it's located at mypy.ini, .mypy.ini, setup.cfg, or pyproject.toml.

Otherwise, you must set the option [mypy].config for Pants to include the config file in the process's sandbox and to instruct MyPy to load it.

pants.toml
[mypy]
config = "build-support/mypy.ini"

Change the MyPy version​

Use the install_from_resolve option in the [mypy] scope:

pants.toml
[python.resolves]
mypy = "3rdparty/python/mypy.lock"

[mypy]
install_from_resolve = "mypy"

See Lockfiles for tools.

Incrementally adopt MyPy with skip_mypy=True​

You can tell Pants to skip running MyPy on certain files by adding skip_mypy=True to the relevant targets.

project/BUILD
# Skip MyPy for all the Python files in this directory
# (both test and non-test files).
python_sources(name="lib", skip_mypy=True)
python_tests(name="tests", skip_mypy=True)

# To only skip certain files, use the `overrides` field.
python_sources(
    name="lib",
    overrides={
        "util.py": {"skip_mypy": True},
        # Use a tuple to specify multiple files.
        ("user.py", "admin.py"): {"skip_mypy": True},
    },
)

When you run pants check ::, Pants will skip any files belonging to skipped targets.

MyPy may still try to check the skipped files!

The skip_mypy field only tells Pants not to provide the skipped files as direct input to MyPy. But MyPy, by default, will still try to check files that are dependencies of the direct inputs. So if your skipped files are dependencies of unskipped files, they may still be checked.

To change this behavior, use MyPy's --follow-imports option, typically by setting it to silent. You can do so either by adding it to the args option in the [mypy] section of your Pants config file, or by setting it in mypy.ini.

First-party type stubs (.pyi files)​

You can use .pyi files for both first-party and third-party code. Include the .pyi files in the sources field for python_source / python_sources and python_test / python_tests targets. MyPy will use these stubs rather than looking at the implementation.

Pants's dependency inference knows to infer a dependency both on the implementation and the type stub. You can verify this by running pants dependencies path/to/file.py.

When writing stubs for third-party libraries, you may need to set up the [source].root_patterns option so that source roots are properly stripped. For example:

[source]
root_patterns = ["mypy-stubs", "src/python"]

Third-party type stubs​

You can install third-party type stubs (for example, types-requests) like normal Python requirements. Pants will infer a dependency on both the type stub and the actual dependency, for example, both types-requests and requests, which you can confirm by running pants dependencies path/to/f.py.

If you install MyPy from a custom lockfile you can also add type stub requirements to that lockfile. This ensures that the stubs are only used when running MyPy and are not included when, for example, packaging a PEX.

Add a third-party plugin​

Add any third-party MyPy plugins to a custom lockfile:

[python.resolves]
mypy = "3rdparty/python/mypy-lock.txt"

[mypy]
install_from_resolve = "mypy"

Then update your mypy.ini to load the plugin:

mypy.ini
[mypy]
plugins =
    pydantic.mypy

For some plugins, such as django-stubs, you may need to always load certain source files, such as a settings.py file. You can make sure that this source file is always used by hijacking the source_plugins option, which allows you to specify targets whose sources should always be used when running MyPy. See the section below for more information about source plugins.

For example, to fully use the django-stubs plugin, your setup might look like this:

[source]
root_patterns = ["src/python"]

[mypy]
install_from_resolve = "mypy"
source_plugins = ["src/python/project:django_settings"]
Importing type stubs

Type stubs specified in the MyPy custom lockfile are not visible to the python-infer subsystem, and cannot be referenced as explicit dependencies. If you import from a stubs module in your code, and it does not have a corresponding implementation python_requirement target that provides the imported module, you may see a warning/error depending on the value you've configured for [python-infer].unowned_dependency_behavior. Goals other than check will also raise ImportErrors if the import isn't conditional on the value of typing.TYPE_CHECKING:

[python-infer]
unowned_dependency_behavior = "warning"
[mypy]
install_from_resolve = "mypy"

For these reasons, it's recommended to load any type-stub libraries that require explicit imports as part of your normal third-party dependencies. Alternatively, you can set # pants: no-infer-dep on the lines of type-stub imports "guarded" by a check of if TYPE_CHECKING.

MyPy Protobuf support

Add mypy_plugin = true to the [python-protobuf] scope. See Protobuf for more information.

Add a first-party plugin​

To add a MyPy plugin you wrote, add a python_source or python_sources target with the plugin's Python file(s) included in the sources field.

Then, add plugins = path.to.module to your MyPy config file, using the name of the module without source roots. For example, if your Python file is called pants-plugins/mypy_plugins/custom_plugin.py, and you set pants-plugins as a source root, then set plugins = mypy_plugins.custom_plugin. Set the config option in the [mypy] scope in your pants.toml to point to your MyPy config file.

Finally, set the option source_plugins in the [mypy] scope to include this target's address, e.g. source_plugins = ["pants-plugins/mypy_plugins:plugin"]. This will ensure that your plugin's sources are always included in the subprocess.

For example:

[mypy]
source_plugins = ["pants-plugins/mypy_plugins:plugin"]

Because this is a python_source or python_sources target, Pants will treat this code like your other Python files, such as running linters on it or allowing you to write a python_distribution target to distribute the plugin externally.

Reports​

MyPy can generate various report files.

For Pants to properly preserve the reports, instruct MyPy to write to the reports/ folder by updating its config file or --mypy-args. For example, in your pants.toml:

pants.toml
[mypy]
args = ["--linecount-report=reports"]

Pants will copy all reports into the folder dist/check/mypy.

Known limitations​

Performance is sometimes slower than normal​

Pants 2.14 added support for leveraging MyPy's cache, making subsequent runs of MyPy extremely performant. The support, however, requires features that were added to MyPy in version 0.700, and requires that python_version isn't set in MyPy's config or in [mypy].args.

If you're using a version of MyPy older than 0.700, consider upgrading to unlock super-speedy subsequent runs of MyPy. Additionally consider not providing python_version in your config or args.

Tip: only run over changed files and their dependents​

When changing type hints code, you not only need to run over the changed files, but also any code that depends on the changed files:

$ pants --changed-since=HEAD --changed-dependents=transitive check

See Advanced target selection for more information.