# Setting up coverage

As of Bazel 6, the Python toolchains and bootstrap logic supports providing
coverage information using the `coverage` library.

As of `rules_python` version `0.18.1`, builtin coverage support can be enabled
when configuring toolchains.

## Enabling `rules_python` coverage support

Enabling the coverage support bundled with `rules_python` just requires setting an
argument when registering toolchains.

For Bzlmod:

```starlark
python.toolchain(
    "@python3_9_toolchains//:all",
    configure_coverage_tool = True,
)
```

For WORKSPACE configuration:

```starlark
python_register_toolchains(
   register_coverage_tool = True,
)
```

:::{note}
This will implicitly add the version of `coverage` bundled with
`rules_python` to the dependencies of `py_test` rules when `bazel coverage` is
run. If a target already transitively depends on a different version of
`coverage`, then the behavior is undefined -- it is undefined which version comes
first in the import path. If you find yourself in this situation, then you'll
need to manually configure coverage (see below).
:::

:::{note}
The bundled `coverage` wheel set covers CPython 3.9 through 3.14 (with
freethreaded variants for 3.13+). For Python versions outside that range,
`configure_coverage_tool = True` is a silent no-op and `bazel coverage` will
produce empty lcov data; manually configure coverage (see below) instead.
:::

## Manually configuring coverage

To manually configure coverage support, you'll need to set the
`py_runtime.coverage_tool` attribute. This attribute is a target that specifies
the coverage entry point file and, optionally, client libraries that are added
to `py_test` targets. Typically, this would be a `filegroup` that looked like:

```starlark
filegroup(
  name = "coverage",
  srcs = ["coverage_main.py"],
  data = ["coverage_lib1.py", ...]
)
```

Using `filegroup` isn't required, nor are including client libraries. The
important behaviors of the target are:

*   It provides a single output file OR it provides an executable output; this
    output is treated as the coverage entry point.
*   If it provides runfiles, then `runfiles.files` are included into `py_test`.