python/runfiles/README.md

# bazel-runfiles library

This is a Bazel Runfiles lookup library for Bazel-built Python binaries and tests.

Learn about runfiles: read [Runfiles guide](https://bazel.build/extending/rules#runfiles)
or watch [Fabian's BazelCon talk](https://www.youtube.com/watch?v=5NbgUMH1OGo).

## Importing

The Runfiles API is available from two sources, a direct Bazel target, and a [pypi](https://pypi.org/) package.

## Pure Bazel imports

1. Depend on this runfiles library from your build rule, like you would other third-party libraries:

    ```python
    py_binary(
        name = "my_binary",
        # ...
        deps = ["@rules_python//python/runfiles"],
    )
    ```

2. Import the runfiles library:

    ```python
        from python.runfiles import Runfiles
    ```

## Pypi imports

1. Add the 'bazel-runfiles' dependency along with other third-party dependencies, for example in your `requirements.txt` file.

2. Depend on this runfiles library from your build rule, like you would other third-party libraries:
    ```python
    load("@pip_deps//:requirements.bzl", "requirement")
    
    py_binary(
        name = "my_binary",
        ...
        deps = [requirement("bazel-runfiles")],
    )
    ```

3. Import the runfiles library:
    ```python
    from runfiles import Runfiles
    ```

## Typical Usage

Create a `Runfiles` object and use `Rlocation` to look up runfile paths:

```python
r = Runfiles.Create()
# ...
with open(r.Rlocation("my_workspace/path/to/my/data.txt"), "r") as f:
    contents = f.readlines()
    # ...
```

The code above creates a manifest- or directory-based implementation based on the environment variables in `os.environ`. See `Runfiles.Create()` for more info.

If you want to explicitly create a manifest- or directory-based
implementation, you can do so as follows:

```python
r1 = Runfiles.CreateManifestBased("path/to/foo.runfiles_manifest")

r2 = Runfiles.CreateDirectoryBased("path/to/foo.runfiles/")
```

If you want to start subprocesses, and the subprocess can't automatically
find the correct runfiles directory, you can explicitly set the right
environment variables for them:

```python
import subprocess
from python.runfiles import Runfiles

r = Runfiles.Create()
env = {}
# ...
env.update(r.EnvVars())
p = subprocess.run(
    [r.Rlocation("path/to/binary")],
    env=env,
    # ...
)
```