feat(runfiles): create pathlib api for runfiles library (#3694)

This adds a pathlib.Path-based API for interacting with runfiles. This
makes it easier to interact with runfiles.

Fixes #3296

---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: rickeylev

CHANGELOG.md

@@ -47,6 +47,28 @@ BEGIN_UNRELEASED_TEMPLATE
 END_UNRELEASED_TEMPLATE
 -->
 
+{#v0-0-0}
+## Unreleased
+
+[0.0.0]: https://github.com/bazel-contrib/rules_python/releases/tag/0.0.0
+
+{#v0-0-0-removed}
+### Removed
+* Nothing removed.
+
+{#v0-0-0-changed}
+### Changed
+* Nothing changed.
+
+{#v0-0-0-fixed}
+### Fixed
+* Nothing fixed.
+
+{#v0-0-0-added}
+### Added
+* (runfiles) Added a pathlib-compatible API: {obj}`Runfiles.root()`
+  Fixes [#3296](https://github.com/bazel-contrib/rules_python/issues/3296).
+
 {#v2-0-0}
 ## [2.0.0] - 2026-04-09
 

python/runfiles/runfiles.py

@@ -25,10 +25,11 @@
 import collections.abc
 import inspect
 import os
+import pathlib
 import posixpath
 import sys
 from collections import defaultdict
-from typing import Dict, Optional, Tuple, Union
+from typing import Dict, List, Optional, Tuple, Union
 
 
 class _RepositoryMapping:
@@ -137,7 +138,127 @@ def is_empty(self) -> bool:
         Returns:
             True if there are no mappings, False otherwise
         """
-        return len(self._exact_mappings) == 0 and len(self._grouped_prefixed_mappings) == 0
+        return (
+            len(self._exact_mappings) == 0 and len(self._grouped_prefixed_mappings) == 0
+        )
+
+
+class Path(pathlib.PurePath):
+    """A pathlib-like path object for runfiles.
+
+    This class extends `pathlib.PurePath` and resolves paths
+    using the associated `Runfiles` instance when converted to a string.
+    """
+
+    # For Python < 3.12 compatibility when subclassing PurePath directly
+    _flavour = getattr(type(pathlib.PurePath()), "_flavour", None)
+
+    def __new__(
+        cls,
+        *args: Union[str, os.PathLike],
+        runfiles: Optional["Runfiles"] = None,
+        source_repo: Optional[str] = None,
+    ) -> "Path":
+        """Private constructor. Use Runfiles.root() to create instances."""
+        obj = super().__new__(cls, *args)
+        # Type checkers might complain about adding attributes to PurePath,
+        # but this is standard for pathlib subclasses.
+        obj._runfiles = runfiles  # type: ignore
+        obj._source_repo = source_repo  # type: ignore
+        return obj
+
+    def __init__(
+        self,
+        *args: Union[str, os.PathLike],
+        runfiles: Optional["Runfiles"] = None,
+        source_repo: Optional[str] = None,
+    ) -> None:
+        pass
+
+    def with_segments(self, *pathsegments: Union[str, os.PathLike]) -> "Path":
+        """Used by Python 3.12+ pathlib to create new path objects."""
+        return type(self)(
+            *pathsegments,
+            runfiles=self._runfiles,  # type: ignore
+            source_repo=self._source_repo,  # type: ignore
+        )
+
+    # For Python < 3.12
+    @classmethod
+    def _from_parts(cls, args: Tuple[str, ...]) -> "Path":
+        obj = super()._from_parts(args)  # type: ignore
+        # These will be set by the calling instance later, or we can't set them here
+        # properly without context. Usually pathlib calls this from an instance
+        # method like _make_child, which we also might need to override.
+        return obj
+
+    def _make_child(self, args: Tuple[str, ...]) -> "Path":
+        obj = super()._make_child(args)  # type: ignore
+        obj._runfiles = self._runfiles  # type: ignore
+        obj._source_repo = self._source_repo  # type: ignore
+        return obj
+
+    @classmethod
+    def _from_parsed_parts(cls, drv: str, root: str, parts: List[str]) -> "Path":
+        obj = super()._from_parsed_parts(drv, root, parts)  # type: ignore
+        return obj
+
+    def _make_child_relpath(self, part: str) -> "Path":
+        obj = super()._make_child_relpath(part)  # type: ignore
+        obj._runfiles = self._runfiles  # type: ignore
+        obj._source_repo = self._source_repo  # type: ignore
+        return obj
+
+    @property
+    def parents(self) -> Tuple["Path", ...]:
+        return tuple(
+            type(self)(
+                p,
+                runfiles=getattr(self, "_runfiles", None),
+                source_repo=getattr(self, "_source_repo", None),
+            )
+            for p in super().parents
+        )
+
+    @property
+    def parent(self) -> "Path":
+        return type(self)(
+            super().parent,
+            runfiles=getattr(self, "_runfiles", None),
+            source_repo=getattr(self, "_source_repo", None),
+        )
+
+    def with_name(self, name: str) -> "Path":
+        return type(self)(
+            super().with_name(name),
+            runfiles=getattr(self, "_runfiles", None),
+            source_repo=getattr(self, "_source_repo", None),
+        )
+
+    def with_suffix(self, suffix: str) -> "Path":
+        return type(self)(
+            super().with_suffix(suffix),
+            runfiles=getattr(self, "_runfiles", None),
+            source_repo=getattr(self, "_source_repo", None),
+        )
+
+    def __repr__(self) -> str:
+        return 'runfiles.Path({!r})'.format(super().__str__())
+
+    def __str__(self) -> str:
+        path_posix = super().__str__().replace("\\", "/")
+        if not path_posix or path_posix == ".":
+            # pylint: disable=protected-access
+            return self._runfiles._python_runfiles_root  # type: ignore
+        resolved = self._runfiles.Rlocation(path_posix, source_repo=self._source_repo)  # type: ignore
+        return resolved if resolved is not None else super().__str__()
+
+    def __fspath__(self) -> str:
+        return str(self)
+
+    def runfiles_root(self) -> "Path":
+        """Returns a Path object representing the runfiles root."""
+        return self._runfiles.root(source_repo=self._source_repo)  # type: ignore
 
 
 class _ManifestBased:
@@ -254,6 +375,16 @@ def __init__(self, strategy: Union[_ManifestBased, _DirectoryBased]) -> None:
             strategy.RlocationChecked("_repo_mapping")
         )
 
+    def root(self, source_repo: Optional[str] = None) -> Path:
+        """Returns a Path object representing the runfiles root.
+
+        The repository mapping used by the returned Path object is that of the
+        caller of this method.
+        """
+        if source_repo is None and not self._repo_mapping.is_empty():
+            source_repo = self.CurrentRepository(frame=2)
+        return Path(runfiles=self, source_repo=source_repo)
+
     def Rlocation(self, path: str, source_repo: Optional[str] = None) -> Optional[str]:
         """Returns the runtime path of a runfile.
 
@@ -325,9 +456,7 @@ def Rlocation(self, path: str, source_repo: Optional[str] = None) -> Optional[st
 
         # Look up the target repository using the repository mapping
         if target_canonical is not None:
-            return self._strategy.RlocationChecked(
-                target_canonical + "/" + remainder
-            )
+            return self._strategy.RlocationChecked(target_canonical + "/" + remainder)
 
         # No mapping found - assume target_repo is already canonical or
         # we're not using Bzlmod
@@ -396,10 +525,9 @@ def CurrentRepository(self, frame: int = 1) -> str:
             # TODO: This doesn't cover the case of a script being run from an
             #       external repository, which could be heuristically detected
             #       by parsing the script's path.
-            if (
-                (sys.version_info.minor <= 10 or sys.platform == "win32")
-                and sys.path[0] != self._python_runfiles_root
-            ):
+            if (sys.version_info.minor <= 10 or sys.platform == "win32") and sys.path[
+                0
+            ] != self._python_runfiles_root:
                 return ""
             raise ValueError(
                 "{} does not lie under the runfiles root {}".format(

tests/runfiles/BUILD.bazel

@@ -14,6 +14,12 @@ py_test(
     deps = ["//python/runfiles"],
 )
 
+py_test(
+    name = "pathlib_test",
+    srcs = ["pathlib_test.py"],
+    deps = ["//python/runfiles"],
+)
+
 build_test(
     name = "publishing",
     targets = [

tests/runfiles/pathlib_test.py

@@ -0,0 +1,105 @@
+import os
+import pathlib
+import tempfile
+import unittest
+
+from python.runfiles import runfiles
+
+
+class PathlibTest(unittest.TestCase):
+    def setUp(self) -> None:
+        self.tmpdir = tempfile.TemporaryDirectory(dir=os.environ.get("TEST_TMPDIR"))
+        # Runfiles paths are expected to be posix paths internally when we construct the strings for assertions
+        self.root_dir = pathlib.Path(self.tmpdir.name).as_posix()
+
+    def tearDown(self) -> None:
+        self.tmpdir.cleanup()
+
+    def test_path_api(self) -> None:
+        r = runfiles.Create({"RUNFILES_DIR": self.root_dir})
+        assert r is not None
+        root = r.root()
+
+        # Test basic joining
+        p = root / "repo/pkg/file.txt"
+        self.assertEqual(str(p), f"{self.root_dir}/repo/pkg/file.txt")
+
+        # Test PurePath API
+        self.assertEqual(p.name, "file.txt")
+        self.assertEqual(p.suffix, ".txt")
+        self.assertEqual(p.parent.name, "pkg")
+        self.assertEqual(p.parts, ("repo", "pkg", "file.txt"))
+        self.assertEqual(p.stem, "file")
+        self.assertEqual(p.suffixes, [".txt"])
+
+        # Test multiple joins
+        p2 = root / "repo" / "pkg" / "file.txt"
+        self.assertEqual(p, p2)
+
+        # Test joins with pathlib objects
+        p3 = root / pathlib.PurePath("repo/pkg/file.txt")
+        self.assertEqual(p, p3)
+
+    def test_root(self) -> None:
+        r = runfiles.Create({"RUNFILES_DIR": self.root_dir})
+        assert r is not None
+        self.assertEqual(str(r.root()), self.root_dir)
+
+    def test_runfiles_root_method(self) -> None:
+        r = runfiles.Create({"RUNFILES_DIR": self.root_dir})
+        assert r is not None
+        p = r.root() / "foo/bar"
+        self.assertEqual(p.runfiles_root(), r.root())
+        self.assertEqual(str(p.runfiles_root()), self.root_dir)
+
+    def test_os_path_like(self) -> None:
+        r = runfiles.Create({"RUNFILES_DIR": self.root_dir})
+        assert r is not None
+        p = r.root() / "foo"
+        self.assertEqual(os.fspath(p), f"{self.root_dir}/foo")
+
+    def test_equality_and_hash(self) -> None:
+        r = runfiles.Create({"RUNFILES_DIR": self.root_dir})
+        assert r is not None
+        p1 = r.root() / "foo"
+        p2 = r.root() / "foo"
+        p3 = r.root() / "bar"
+
+        self.assertEqual(p1, p2)
+        self.assertNotEqual(p1, p3)
+        self.assertEqual(hash(p1), hash(p2))
+
+    def test_join_path(self) -> None:
+        r = runfiles.Create({"RUNFILES_DIR": self.root_dir})
+        assert r is not None
+        p = r.root().joinpath("repo", "file")
+        self.assertEqual(str(p), f"{self.root_dir}/repo/file")
+
+    def test_parents(self) -> None:
+        r = runfiles.Create({"RUNFILES_DIR": self.root_dir})
+        assert r is not None
+        p = r.root() / "a/b/c"
+        parents = list(p.parents)
+        self.assertEqual(len(parents), 3)
+        self.assertEqual(str(parents[0]), f"{self.root_dir}/a/b")
+        self.assertEqual(str(parents[1]), f"{self.root_dir}/a")
+        self.assertEqual(str(parents[2]), self.root_dir)
+
+    def test_with_methods(self) -> None:
+        r = runfiles.Create({"RUNFILES_DIR": self.root_dir})
+        assert r is not None
+        p = r.root() / "foo/bar.txt"
+        self.assertEqual(str(p.with_name("baz.py")), f"{self.root_dir}/foo/baz.py")
+        self.assertEqual(str(p.with_suffix(".dat")), f"{self.root_dir}/foo/bar.dat")
+
+    def test_match(self) -> None:
+        r = runfiles.Create({"RUNFILES_DIR": self.root_dir})
+        assert r is not None
+        p = r.root() / "foo/bar.txt"
+        self.assertTrue(p.match("*.txt"))
+        self.assertTrue(p.match("foo/*.txt"))
+        self.assertFalse(p.match("bar/*.txt"))
+
+
+if __name__ == "__main__":
+    unittest.main()