pathlib: add analogues to py.path.local's bestrelpath and common

An equivalent for these py.path.local functions is needed for some
upcoming py.path -> pathlib conversions.

src/_pytest/pathlib.py

@@ -569,3 +569,35 @@ def visit(
     for entry in entries:
         if entry.is_dir(follow_symlinks=False) and recurse(entry):
             yield from visit(entry.path, recurse)
+
+
+def commonpath(path1: Path, path2: Path) -> Optional[Path]:
+    """Return the common part shared with the other path, or None if there is
+    no common part."""
+    try:
+        return Path(os.path.commonpath((str(path1), str(path2))))
+    except ValueError:
+        return None
+
+
+def bestrelpath(directory: Path, dest: Path) -> str:
+    """Return a string which is a relative path from directory to dest such
+    that directory/bestrelpath == dest.
+
+    If no such path can be determined, returns dest.
+    """
+    if dest == directory:
+        return os.curdir
+    # Find the longest common directory.
+    base = commonpath(directory, dest)
+    # Can be the case on Windows.
+    if not base:
+        return str(dest)
+    reldirectory = directory.relative_to(base)
+    reldest = dest.relative_to(base)
+    return os.path.join(
+        # Back from directory to base.
+        *([os.pardir] * len(reldirectory.parts)),
+        # Forward from base to dest.
+        *reldest.parts,
+    )

testing/test_pathlib.py

@@ -6,6 +6,8 @@ from textwrap import dedent
 import py
 
 import pytest
+from _pytest.pathlib import bestrelpath
+from _pytest.pathlib import commonpath
 from _pytest.pathlib import ensure_deletable
 from _pytest.pathlib import fnmatch_ex
 from _pytest.pathlib import get_extended_length_path_str
@@ -381,3 +383,21 @@ def test_suppress_error_removing_lock(tmp_path):
     # check now that we can remove the lock file in normal circumstances
     assert ensure_deletable(path, consider_lock_dead_if_created_before=mtime + 30)
     assert not lock.is_file()
+
+
+def test_bestrelpath() -> None:
+    curdir = Path("/foo/bar/baz/path")
+    assert bestrelpath(curdir, curdir) == "."
+    assert bestrelpath(curdir, curdir / "hello" / "world") == "hello" + os.sep + "world"
+    assert bestrelpath(curdir, curdir.parent / "sister") == ".." + os.sep + "sister"
+    assert bestrelpath(curdir, curdir.parent) == ".."
+    assert bestrelpath(curdir, Path("hello")) == "hello"
+
+
+def test_commonpath() -> None:
+    path = Path("/foo/bar/baz/path")
+    subpath = path / "sampledir"
+    assert commonpath(path, subpath) == path
+    assert commonpath(subpath, path) == path
+    assert commonpath(Path(str(path) + "suffix"), path) == path.parent
+    assert commonpath(path, path.parent.parent) == path.parent.parent