Suppress a possible relative_to() failure, fixes #9894

AUTHORS

@@ -312,6 +312,7 @@ Stefan Scherfke
 Stefan Zimmermann
 Stefano Taschini
 Steffen Allner
+Stephan Loyd
 Stephan Obermann
 Sven-Hendrik Haase
 Sylvain Marié

changelog/9894.improvement.rst

@@ -0,0 +1 @@
+Suppress a potential pathlib relative_to() error, which is possible to be raised from ExceptionInfo.getexpr() when running Python from double-slashed path (like //usr/bin/python3) and single-slashed tests path.

src/_pytest/pathlib.py

@@ -707,8 +707,12 @@ def bestrelpath(directory: Path, dest: Path) -> str:
     # Can be the case for a relative path and an absolute path.
     if not base:
         return str(dest)
+    try:
         reldirectory = directory.relative_to(base)
         reldest = dest.relative_to(base)
+    except ValueError:
+        # GH 9894
+        return str(dest)
     return os.path.join(
         # Back from directory to base.
         *([os.pardir] * len(reldirectory.parts)),

testing/test_pathlib.py

@@ -413,6 +413,11 @@ def test_bestrelpath() -> None:
     assert bestrelpath(curdir, curdir.parent) == ".."
     assert bestrelpath(curdir, Path("hello")) == "hello"
 
+    # GH 9894
+    directory = Path("/var/tmp/pytest-issue")
+    dest = Path("//usr/lib/python3.9/importlib/__init__.py")
+    assert bestrelpath(directory, dest) == str(dest)
+
 
 def test_commonpath() -> None:
     path = Path("/foo/bar/baz/path")