[3.7] bpo-38878: Fix os.PathLike __subclasshook__ (GH-17336) (GH-17685)

https://bugs.python.org/issue38878

3 files changed, 15 insertions, 1 deletions

diff --git a/Lib/os.py b/Lib/os.py
index b93f95d98e..9853e37c61 100644
--- a/Lib/os.py
+++ b/Lib/os.py
@@ -26,6 +26,8 @@ import abc
 import sys
 import stat as st
 
+from _collections_abc import _check_methods
+
 _names = sys.builtin_module_names
 
 # Note:  more names are added to __all__ later.
@@ -1076,4 +1078,6 @@ class PathLike(abc.ABC):
 
     @classmethod
     def __subclasshook__(cls, subclass):
-        return hasattr(subclass, '__fspath__')
+        if cls is PathLike:
+            return _check_methods(subclass, '__fspath__')
+        return NotImplemented
diff --git a/Lib/test/test_os.py b/Lib/test/test_os.py
index df4bad7a8c..411e5aa507 100644
--- a/Lib/test/test_os.py
+++ b/Lib/test/test_os.py
@@ -3745,6 +3745,14 @@ class TestPEP519(unittest.TestCase):
         self.assertRaises(ZeroDivisionError, self.fspath,
                           FakePath(ZeroDivisionError()))
 
+    def test_pathlike_subclasshook(self):
+        # bpo-38878: subclasshook causes subclass checks
+        # true on abstract implementation.
+        class A(os.PathLike):
+            pass
+        self.assertFalse(issubclass(FakePath, A))
+        self.assertTrue(issubclass(FakePath, os.PathLike))
+
 
 class TimesTests(unittest.TestCase):
     def test_times(self):
diff --git a/Misc/NEWS.d/next/Library/2019-11-22-12-08-52.bpo-38878.EJ0cFf.rst b/Misc/NEWS.d/next/Library/2019-11-22-12-08-52.bpo-38878.EJ0cFf.rst
new file mode 100644
index 0000000000..9cbdf08dd5
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2019-11-22-12-08-52.bpo-38878.EJ0cFf.rst
@@ -0,0 +1,2 @@
+Fixed __subclasshook__ of :class:`os.PathLike` to return a correct result
+upon inheritence. Patch by Bar Harel.