gh-139001: Fix thread-safety issue in pathlib.Path

Lib/pathlib/__init__.py

@@ -335,13 +335,8 @@ def _raw_path(self):
             return paths[0]
         elif paths:
             # Join path segments from the initializer.
-            path = self.parser.join(*paths)
-            # Cache the joined path.
-            paths.clear()
-            paths.append(path)
-            return path
+            return self.parser.join(*paths)
         else:
-            paths.append('')
             return ''
 
     @property

Lib/test/test_pathlib/test_join.py

@@ -3,6 +3,8 @@
 """
 
 import unittest
+import threading
+from test.support import threading_helper
 
 from .support import is_pypi
 from .support.lexical_path import LexicalPath
@@ -158,6 +160,26 @@ def test_parts(self):
         parts = p.parts
         self.assertEqual(parts, (sep, 'a', 'b'))
 
+    @threading_helper.requires_working_threading()
+    def test_parts_multithreaded(self):
+        P = self.cls
+
+        NUM_THREADS = 10
+        NUM_ITERS = 10
+
+        for _ in range(NUM_ITERS):
+            b = threading.Barrier(NUM_THREADS)
+            path = P('a') / 'b' / 'c' / 'd' / 'e'
+            expected = ('a', 'b', 'c', 'd', 'e')
+
+            def check_parts():
+                b.wait()
+                self.assertEqual(path.parts, expected)
+
+            threads = [threading.Thread(target=check_parts) for _ in range(NUM_THREADS)]
+            with threading_helper.start_threads(threads):
+                pass
+
     def test_parent(self):
         # Relative
         P = self.cls

Misc/NEWS.d/next/Library/2025-09-17-12-07-21.gh-issue-139001.O6tseN.rst

@@ -0,0 +1,2 @@
+Fix race condition in :class:`pathlib.Path` on the internal ``_raw_paths``
+field.