From b57105ae33e1f61e6bdf0eec45c4135d067b9b22 Mon Sep 17 00:00:00 2001
From: Barney Gale <barney.gale@gmail.com>
Date: Tue, 11 Apr 2023 17:26:45 +0100
Subject: [PATCH] GH-103220: Fix `ntpath.join()` of partial UNC drive with
 trailing slash (GH-103221)

---
 Lib/ntpath.py                                                | 2 +-
 Lib/test/test_ntpath.py                                      | 5 +++++
 .../Library/2023-04-03-21-08-53.gh-issue-103220.OW_Bj5.rst   | 2 ++
 3 files changed, 8 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Library/2023-04-03-21-08-53.gh-issue-103220.OW_Bj5.rst

diff --git a/Lib/ntpath.py b/Lib/ntpath.py
index 6e2da79c85d..0f3674fe11e 100644
--- a/Lib/ntpath.py
+++ b/Lib/ntpath.py
@@ -142,7 +142,7 @@ def join(path, *paths):
             result_path = result_path + p_path
         ## add separator between UNC and non-absolute path
         if (result_path and not result_root and
-            result_drive and result_drive[-1:] != colon):
+            result_drive and result_drive[-1:] not in colon + seps):
             return result_drive + sep + result_path
         return result_drive + result_root + result_path
     except (TypeError, AttributeError, BytesWarning):
diff --git a/Lib/test/test_ntpath.py b/Lib/test/test_ntpath.py
index 4e755d15403..42b9587ca18 100644
--- a/Lib/test/test_ntpath.py
+++ b/Lib/test/test_ntpath.py
@@ -300,6 +300,11 @@ class TestNtpath(NtpathTestCase):
         tester("ntpath.join('//computer/share', 'a', 'b')", '//computer/share\\a\\b')
         tester("ntpath.join('//computer/share', 'a/b')", '//computer/share\\a/b')
 
+        tester("ntpath.join('\\\\', 'computer')", '\\\\computer')
+        tester("ntpath.join('\\\\computer\\', 'share')", '\\\\computer\\share')
+        tester("ntpath.join('\\\\computer\\share\\', 'a')", '\\\\computer\\share\\a')
+        tester("ntpath.join('\\\\computer\\share\\a\\', 'b')", '\\\\computer\\share\\a\\b')
+
     def test_normpath(self):
         tester("ntpath.normpath('A//////././//.//B')", r'A\B')
         tester("ntpath.normpath('A/./B')", r'A\B')
diff --git a/Misc/NEWS.d/next/Library/2023-04-03-21-08-53.gh-issue-103220.OW_Bj5.rst b/Misc/NEWS.d/next/Library/2023-04-03-21-08-53.gh-issue-103220.OW_Bj5.rst
new file mode 100644
index 00000000000..9cf26c26873
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2023-04-03-21-08-53.gh-issue-103220.OW_Bj5.rst
@@ -0,0 +1,2 @@
+Fix issue where :func:`os.path.join` added a slash when joining onto an
+incomplete UNC drive with a trailing slash on Windows.