ntpath.join() incorrectly inserts an additional slash when joining an argument onto an incomplete UNC drive with a trailing slash:

>>> import ntpath
>>> ntpath.join('\\\\server\\share\\foo\\', 'bar')
'\\\\server\\share\\foo\\bar'  # ok
>>> ntpath.join('\\\\server\\share\\', 'foo')
'\\\\server\\share\\foo'  # ok
>>> ntpath.join('\\\\server\\', 'share')
'\\\\server\\\\share'  # wrong!
>>> ntpath.join('\\\\', 'server')
'\\\\\\server'  # wrong!

Before 005e694 (3.12), the last test case succeeds because splitdrive() doesn't identify '\\\\' as a UNC drive. But the third test case is reproducible going back to 3.11 and 3.10.

Linked PRs

• GH-103220: Fix ntpath.join() of partial UNC drive with trailing slash #103221