I just discovered that perhaps ntpath.join should be even more clever. Windows supports current directories for every drive separately, so perhaps ntpath.join('c:/x', 'd:/y', 'c:z') should return 'c:/x\\z', not 'c:/z'.

Could anyone please check it? Create directory x/z on drive c: and directory y on drive d:, then execute following commands:

cd c:/x
cd d:/y
cd c:z

What is resulting current working directory?

Here is a patch which implements this algorithm.