Per @eryksun:

Note that this check is wrong for incomplete UNC paths such as "//", "//spam", "//?/UNC/", and "//?/UNC/spam". Such paths lack a root and don't even have an implicit root. However, they're still absolute because they don't depend on the current working directory. I don't know how or whether you want to handle this edge case.

ntpath.isabs() gets this case right, but it's still wrong for rooted paths such as "/spam".