Phillip> But it doesn't handle the fact that Windows paths are Phillip> case-insensitive, or that Posix paths can have symlinks... or Phillip> that one path might be relative and another absolute... Phillip> As soon as you move away from being a string operation, you get Phillip> an endless series of gotchas... none of which are currently Phillip> documented. Well, then we can document (some of?) the gotchas* and work on a better implementation of commonpathprefix. I don't do Windows. You're lucky I got as far as I did with the Windows side of things. ;-) Skip * I would argue that symlinks should be transparent. By the very nature of the operations and the fact that they might be performed on other platforms (import posixpath on Windows for instance) there is not much, if anything, you can infer about the paths themselves other than their structure.