How to determine if a path is a subdirectory of another?

Question

I am given a list of paths that I need to check files within. Of course, if I am given a root, and a subdirectory, there is no need to process the sub-directory. For example

c:\test  // process this
c:\test\pics // do not process this
c:\test2 // process this

How can I tell (cross platform) that a path is not a subdirectory of the other. Preferably I would want this to be cross platform, and am not worried about symlinks as long as they are not cyclical (worse case is that I end up processing the data twice).

UPDATE: here is the code I ended up using, thanks to @F.J

   def unique_path_roots(paths):
    visited = set()
    paths = list(set(paths))

    for path in sorted(paths,key=cmp_to_key(locale.strcoll)):
        path = normcase(normpath(realpath(path)))

        head, tail = os.path.split(path)
        while head and tail:
            if head in visited:
                break
            head, tail = os.path.split(head)
        else:
            yield path
            visited.add(path)

Answer 1

def is_subdir(path, directory):
    path = os.path.realpath(path)
    directory = os.path.realpath(directory)

    relative = os.path.relpath(path, directory)

    if relative.startswith(os.pardir):
        return False
    else:
        return True