make sure a directory exists and is writeable

def prepare_writeable_dir(tree):
    ''' make sure a directory exists and is writeable '''

    if tree != '/':
        tree = os.path.realpath(os.path.expanduser(tree))
    if not os.path.exists(tree):
        try:
            os.makedirs(tree)
        except (IOError, OSError), e:
            exit("Could not make dir %s: %s" % (tree, e))
    if not os.access(tree, os.W_OK):
        exit("Cannot write to path %s" % tree)