I have to agree with jab. While the way relative paths works are more or less common knowledge, the way umask does and does not work in relation to python isn't.

It is however given a short description at http://docs.python.org/library/os.html#os.makedirs:
...The default mode is 0777 (octal). On some systems, mode is ignored. Where it is used, the current umask value is first masked out.

And as it is documented (briefly) online that the umask *is* used, perhaps it is not necessary to add it to the doc-string?