As discussed in python-dev mailing list, something should be add to os.mkdir() and os.makedirs() to simulate the shell's "mkdir -p" function, that is, suppress the OSError exception if the target directory exists. 

Here is a patch against py3k, with code, test and doc. I add an "ensure_exist" keyword argument for both os.mkdir() and os.makedirs(), indicates weather an OSError is raised if the target directory already exists.

Since I've no windows environment, I only tested the patch on Unix. Hope someone could help test it on windows.