Ah, I was misunderstanding the behavior of mkdir, thank you for the 
explanation.

My misunderstanding stemmed from recently coming across two widely-used 
packages which both pass mode=0750 to os.makedirs. I have no idea why 
they would be doing this (as it effectively throws away part of the 
umask), unless they too are misunderstanding the mode parameter. My 
suspicion is that the mode parameter is widely misunderstood to mean the 
desired permissions of the created directory. I have filed tickets in 
the packages in which I came across this to make sure they're doing what 
they intend.

Could the __doc__ for os.mkdir and os.makedirs be expanded to make this 
clearer?