What would be the best way in Python to determine whether a directory is writeable for the user executing the script? Since this will likely involve using the os module I should mention I'm running it under a *nix environment.
It may seem strange to suggest this, but a common Python idiom is 

It's easier to ask for forgiveness
  than for permission

Following that idiom, one might say:
Try writing to the directory in question, and catch the error if you don't have the permission to do so.
Determining Whether a Directory is Writeable