Is there a portable way to get the current username in Python?

What is a portable way (e.g. for Linux and Windows) to get the current user's username? Something similar to os.getuid() would be nice:

>>> os.getuid()
42

# Does not currently exist in Python
>>> os.getusername()
'slartibartfast'

The pwd module works for Unix only. Some people suggest that getting the username under Windows can be complicated in certain circumstances (e.g., running as a Windows service).