Rather than add a variable to sys that will be empty 99% of the time, I think I'd rather stick a '__command__' constant in the __main__ module namespace when running with '-c' (think of '__file__').  You could then get at it elsewhere with 'from __main__ import __command__' (probably wrapped in a try/except ImportError, since it will usually not exist).

This should probably be discussed on python-ideas.

(Removing all versions but 3.5, as this is a feature request.)