astropy/astropy — pull request #394 by mdboom (2012-09-21 14:07:18+00:00)
Title: Add InvalidUnit concept 

This came out of a discussion in #380 and in a discussion "Invalid unit handling" on the mailing list.

Unit strings that don't parse will warn and then still create an object that allows for round-tripping to strings, but doesn't have any valid unit operations.