[$] Late-bound argument defaults for Python

Python supports default values for arguments to functions, but thosedefaults are evaluated at function-definition time. A proposal to adddefaults that are evaluated when the function is called has been discussedat some length on the python-ideas mailing list. The idea came about, in part,due to yet another resurrection of the proposalfor None-aware operators in Python. Late-bound defaults would helpwith one use case for those operators, but there are other, strongerreasons to consider their addition to the language.