[fatbird]

In his talk at last year's pycon, Hastings said the three constraints GvR laid out are:

1. Can't degrade single-threaded performance

2. Can't break existing extensions

3. Can't make the implementation of cpython much more complicated (i.e., can't raise the barrier to entry to participating in the development of python)

All of these are pretty reasonable, if tough, targets to meet, and Hastings agrees with all of them. For 1 and 2 he was generally looking at making GIL-less cpython a compiled mode so that the default was the single threaded version, thus retaining compatibility and performance, but offering a true multi-threaded binary for those who would use it.