Very simple: this is a PEP to improve the state of "Cpython" (this is the main python implementation right now, and has been since the invention of python). Py

Very simple: this is a PEP to improve the state of "Cpython" (this is the main python implementation right now, and has been since the invention of python). Pypy, on the other hand, is a new implementation of Python (just like ironpython or jython), and it could eventually replace cpython in the future if it proves to be succesfull (although it would be a long way off). In few words: Python, the language, have many implementations. Cpython is the current reference one. Unladen Swallow aims to improve it, not to replace it. Actually, even the US developers acknowledge that Pypy (or any other succesfull implementation that may come up) is the future, because it doesn't have to bear the burden imposed by the c platform.