Stackless Python is a code “fork” to CPython that introduces a parallelism approach of “continuations”, providing a higher performance alternative to Python threading (the Python thread switch is relatively slow).

At the moment, Stackless is 100% binary compatible with CPython.

It is not clear whether Stackless will be included in the primary CPython distribution, as this would encourage development of applications that couldn't be easily ported to other Python implementations, such as Jython.