The greenlet package is a spin-off of Stackless, a version of CPython that
 supports micro-threads called "tasklets". Tasklets run pseudo-concurrently
 (typically in a single or a few OS-level threads) and are synchronized with
 data exchanges on "channels".
 .
 greenlet is the standalone package derived from the py lib, and is used by
 several non-blocking IO packages as a more flexible alternative to Python's
 built in coroutines.