Greenlets are lightweight coroutines for in-process concurrent programming.
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".
A "greenlet", on the other hand, is a still more primitive notion of micro-thread with no implicit scheduling; coroutines, in other words. This is useful when you want to control exactly when your code runs. You can build custom scheduled micro-threads on top of greenlet; however, it seems that greenlets are useful on their own as a way to make advanced control flow structures. For example, we can recreate generators; the difference with Python's own generators is that our generators can call nested functions and the nested functions can yield values too. (Additionally, you don't need a "yield" keyword. See the example in test_generator.py).
Greenlets are provided as a C extension module for the regular unmodified interpreter.
Who is using Greenlet?
There are several libraries that use Greenlet as a more flexible alternative to Python's built in coroutine support:
The easiest way to get Greenlet is to install it with pip:
pip install greenlet
Source code archives and binary distributions are vailable on the python package index at https://pypi.org/project/greenlet
The source code repository is hosted on github: https://github.com/python-greenlet/greenlet
Documentation is available on readthedocs.org: https://greenlet.readthedocs.io