Unladen Swallow was an optimization branch of CPython, the reference implementation of the Python programming language, which incorporated a just-in-time compiler built using LLVM into CPython's virtual machine.
[1][2] Although it fell short of all its published goals, some Unladen Swallow code was added into the main Python implementation, such as improvements to the cPickle module.
[3][1] Unladen Swallow was sponsored by Google, and the project owners, Thomas Wouters, Jeffrey Yasskin, and Collin Winter, were Google employees, though most project contributors were not.
[6] In July 2010, speculation began on whether the project was dead or dying since the 2009 Q4 milestone had not yet been released,[7] and the traffic on Unladen's mailing list had decreased from 500 messages in January 2010 to fewer than 10 in September 2010.
[6] A 2009 Q4 development branch was created in January 2010,[14] but was not advertised on the website, and its milestone was unmet.