CPython

Design

A significant limitation of CPython is the use of a global interpreter lock (GIL) on each CPython interpreter process, which effectively disables concurrent Python threads within one process. Concurrency can only be achieved with separate CPython interpreter processes managed by a multitasking operating system. This complicates communication between concurrent Python processes, though the multiprocessing module mitigates this somewhat. Much discussion took place on whether to remove the GIL from CPython. A set of "free threading" patches to CPython was submitted by Greg Stein, which effectively replaced GIL with fine-grained locking. However the patches were rejected due to the execution overhead they introduced into single-process code.

Distribution

Supported platforms include:

PEP 11 lists platforms which are not supported in CPython by Python Software Foundation. These platforms can still be supported by external ports. These ports include:

External ports not integrated to Python Software Foundation's official version of CPython, with links to its main development site, often include additional modules for platform-specific functionalities, like graphics and sound API for PSP and SMS and camera API for S60. These ports include:

Unladen Swallow

Unladen Swallow was an optimization branch of CPython, intended to be fully compatible and significantly faster. It aimed to achieve its goals by supplementing CPython's custom virtual machine with a just-in-time compiler built using LLVM.

The project had stated a goal of a speed improvement by a factor of five over CPython; this goal was not met.

The project was sponsored by Google, and the project owners, Thomas Wouters, Jeffrey Yasskin, and Collin Winter, are full-time Google employees, however most project contributors are not Google employees. Unladen Swallow is hosted on Google Code.

Like many things regarding the Python language, the name Unladen Swallow is a Monty Python reference, specifically to the joke about the airspeed velocity of unladen swallows in Monty Python and the Holy Grail.

Although it fell short of all published goals, Unladen Swallow did produce some code which got added to the main Python implementation, such as improvements to the cPickle module.

In July 2010, some observers speculated on whether the project was dead or dying, since the 2009 Q4 milestone had not yet been released. The traffic on Unladen's mailing list had decreased from 500 messages in January 2010, to fewer than 10 in September 2010. It has also been reported that Unladen lost Google's funding. In November 2010, one of the main developers announced that "Jeffrey and I have been pulled on to other projects of higher importance to Google".

The 2009 Q4 development branch was created on January 26, 2010, but no advertising was made on the website. Further, regarding the long-term plans, and as the project missed the Python 2.7 release, a Python Enhancement Proposal (PEP) was accepted, which proposed a merge of Unladen Swallow into a special py3k-jit branch of Python's official repository. As of July 2010, this work was ongoing. This merging would have taken some time, since Unladen Swallow was originally based on Python 2.6 with which Python 3 broke compatibility (see Python 3000 for more details). However, the PEP was subsequently withdrawn.

In early 2011, it became clear that the project was stopped.

Alternatives

CPython is one of several "production-quality" Python implementations including: Jython, written in Java for the Java virtual machine (JVM), PyPy, written in RPython and translated into C, and IronPython, which is written in C# for the Common Language Infrastructure. There are also several experimental implementations.