1.4. Python Development and Versions

Python is developed, maintained, and released by a team of core developers headed by Guido van Rossum, Python's inventor, architect, and Benevolent Dictator For Life (BDFL). This title means that Guido has the final say on what becomes part of the Python language and standard libraries. Python's intellectual property is vested in the Python Software Foundation (PSF), a nonprofit corporation devoted to promoting Python, with dozens of individual members (nominated for their contributions to Python, and including all of the Python core team) and corporate sponsors. Most PSF members have commit privileges to Python's SVN repositories (http://svn.python.org/projects/), and most Python SVN committers are members of the PSF.

Proposed changes to Python are detailed in public documents called Python Enhancement Proposals (PEPs), debated (and sometimes advisorily voted on) by Python developers and the wider Python community, and finally approved or rejected by Guido, who takes debates and votes into account but is not bound by them. Many hundreds of people actively contribute to Python development through PEPs, discussion, bug reports, and proposed patches to Python sources, libraries, and documentation.

The Python core team releases minor versions of Python (2.x, for growing values of x), currently at a pace of about once every year or two. Python 2.2 was released in December 2001, 2.3 in July 2003, and 2.4 in November 2004. Python 2.5 is scheduled to be released in the summer of 2006 (at the time of this writing, the first alpha release of 2.5 has just appeared). Each minor release adds features that make Python more powerful and simpler to use, but also takes care to maintain backward compatibility. One day there will be a Python 3.0 release, which will be allowed to break backward compatibility to some extent in order to remove some redundant "legacy" features and simplify the language even further. However, that release is still years in the future, and no specific schedules for it currently exist; the current state of Guido's ideas about Python 3.0 can be studied at http://python.org/peps/pep-3000.html.

Each minor release 2.x starts with alpha releases, tagged as 2.xa0, 2.xa1, and so on. After the alphas comes at least one beta release, 2.xb1, and after the betas, at least one release candidate, 2.xrc1. By the time the final release of 2.x comes out, it is always solid, reliable, and well tested on all major platforms. Any Python programmer can help ensure this by downloading alphas, betas, and release candidates, trying them out extensively, and filing bug reports for any problem that might emerge.

Once a minor release is out, part of the attention of the core team switches to the next minor release. However, a minor release normally gets successive point releases (i.e., 2.x.1, 2.x.2, and so on) that add no functionality but can fix errors, port Python to new platforms, enhance documentation, and add optimizations and tools.

This book focuses on Python 2.4 (and all its point releases), the most stable and widespread release at the time of this writing. I also cover, or at least mention, the changes that are scheduled to appear in Python 2.5, and I document which parts of the language and libraries were first introduced in 2.4 and thus cannot be used with the previous 2.3 release. Whenever I say that a feature is "in 2.4," I mean 2.4 and all following versions (in other words, with this phrasing I mean to include Python 2.5 but to exclude 2.3), unless I immediately continue by explaining some difference that is specific to 2.5.

At the time of this writing, the released version of Jython supports only Python 2.2 (and some, but not all, of Python 2.3), but not the full specifications of Python 2.4. IronPython 1.0 supports Python 2.4.

This book does not address older versions of Python, such as 1.5.2, 2.0, 2.1, 2.2; such versions are over four years old and should not be used for any new development. However, you might have to worry about such legacy versions if they are embedded in some application you need to script. Fortunately, Python's backward compatibility is quite good: current versions of Python are able to properly process just about any valid Python program that was written for Python 1.5.2 or later. You can find code and documentation for all old releases of Python at http://python.org/doc/versions.html.