This page is primarily intended for developers of Mercurial.

Supported Python Versions

The versions of Python supported by Mercurial.

1. Current support

Mercurial currently supports Python versions 2.4 to 2.7, provided they include the following standard library components:

Some versions of Python may have difficulty with some particular features. For instance, Python before 2.5 cannot properly verify SSL certificates.

2. Future support plan

Python 2.3 support was dropped in July 2009 for Mercurial 1.3 when it became difficult for developers to continue to testing compatibility.

We will probably continue to support Python 2.4 as long as it doesn't prevent a significant barrier to development. Given that Python 2.5 and later don't contain any features that we're dying to use, that may be a long time off. Python did however have a lot of problems with HTTP and SSL until 2.6.3. Mercurial on these old versions can thus only create unverified HTTPS connections and has limited support for proxy use.

We now have a suite of automated code tests and a continuous build server that will find most Python 2.4 issues even though most developers now use Python 2.6 or later.

We also will continue to support Python 2.x as long as there is a significant installed base in the form of Red Hat Enterprise Linux and Ubuntu LTS users. RHEL 5, which uses Python 2.4, will reach the end of the "production 2" portion of its lifecycle in Q1 2014 and the end of its regular lifecycle in 2017.

3. Python 3.x support

Python 3.x has proven notoriously difficult to support, due to our pervasive dependence on a byte-based encoding strategy and string manipulation. We have supported two GSoC projects to research 3.x support, after which mpm estimated that it would take at least another person-year of core developer time to make a full 3.x-compatible version. Also, Python 3.x has proven significantly slower in almost all benchmarks that matter to us, especially start-up time.

As there are no Python 3.x features that are of interest to us, and we will need to continue to support 2.x for the foreseeable future, this is not an attractive or high-priority project.