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PEP 430 -- Migrating to Python 3 as the default online documentation

PEP: 430
Title: Migrating to Python 3 as the default online documentation
Author: Nick Coghlan <ncoghlan at>
BDFL-Delegate: Georg Brandl
Status: Final
Type: Informational
Created: 27-Oct-2012


This PEP proposes a strategy for migrating the default version of the Python documentation presented to users of Python when accessing from 2.7 to Python 3.3.

It proposes a backwards compatible scheme that preserves the meaning of existing deep links in to the Python 2 documentation, while still presenting the Python 3 documentation by default, and presenting the Python 2 and 3 documentation in a way that avoids making the Python 3 documentation look like a second-class citizen.


With the transition of the overall Python ecosystem from Python 2 to Python 3 still in progress, one question which arises periodically [ 1 , 2 ] is when and how to handle the change from providing the Python 2 documentation as the default version displayed at the root URL to providing the Python 3 documentation.

Key Concerns

There are a couple of key concerns that any migration proposal needs to address.

Don't Confuse Beginners

Many beginners learn Python through third party resources. These resources, not all of which are online resources, may reference in to the online documentation for additional background and details.

Importantly, even when the online documentation is updated, the "version added" and "version changed" tags usually provide enough information for users to adjust appropriately for the specific version they are using.

While deep links in to the documentation may occasionally break within the Python 2 series, this is very rare.

Migrating to Python 3 is a very different matter. Many links would break due to renames and removals, and the "version added" and "version changed" information for the Python 2 series is completely absent.

Don't Break Useful Resources

There are many useful Python resources out there, such as the mailing list archives on and question-and-answer sites like Stack Overflow, where links are highly unlikely to be updated, no matter how much notice is provided.

Old posts and answers to questions all currently link to expecting to get the Python 2 documentation at unqualified URLs. Links from answers that relate to Python 3 are explicitly qualified with /py3k/ in the path component.


This PEP (based on an idea originally put forward back in May [ 3 ]) is to not migrate the Python 2 specific deep links at all, and instead adopt a scheme where all URLs presented to users on are qualified appropriately with the relevant release series.

Visitors to the root URL at will be automatically redirected to , but links deeper in the version-specific hierarchy, such as to , will instead be redirected to a Python 2 specific link such as .

The specific subpaths which will be redirected to explicitly qualified paths for the Python 2 docs are:

  • /c-api/
  • /distutils/
  • /extending/
  • /faq/
  • /howto/
  • /library/
  • /reference/
  • /tutorial/
  • /using/
  • /whatsnew/
  • /about.html
  • /bugs.html
  • /contents.html
  • /copyright.html
  • /license.html
  • /genindex.html
  • /glossary.html
  • /py-modindex.html
  • /search.html

The existing /py3k/ subpath will be redirected to the new /3/ subpath.

Presented URLs

With this scheme, the following URLs would be presented to users after resolution of any aliasing and rewriting rules:


The /x/ URLs mean "give me the latest documentation for a released version in this release series". It will draw the documentation from the relevant maintenance branch in source control (this will always be the 2.7 branch for Python 2 and is currently 3.3 for Python 3). Differences relative to previous versions in the release series will be available through "version added" and "version changed" markers.

The /x.y/ URLs mean "give me the latest documentation for this release". It will draw the documentation from the relevant maintenance branch in source control (or the default branch for the currently in development version). It differs from the status quo in that the URLs will actually remain available in the user's browser for easy copy and pasting. (Currently, references to specific versions that are not the latest in their release series will resolve to a stable URL for a specific maintenance version in the "release" hierarchy, while the current latest version in the release series resolves to the release series URL. This makes it hard to get a "latest version specific URL", since it is always necessary to construct them manually).

The /dev/ URL means the documentation for the default branch in source control.

The /release/x.y.x/ URLs will refer to the documentation of those releases, exactly as it was at the time of the release.

The developer's guide is not version specific, and thus retains its own stable /devguide/ URL.


There is some desire to switch the unqualified references to mean Python 3 as a sign of confidence in Python 3. Such a move would either break a lot of things, or else involve an awful lot of work to avoid breaking things.

I believe we can get much the same effect without breaking the world by:

  1. Deprecating the use of unqualified references to the online documentation (while promising to preserve the meaning of such references indefinitely)
  2. Updating all and python-dev controlled links to use qualified references (excluding archived email)
  3. Redirecting visitors to the root of to

Most importantly, because this scheme doesn't alter the behaviour of any existing deep links, it could be implemented with a significantly shorter warning period than would be required for a scheme that risked breaking deep links, or started to redirect unqualified links to Python 3. The only part of the scheme which would require any warning at all is the step of redirecting the " " landing page to the Python 3.3 documentation.

Namespaces are one honking great idea - let's do more of those.

Note that the approach described in this PEP gives two ways to access the content of the default branch: as /dev/ or using the appropriate /x.y/ reference. This is deliberate, as the default branch is referenced for two different purposes:

  • to provide additional information when discussing an upcoming feature of the next release (a /x.y/ URL is appropriate)
  • to provide a stable destination for developers to access the documentation of the next feature release, regardless of the version (a /dev/ URL is appropriate)


The URLs on are controlled by the infrastructure team rather than through the CPython source repo, so acceptance and implementation of the ideas in this PEP will be up to the team.