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PEP 411 -- Provisional packages in the Python standard library

Title:Provisional packages in the Python standard library
Author:Nick Coghlan <ncoghlan at>, Eli Bendersky <eliben at>
Post-History:2012-02-10, 2012-03-24


The process of including a new package into the Python standard library is hindered by the API lock-in and promise of backward compatibility implied by a package being formally part of Python. This PEP describes a methodology for marking a standard library package "provisional" for the period of a single feature release. A provisional package may have its API modified prior to "graduating" into a "stable" state. On one hand, this state provides the package with the benefits of being formally part of the Python distribution. On the other hand, the core development team explicitly states that no promises are made with regards to the stability of the package's API, which may change for the next release. While it is considered an unlikely outcome, such packages may even be removed from the standard library without a deprecation period if the concerns regarding their API or maintenance prove well-founded.

Proposal - a documented provisional state

Whenever the Python core development team decides that a new package should be included into the standard library, but isn't entirely sure about whether the package's API is optimal, the package can be included and marked as "provisional".

In the next feature release, the package may either be "graduated" into a normal "stable" state in the standard library, remain in provisional state, or be rejected and removed entirely from the Python source tree. If the package ends up graduating into the stable state after being provisional, its API may be changed according to accumulated feedback. The core development team explicitly makes no guarantees about API stability and backward compatibility of provisional packages.

Marking a package provisional

A package will be marked provisional by a notice in its documentation page and its docstring. The following paragraph will be added as a note at the top of the documentation page:

The <X> package has been included in the standard library on a provisional basis. Backwards incompatible changes (up to and including removal of the package) may occur if deemed necessary by the core developers.

The phrase "provisional basis" will then be a link to the glossary term "provisional package", defined as:

A provisional package is one which has been deliberately excluded from the standard library's backwards compatibility guarantees. While major changes to such packages are not expected, as long as they are marked provisional, backwards incompatible changes (up to and including removal of the package) may occur if deemed necessary by core developers. Such changes will not be made gratuitously -- they will occur only if serious flaws are uncovered that were missed prior to the inclusion of the package.

This process allows the standard library to continue to evolve over time, without locking in problematic design errors for extended periods of time. See PEP 411 for more details.

The following will be added to the start of the package's docstring:

The API of this package is currently provisional. Refer to the documentation for details.

Moving a package from the provisional to the stable state simply implies removing these notes from its documentation page and docstring.

Which packages should go through the provisional state

We expect most packages proposed for addition into the Python standard library to go through a feature release in the provisional state. There may, however, be some exceptions, such as packages that use a pre-defined API (for example lzma, which generally follows the API of the existing bz2 package), or packages with an API that has wide acceptance in the Python development community.

In any case, packages that are proposed to be added to the standard library, whether via the provisional state or directly, must fulfill the acceptance conditions set by PEP 2.

Criteria for "graduation"

In principle, most provisional packages should eventually graduate to the stable standard library. Some reasons for not graduating are:

  • The package may prove to be unstable or fragile, without sufficient developer support to maintain it.
  • A much better alternative package may be found during the preview release.

Essentially, the decision will be made by the core developers on a per-case basis. The point to emphasize here is that a package's inclusion in the standard library as "provisional" in some release does not guarantee it will continue being part of Python in the next release. At the same time, the bar for making changes in a provisional package is quite high. We expect that most of the API of most provisional packages will be unchanged at graduation. Withdrawals are expected to be rare.


Benefits for the core development team

Currently, the core developers are really reluctant to add new interfaces to the standard library. This is because as soon as they're published in a release, API design mistakes get locked in due to backward compatibility concerns.

By gating all major API additions through some kind of a provisional mechanism for a full release, we get one full release cycle of community feedback before we lock in the APIs with our standard backward compatibility guarantee.

We can also start integrating provisional packages with the rest of the standard library early, so long as we make it clear to packagers that the provisional packages should not be considered optional. The only difference between provisional APIs and the rest of the standard library is that provisional APIs are explicitly exempted from the usual backward compatibility guarantees.

Benefits for end users

For future end users, the broadest benefit lies in a better "out-of-the-box" experience - rather than being told "oh, the standard library tools for task X are horrible, download this 3rd party library instead", those superior tools are more likely to be just be an import away.

For environments where developers are required to conduct due diligence on their upstream dependencies (severely harming the cost-effectiveness of, or even ruling out entirely, much of the material on PyPI), the key benefit lies in ensuring that all packages in the provisional state are clearly under python-dev's aegis from at least the following perspectives:

  • Licensing: Redistributed by the PSF under a Contributor Licensing Agreement.
  • Documentation: The documentation of the package is published and organized via the standard Python documentation tools (i.e. ReST source, output generated with Sphinx and published on
  • Testing: The package test suites are run on the buildbot fleet and results published via
  • Issue management: Bugs and feature requests are handled on
  • Source control: The master repository for the software is published on

Candidates for provisional inclusion into the standard library

For Python 3.3, there are a number of clear current candidates:

Other possible future use cases include:

  • Improved HTTP modules (e.g. requests)
  • HTML 5 parsing support (e.g. html5lib)
  • Improved URL/URI/IRI parsing
  • A standard image API (PEP 368)
  • Improved encapsulation of import state (PEP 406)
  • Standard event loop API (PEP 3153)
  • A binary version of WSGI for Python 3 (e.g. PEP 444)
  • Generic function support (e.g. simplegeneric)