PEP: 408 Title: Standard library __preview__ package Version: $Revision$
Last-Modified: $Date$ Author: Alyssa Coghlan <ncoghlan@gmail.com>, Eli
Bendersky <eliben@gmail.com> Status: Rejected Type: Standards Track
Content-Type: text/x-rst Created: 07-Jan-2012 Python-Version: 3.3
Post-History: 27-Jan-2012 Resolution:
https://mail.python.org/pipermail/python-dev/2012-January/115962.html

Abstract

The process of including a new module into the Python standard library
is hindered by the API lock-in and promise of backward compatibility
implied by a module being formally part of Python. This PEP proposes a
transitional state for modules - inclusion in a special __preview__
package for the duration of a minor release (roughly 18 months) prior to
full acceptance into the standard library. On one hand, this state
provides the module 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 module's
eventual full inclusion into the standard library, or to the stability
of its API, which may change for the next release.

PEP Rejection

Based on his experience with a similar "labs" namespace in Google App
Engine, Guido has rejected this PEP[1] in favour of the simpler
alternative of explicitly marking provisional modules as such in their
documentation.

If a module is otherwise considered suitable for standard library
inclusion, but some concerns remain regarding maintainability or certain
API details, then the module can be accepted on a provisional basis.
While it is considered an unlikely outcome, such modules may be removed
from the standard library without a deprecation period if the lingering
concerns prove well-founded.

As part of the same announcement, Guido explicitly accepted Matthew
Barnett's 'regex' module[2] as a provisional addition to the standard
library for Python 3.3 (using the 'regex' name, rather than as a drop-in
replacement for the existing 're' module).

Proposal - the __preview__ package

Whenever the Python core development team decides that a new module
should be included into the standard library, but isn't entirely sure
about whether the module's API is optimal, the module can be placed in a
special package named __preview__ for a single minor release.

In the next minor release, the module may either be "graduated" into the
standard library (and occupy its natural place within its namespace,
leaving the __preview__ package), or be rejected and removed entirely
from the Python source tree. If the module ends up graduating into the
standard library after spending a minor release in __preview__, its API
may be changed according to accumulated feedback. The core development
team explicitly makes no guarantees about API stability and backward
compatibility of modules in __preview__.

Entry into the __preview__ package marks the start of a transition of
the module into the standard library. It means that the core development
team assumes responsibility of the module, similarly to any other module
in the standard library.

Which modules should go through __preview__

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

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

It is important to stress that the aim of this proposal is not to make
the process of adding new modules to the standard library more
difficult. On the contrary, it tries to provide a means to add more
useful libraries. Modules which are obvious candidates for entry can be
added as before. Modules which due to uncertainties about the API could
be stalled for a long time now have a means to still be distributed with
Python, via an incubation period in the __preview__ package.

Criteria for "graduation"

In principle, most modules in the __preview__ package should eventually
graduate to the stable standard library. Some reasons for not graduating
are:

-   The module may prove to be unstable or fragile, without sufficient
    developer support to maintain it.
-   A much better alternative module 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 module's
appearance in the __preview__ package in some release does not guarantee
it will continue being part of Python in the next release.

Example

Suppose the example module is a candidate for inclusion in the standard
library, but some Python developers aren't convinced that it presents
the best API for the problem it intends to solve. The module can then be
added to the __preview__ package in release 3.X, importable via:

    from __preview__ import example

Assuming the module is then promoted to the standard library proper in
release 3.X+1, it will be moved to a permanent location in the library:

    import example

And importing it from __preview__ will no longer work.

Rationale

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

Essentially, the __preview__ package is intended to lower the risk of
locking in minor API design mistakes for extended periods of time.
Currently, this concern can block new additions, even when the core
development team consensus is that a particular addition is a good idea
in principle.

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 anything in the __preview__ package is
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 module is published and
    organized via the standard Python documentation tools (i.e. ReST
    source, output generated with Sphinx and published on
    http://docs.python.org).
-   Testing: The module test suites are run on the python.org buildbot
    fleet and results published via http://www.python.org/dev/buildbot.
-   Issue management: Bugs and feature requests are handled on
    http://bugs.python.org
-   Source control: The master repository for the software is published
    on http://hg.python.org.

Candidates for inclusion into __preview__

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

-   regex (http://pypi.python.org/pypi/regex)
-   daemon (PEP 3143)
-   ipaddr (PEP 3144)

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)
-   Encapsulation of the import state (PEP 368)
-   Standard event loop API (PEP 3153)
-   A binary version of WSGI for Python 3 (e.g. PEP 444)
-   Generic function support (e.g. simplegeneric)

Relationship with PEP 407

PEP 407 proposes a change to the core Python release cycle to permit
interim releases every 6 months (perhaps limited to standard library
updates). If such a change to the release cycle is made, the following
policy for the __preview__ namespace is suggested:

-   For long-term support releases, the __preview__ namespace would
    always be empty.
-   New modules would be accepted into the __preview__ namespace only in
    interim releases that immediately follow a long-term support
    release.
-   All modules added will either be migrated to their final location in
    the standard library or dropped entirely prior to the next long-term
    support release.

Rejected alternatives and variations

Using __future__

Python already has a "forward-looking" namespace in the form of the
__future__ module, so it's reasonable to ask why that can't be re-used
for this new purpose.

There are two reasons why doing so not appropriate:

1. The __future__ module is actually linked to a separate compiler
directives feature that can actually change the way the Python
interpreter compiles a module. We don't want that for the preview
package - we just want an ordinary Python package.

2. The __future__ module comes with an express promise that names will
be maintained in perpetuity, long after the associated features have
become the compiler's default behaviour. Again, this is precisely the
opposite of what is intended for the preview package - it is almost
certain that all names added to the preview will be removed at some
point, most likely due to their being moved to a permanent home in the
standard library, but also potentially due to their being reverted to
third party package status (if community feedback suggests the proposed
addition is irredeemably broken).

Versioning the package

One proposed alternative[3] was to add explicit versioning to the
__preview__ package, i.e. __preview34__. We think that it's better to
simply define that a module being in __preview__ in Python 3.X will
either graduate to the normal standard library namespace in Python 3.X+1
or will disappear from the Python source tree altogether. Versioning the
_preview__ package complicates the process and does not align well with
the main intent of this proposal.

Using a package name without leading and trailing underscores

It was proposed[4] to use a package name like preview or exp, instead of
__preview__. This was rejected in the discussion due to the special
meaning a "dunder" package name (that is, a name with leading and
trailing double-underscores) conveys in Python. Besides, a non-dunder
name would suggest normal standard library API stability guarantees,
which is not the intention of the __preview__ package.

Preserving pickle compatibility

A pickled class instance based on a module in __preview__ in release 3.X
won't be unpickle-able in release 3.X+1, where the module won't be in
__preview__. Special code may be added to make this work, but this goes
against the intent of this proposal, since it implies backward
compatibility. Therefore, this PEP does not propose to preserve pickle
compatibility.

Credits

Dj Gilcrease initially proposed the idea of having a __preview__ package
in Python[5]. Although his original proposal uses the name
__experimental__, we feel that __preview__ conveys the meaning of this
package in a better way.

References

Copyright

This document has been placed in the public domain.

[1] Guido's decision:
https://mail.python.org/pipermail/python-dev/2012-January/115962.html

[2] Proposal for inclusion of regex: http://bugs.python.org/issue2636

[3] Discussed in this thread:
https://mail.python.org/pipermail/python-ideas/2012-January/013246.html

[4] Discussed in this thread:
https://mail.python.org/pipermail/python-ideas/2012-January/013246.html

[5] https://mail.python.org/pipermail/python-ideas/2011-August/011278.html