Hi, in the past I suggested to document our deprecation policy on a PEP. Since the issue came up again on a few issues on the tracker and on #python-dev, I adapted the content of my previous email and wrote a PEP.

Attached you can find the full text, including references to the previous discussions on python-dev and related issues.

Best Regards, Ezio Melotti

PEP: XXX Title: Deprecation Policy Version: $Revision$ Last-Modified: $Date$ Author: Ezio Melotti ezio.melotti@gmail.com Status: Draft Type: Process Content-Type: text/x-rst Created: 29-Jan-2016 Post-History:

Abstract

The goal of this PEP is to document when and how to deprecate APIs in the Python language and in the standard library.

Rationale

Python doesn't currently have a documented policy regarding deprecations. This leads to repeated discussions and inconsistencies in how we handle and document deprecations [#]_ [#]_ [#]_ [#]_ [#]_ [#]_ [#]_.

What and when to deprecate

• An API can be deprecated if a better alternative exists, if the implementation is incorrect or obsolete, or if the name or module has been changed. If possible, an alternative should be provided.
• If the API is public, it must go through the deprecation process. If the API is private or provisional, it might.
• The number of releases before an API is removed is decided on a case-by-case basis depending on widely used the API is, in which Python versions the alternative is available, which Python versions are currently supported by different operating systems, distros, and projects, and how easy it is to replace the API.
• In general it's better to be conservative, and if the API is deprecated in 3.X, it shouldn't be removed before 3.X+2. This should also take into account which Python versions are currently .

Porting from 2.x to 3.x

Some APIs that are available in Python 2.7 might get deprecated in Python 3, and people that upgrade directly from 2.7 to 3.X might not see any warnings.

In order to make porting code to 3.X easier:

• nothing that is available and not deprecated in 2.7 should be removed from Python 3 as long as 2.7 is officially supported;
• deprecation warnings can be backported to 2.7 and enabled using the -3 flag;

Deprecation Warnings

Python offers two kinds of deprecation warnings:

• PendingDeprecationWarning
• DeprecationWarning

Initially, only PendingDeprecationWarning\ s were silenced by default. Starting from Python 2.7, DeprecationWarning\ s are also silenced by default [#]_ [#]_.

Since this distinction is no longer true:

• PendingDeprecationWarning should not be used
• DeprecationWarning will be used for all deprecations

PendingDeprecationWarning won't be removed, and 3rd-party projects are allowed to use it as they see fit.

Deprecation Progression

In the past, the following deprecation progression has been used:

1. in release 3.X a PendingDeprecationWarning is added
2. in release 3.X+1 it becomes a DeprecationWarning
3. in release 3.X+2 the API is removed

The warning were occasionally left for more than one release, either intentionally or because no one remembered to update them.

Since PendingDeprecationWarning should no longer be used, this can be simplified to:

1. in release 3.X a DeprecationWarning is added
2. in release 3.X+N the API is removed

N can be >=1, should be decided beforehand, and should be documented explicitly.

Deprecation Process

These are the steps required to deprecate and remove an API:

1. propose to deprecate an API on a tracker issue or on python-dev and decide in which version it will be removed.

2. attach to the issue a patch to deprecate the API that:

• adds a DeprecationWarning to the code
• adds the deprecation to the documentation
• adds a test to verify that the warning is raised
• possibly updates code/examples that uses the deprecated API

3. after review, apply the patch to the current in-development branch and close the issue.

4. attach to a separate issue a patch to remove the API that:

• removes the API and the warning
• removes the tests for the API and for the deprecation
• removes the API documentation

5. once the designated branch is available, apply the patch and close the issue.

Deprecation Messages

When a deprecated API is used, a DeprecationWarning should be raised. The message should mention that the API is deprecated, and if possible it should indicate when it will be removed and what should be used instead.

These messages are intended for developers, and are therefore hidden by default to prevent end-users to see them.

These messages should be enabled by default in places where only developers will see them, such as tests [#]_ and in the interactive interpreter [#]_.

Documenting the deprecations

• All deprecations should be documented in the docs, using the deprecated-removed directive.
• If an alternative is available, it should be mentioned, possibly including examples.
• Each "what's new" document should include either a section or a link to a separate document [#]_ listing all the new deprecations, the APIs that have been removed and possibly the planned removals for the upcoming releases. This could be generated automatically with Sphinx.
• Simply removing the documentation for deprecated APIs is not acceptable, since people that see the API used in the code won't know if they can still use the API or not.

Deprecation warnings rendering

On one hand deprecation warnings should be clearly visible, on the other hand we want to avoid riddling the docs with red boxes [#]_.

In order to find a balance between the two:

• Individual functions/methods/attributes should have a label next to the name to indicate the deprecation, and a more verbose description underneath. The label will be red, the description doesn't have to [#]_.
• Modules and classes can use a red warning box. If the documentation spans more than a single screen of text, additional warning labels can be added to the individual functions/methods.
• Links to deprecated objects should be marked differently.

This will require some tweak to the deprecated-removed directive, in order to produce different CSS classes for modules/classes, functions/methods/attributes, and links.

Updating deprecated code

As mentioned above, the error messages and documentation should provide an alternative whenever possible. When the alternative is not straightforward, more complex examples or a new section can added to the documentation to explain how to convert the code.

Another option is to write scripts that can automatically update Python code to use the new alternative. This requires:

1. writing a 3to3 tool similar to (and possibly based on) 2to3;
2. documenting clearly its API and providing several examples;
3. providing fixers for deprecated APIs that can be used to automatically update deprecated code;

This can be done as a GSoC project, and if done correctly, it will provide an easy way for people to upgrade their codebases.

See also

• :pep:387 -- Backwards Compatibility Policy
• :pep:4 -- Deprecation of Standard Modules

References

.. [#] Deprecation Policy (https://mail.python.org/pipermail/python-dev/2011-October/114199.html)

.. [#] Deprecation Policy (https://mail.python.org/pipermail/python-dev/2014-January/132069.html)

.. [#] deprecated in 3.2/3.3, should be removed in 3.5 or ??? (https://bugs.python.org/issue13248)

.. [#] DeprecationWarning for PEP 479 (generator_stop) (http://bugs.python.org/issue26136)

.. [#] Deprecate threading.Thread.isDaemon etc (http://bugs.python.org/issue24203)

.. [#] Remove the Deprecated API in trace module (http://bugs.python.org/issue26069)

.. [#] Resurrect inspect.getargspec() in 3.6 (http://bugs.python.org/issue25486)

.. [#] What's New in Python 2.7 -- Changes to the Handling of Deprecation Warnings (https://docs.python.org/3/whatsnew/2.7.html)

.. [#] Silence DeprecationWarning by default (https://bugs.python.org/issue7319)

.. [#] Enable warnings by default in unittest (https://bugs.python.org/issue10535)

.. [#] DeprecationWarnings should be visible by default in the interactive REPL (https://bugs.python.org/issue24294)

.. [#] Django has a "Django Deprecation Timeline" page (https://docs.djangoproject.com/en/dev/internals/deprecation/)

.. [#] Consistent documentation practices for security concerns and considerations (https://bugs.python.org/issue13515)

.. [#] Put "deprecated" warnings first (http://bugs.python.org/issue25467)

Copyright

This document has been placed in the public domain.