PEP 11: Unsupported platforms & POSIX

peps/pep-0011.rst

@@ -9,6 +9,7 @@ Post-History: `18-Aug-2007 <https://mail.python.org/archives/list/python-dev@pyt
               `14-May-2014 <https://mail.python.org/archives/list/[email protected]/thread/T7WTUJ6TD3IGYGWV3M4PHJWNLM2WPZAW/>`__,
               `20-Feb-2015 <https://mail.python.org/archives/list/[email protected]/thread/OEQHRR2COYZDL6LZ42RBZOMIUB32WI34/>`__,
               `10-Mar-2022 <https://mail.python.org/archives/list/[email protected]/thread/K757345KX6W5ZLTWYBUXOXQTJJTL7GW5/>`__,
+              `21-Nov-2025 <https://discuss.python.org/t/104986>`__,
 
 
 Abstract
@@ -35,15 +36,20 @@ unmaintainability: without having experts for a large number of
 platforms, it is not possible to determine whether a certain
 change to the CPython source code will work on all supported
 platforms.
-
 To reduce this risk, this PEP specifies what is required for a
-platform to be considered supported by CPython as well as providing a
-procedure to remove code for platforms with few or no CPython
-users.
+platform to be considered supported by the CPython core team,
+as well as providing a procedure to remove code for platforms
+with few or no CPython users.
+
+On the other hand, allowing these fragments in the main repository
+can promote collaboration, can help identify non-portable parts of
+the code base, and is necessary for bootstrapping support for
+a "new" platform.
+This PEP specifies what it means for a platform to be "unsupported",
+and how the core team handles code for such platforms.
 
-This PEP also lists what platforms *are* supported by the CPython
-interpreter. This lets people know what platforms are directly
-supported by the CPython development team.
+This PEP also explicitly lists what platforms are directly
+*supported* by the CPython development team.
 
 
 Support tiers
@@ -135,17 +141,43 @@ x86_64-unknown-freebsd           BSD libc, clang             Victor Stinner
 ================================ =========================== ========
 
 
-All other platforms
--------------------
+Unsupported platforms
+---------------------
+
+All platforms not listed in the above tiers are *unsupported* by the core team.
+The core team does not develop and test on such platforms, and so they
+cannot provide any promises that Python will work on them.
+
+However, the code base does include unsupported code -- that is, code
+specific to unsupported platforms.
+Contributions in this area are welcome as long as they:
+
+- pose a minimal maintenance burden to the core team, and
+- benefit substantially more people than the contributor.
+
+We assume contributors are able to maintain modifications/patches,
+test patched builds, redistribute modified code, make promises to their users,
+and otherwise support "their" platform.
+With that in mind, it is generally unnecessary to backport unsupported
+fixes to CPython's maintenance branches.
+
+Unsupported code that *does* cause a maintenance burden, or obstructs
+general improvements, may be rejected or removed from the code base
+without a deprecation process.
+Core team members that do this intentionally are encouraged to notify people
+listed in the `Platforms experts list`_ in the CPython contributor's guide,
+to review any submitted fixes (if inobtrusive), and to consider adding
+configuration or extension capabilities necessary for workarounds.
+
+People interested in unsupported platforms may add themselves to the
+`Platforms experts list`_ to request that they be notified on issues
+related to "their" platform.
+There is, however, no formal guarantee that they *will* be notified.
 
-Support for a platform may be partial within the code base, such as
-from active development around platform support or accidentally.
-Code changes to platforms not listed in the above tiers may be rejected
-or removed from the code base without a deprecation process if they
-cause a maintenance burden or obstruct general improvements.
+.. _Platforms experts list: https://devguide.python.org/core-team/experts/#platforms
 
-Platforms not listed here may be supported by the wider Python
-community in some way. If your desired platform is not listed above,
+Platforms not listed in this PEP may also be supported by the wider Python
+community in other ways. If your desired platform is not listed above,
 please perform a search online to see if someone is already providing
 support in some form.
 
@@ -207,6 +239,27 @@ source tree 3 years after the extended support for the compiler has
 ended (but continue to remain available in revision control).
 
 
+POSIX
+'''''
+
+Features specified in POSIX are expected to work according to the standard.
+This cuts two ways:
+
+- If a POSIX feature is available, it is expected to conform to POSIX.
+
+  Workarounds for out-of-spec platforms are acceptable.
+  For unsupported platforms, disabling functionality is
+  preferred over a non-trivial workaround.
+
+- CPython should make no assumptions about POSIX features beyond what's
+  specified in POSIX.
+
+  For example, while POSIX specifies ``errno`` as an ``int`` with no
+  restrictions, error codes on all supported platforms happen to be positive.
+  Relying on this would be considered a bug, even if it only manifests on
+  unsupported platforms.
+
+
 Legacy C Locale
 '''''''''''''''
 
@@ -373,6 +426,9 @@ No-longer-supported platforms
 Discussions
 ===========
 
+* November 2025: `Policy for unsupported platforms
+  <https://discuss.python.org/t/104986>`_
+  (Petr Viktorin)
 * April 2022: `Consider adding a Tier 3 to tiered platform support
   <https://mail.python.org/archives/list/[email protected]/thread/V3OZPJGA5VJFYM6XYGPZIVPOIYKX6KTD/>`_
   (Victor Stinner)