PEP 775: Make zlib required to build CPython

.github/CODEOWNERS

@@ -653,6 +653,7 @@ peps/pep-0771.rst  @pradyunsg
 peps/pep-0772.rst  @warsaw @pradyunsg
 peps/pep-0773.rst  @zooba
 peps/pep-0774.rst  @savannahostrowski
+peps/pep-0775.rst  @stanfromireland @encukou
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 peps/pep-0777.rst  @warsaw
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peps/pep-0775.rst

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+PEP: 775
+Title: Make zlib required to build Python
+Author: Stan Ulbrych <stanulbrych@gmail.com>,
+        Petr Viktorin <encukou@gmail.com>
+Discussions-To: https://discuss.python.org/t/lets-make-zlib-required-rather-than-optional-to-build-cpython/23062
+Status: Draft
+Type: Standards Track
+Created: 20-Feb-2025
+Python-Version: 3.14
+Post-History: `23-Jan-2023 <https://discuss.python.org/t/23062>`__
+
+Abstract
+========
+
+Building CPython on systems without the ``zlib`` compression library, except WASI,
+is no longer supported.
+
+The ``zlib`` module is made a required part of the standard library,
+except on WASI.
+
+Builds without ``zlib`` will still be possible, but formally unsupported.
+
+
+Motivation
+==========
+
+The ``zlib`` library, which powers the ``zlib`` Python module,
+is available on virtually all systems.
+
+Many wheels on PyPI, including the `pip`_ installer, require ``zlib``.
+Users of *pip* would consider CPython without ``zlib`` to be broken,
+but mostly don't notice because all major builds of CPython include ``zlib``.
+
+.. _pip: https://pypi.org/project/pip/
+
+CPython developers don't really notice either. It turns out that at the time
+of this writing, at least one CPython test fails without ``zlib`` (the “skip”
+decorator in ``test_peg_generator.test_c_parser`` is applied too late),
+but our CI didn't catch this.
+
+This PEP treats this as an issue in documentation and messaging.
+In practice, we already don't support building CPython without ``zlib``; we
+should just say so.
+
+
+Rationale
+=========
+
+There are possible use cases for zlib-less builds, such as embedding and
+bootstrapping, as well as unforeseen ones.
+Therefore, we don't *remove* support for zlib-less systems; we mark them
+unsupported and invite affected users to do their own testing, or to share
+use cases that can make us reconsider this decision.
+
+``zlib`` is not yet used by default on the WASI platform -- mostly because
+adding it hasn't yet been a priority there. (Note that `Pyodide`_, the main
+“real-world” CPython distribution for WASI, does include ``zlib``.)
+We take this as an opportunity to  continue testing a platform without
+``zlib``, so that we don't unintentionally break unsupported builds yet.
+
+
+Specification
+=============
+
+In standard library modules that use ``zlib`` for optional functionality,
+that functionality will raise ``ImportError`` when used.
+Code to generate more “friendly” error messages, or to pre-check whether
+``zlib`` is available, will be removed.
+All non-``zlib``-related functionality will still be usable if ``zlib`` is
+missing.
+
+This affects the following modules, and more that depend on these
+transitively:
+
+* ``shutil`` (``gztar`` and ``zip`` archive formats)
+* ``tarfile``, ``zipfile``, ``zipimport``, ``zipapp`` (archive compression)
+* ``codecs`` (``zlib_codec``)
+
+``shutil.get_archive_formats()`` will always include ``zip`` and ``gztar``
+as registered formats, even if they are unusable due to missing ``zlib``.
+
+The ``configure`` script will issue a warning when `zlib`_ is not found on
+platforms other than WASI.
+
+``test_zlib`` will fail on platforms other than WASI.
+All other tests will continue to be skipped -- that is, uses of
+``@test.support.requires_zlib`` will be kept in place -- for the benefit
+of WASI, unsupported builds, and any possible reverts.
+
+PEP 11 will be adjusted to mark "Systems without zlib, except WASI" as
+unsupported.
+
+
+Backwards Compatibility
+=======================
+
+In practice, nothing major changes, except in error cases -- for example,
+attempts to use tar compression withoutt ``zlib`` available will raise
+``ImportError`` and not ``CompressionError``.
+
+
+Security Implications
+=====================
+
+None known.
+
+
+How to Teach This
+=================
+
+We don't expect that any instructions will need to change, as ``zlib`` is
+already available in all relevant contexts.
+
+
+Reference Implementation
+========================
+
+https://github.com/python/cpython/pull/130297
+
+
+Future work
+===========
+
+In the future, if no use cases for ``zlib``-less  builds are found,
+``zlib`` may be made fully required.
+The main changes needed for that would be making the ``configure`` script
+raise a hard error, and removing ``@test.support.requires_zlib``.
+
+
+Copyright
+=========
+
+This document is placed in the public domain or under the
+CC0-1.0-Universal license, whichever is more permissive.