PEP 775: Make zlib required to build CPython (#4281)

Co-authored-by: Petr Viktorin <[email protected]>
Co-authored-by: Hugo van Kemenade <[email protected]>
Co-authored-by: Adam Turner <[email protected]>

Author: 4 people

.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  @encukou
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peps/pep-0775.rst

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+PEP: 775
+Title: Make zlib required to build CPython
+Author: Gregory P. Smith <[email protected]>,
+        Stan Ulbrych <[email protected]>,
+        Petr Viktorin <[email protected]>
+Discussions-To: https://discuss.python.org/t/23062
+Status: Draft
+Type: Standards Track
+Created: 24-Feb-2025
+Python-Version: 3.14
+Post-History: `23-Jan-2023 <https://discuss.python.org/t/23062>`__
+
+
+Abstract
+========
+
+Building CPython without the `zlib <https://zlib.net>`_ compression library
+will no be longer supported, and the :mod:`zlib` module will be required in
+the standard library.
+The only exception is `WASI <https://wasi.dev>`_, as zlib is not currently
+supported in CPython on WASI.
+Building the interpreter without zlib may still be possible,
+but formally unsupported.
+
+
+Motivation
+==========
+
+The zlib library, which powers the :mod:`!zlib` Python module,
+is available on all supported systems except WASI.
+
+Many wheels on PyPI, including the :pypi:`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.
+
+CPython developers don't really notice either. It turns out that at the time
+of 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.
+
+.. _Pyodide: https://pyodide.org
+
+
+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 functionality unrelated to zlib will still be usable if zlib is
+missing.
+
+This affects the following modules, and more that depend on these
+transitively:
+
+* :mod:`shutil` (``gztar`` and ``zip`` archive formats)
+* :mod:`tarfile`, :mod:`zipfile`, :mod:`zipimport`,
+  :mod:`zipapp` (archive compression)
+* :mod:`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 without 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
+========================
+
+A reference implementation may be found in a pull request to the CPython
+repository, `python/cpython#130297
+<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.