Abstract

Building CPython without the zlib compression library will no be longer supported, and the zlib module will be required in the standard library. The only exception is WASI, 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 zlib Python module, is available on all supported systems except WASI.

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.

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.

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:

• 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 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

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.