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gh-91246: Implement PEP 775: Make zlib required on platforms other than wasi #130297

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@StanFromIreland StanFromIreland commented Feb 19, 2025 •
edited by hugovk
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According to Petr's plan:

You need zlib to install pip, as both the wheel and sdist are compressed. The problem is not 300 extra lines of code. It’s that practically, for most users, CPython without zlib is broken.
But,

  • zlib-less builds of CPython might still be useful in some special cases.
  • Requiring zlib for WASI is not (and should not be) a priority. I haven’t looked at Android and iOS; this discussion might predate their support.

Let’s make that official. I propose to change our messaging, not (much) code.

Here’s a concrete proposal. Where I say WASI, Android/iOS (or any other platform) can be added if needed.

The 3.14 What’s New should say:

zlib is now officially required to build CPython.
It is still technically possible to build CPython without it for special needs, like bootstrapping. Such builds are not supported, but we can accept pull requests to keep them working.
As an exception, zlib is not required on WASI.

Lib/test/test_zlib.py should add the following comment, and a zlib-less build should fail with an error message that points to it. This serves as a deprecation warning. (On WASI the tests should instead continue to be skipped.)

Building CPython without zlib is not supported.
Anyone who wants build CPython this way should be prepared to patch it, but the core team may help getting those patches to the main branch (as that’s the place where multiple third parties can cooperate).

For tests to pass without zlib, this file needs to be removed.

Existing tests skips (e.g. @requires_zlib()) stay in (except in test_zlib.py itself, as above).
binascii’s reimplementation of crc32 is removed; corresponding tests are skipped.
Code that only generates nice error messages in zlib-less builds is removed. Corresponding tests are adjusted.

PEP 11 gets a new entry:

  • Name: Systems without zlib, except WASI
  • Unsupported in: Python 3.14
  • Code removed in: Unknown (see Lib/test/test_zlib.py for details)

configure is changed to produce a warning on systems without zlib, except WASI.

Note that the WASI exception means that we’ll still test some zlib-less build. That’s a good thing.
Getting zlib there is the opposite of a blocker issue: I’d prefer that to happen well after this change.

However binascii's re-implementation of crc32 cannot be removed till WASI supports zlib.

📚 Documentation preview 📚: https://cpython-previews--130297.org.readthedocs.build/

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@StanFromIreland StanFromIreland marked this pull request as draft February 19, 2025 10:44
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@StanFromIreland StanFromIreland marked this pull request as ready for review February 19, 2025 11:37
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@StanFromIreland StanFromIreland changed the title gh-91246: Make zlib required to build python gh-91246: Make zlib required to build python on platforms other than WASI Feb 19, 2025
@hugovk hugovk mentioned this pull request Feb 19, 2025
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@StanFromIreland @gpshead
Remove fancy error messages for zlib import errors
57d3520 
Co-authored-by: Gregory P. Smith <greg@krypto.org>
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encukou commented Feb 21, 2025 •
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Please add zlib to build requirements. The version notes from zlib should be added moved there.
ZLIB_LIBS should mention zlib, not gzip.

Please also adjust docs for shutil.get_archive_formats, shutil.make_archive, and any others you find. Add .. versionchanged:: next notes.

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Lib/test/test_zlib.py
@@ -7,9 +7,22 @@
import random
import sys
from test.support import bigmemtest, _1G, _4G, is_s390x
import importlib
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Keep imports sorted

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They are all technically "Standard library imports"...

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The comment was about alphabetically sorted, your reply about grouped by type 🙂

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I'm sorry but where in PEP 8 is that specified, I cannot seem to find any mention of it?

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It’s a very common practice. But another one is to always import os and sys at the top.
Yet another is to sort by module length to get a nice pyramid drawing 📐

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Doc/whatsnew/3.14.rst
@@ -1310,6 +1310,10 @@ Build changes
* GNU Autoconf 2.72 is now required to generate :file:`configure`.
(Contributed by Erlend Aasland in :gh:`115765`.)

* zlib is now required to build CPytho on all platforms except WASI.
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Suggested change
* zlib is now required to build CPytho on all platforms except WASI.
* zlib is now required to build CPython on all platforms except WASI.

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PEP 775 was withdrawn.

@StanFromIreland StanFromIreland changed the title gh-91246: Make zlib required to build python on platforms other than WASI gh-91246: Implement PEP 775 May 9, 2025
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merwok commented May 9, 2025

@StanFromIreland could you keep the previous title and add the mention of the PEP if you want?

@StanFromIreland StanFromIreland deleted the enfore-zlib branch May 9, 2025 19:39
@StanFromIreland StanFromIreland changed the title gh-91246: Implement PEP 775 gh-91246: Implement PEP 775: Make zlib required on platforms other than wasi May 9, 2025
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@merwok Is this sufficient?

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