gh-115119: Removed bundled copy of the libmpdec by skirpichev · Pull Request #133964 · python/cpython

skirpichev · 2025-05-13T09:40:09Z

Issue: Unvendor libmpdec sources #115119

📚 Documentation preview 📚: https://cpython-previews--133964.org.readthedocs.build/

AA-Turner

quick comments, haven't looked at build system changes

Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>

ned-deily

Mac/Buildscript/build-installer.py change LGTM

zware · 2026-05-09T01:59:05Z

+* Remove the bundled libmpdec_ decimal library from the CPython source tree,
+  to simplify maintenence and updates. The :mod:`decimal` module will now
+  unconditionally use the system's libmpdec decimal library. Also remove the
+  now unused :option:`!--with-system-libmpdec` :program:`configure` flag.


We should probably mention that binary distributions will still include the _decimal module with libmpdec compiled in.

zware · 2026-05-09T02:07:09Z

 decimal.so is not built from a static libmpdec.a since doing so led to
 failures on AIX (user report) and Windows (mixing static and dynamic CRTs
 causes locale problems and more).
-


Nit: most of this is out of date, and there are more extra blank lines at the top of the file.

Are you OK with removal of this?

Either removal or paring it back to just the About section would be my preference.

~~There's also a Modules/_decimal/windows/ directory to remove, and arguably Modules/_decimal/tests/.~~ Scratch that, we still use Modules/_decimal/windows/mpdecimal.h. Still on the fence about tests/, though.

Still on the fence about tests/, though.

Why? We don't have any tests for the decimal module in th pybenchmark. Yet, have severe regressions since 3.9+. I don't think that removal of tests is a good idea.

At least, that should go to another pr.

As far as I can tell, these tests are not run by regrtest and are mostly relevant to libmpdec rather than _decimal. I mostly just don't see why they're here. If they're important enough to keep, they're important enough to wire into regrtest.

Co-authored-by: Zachary Ware <zachary.ware@gmail.com>

sethmlarson

Reviewed the changes to sbom.spdx.json and generate_sbom.py and those changes LGTM.

zware

Quite close, I think, just a few more cleanups.

zware · 2026-05-11T16:31:01Z

 decimal.so is not built from a static libmpdec.a since doing so led to
 failures on AIX (user report) and Windows (mixing static and dynamic CRTs
 causes locale problems and more).
-


Either removal or paring it back to just the About section would be my preference.

zware · 2026-05-11T16:47:08Z

 decimal.so is not built from a static libmpdec.a since doing so led to
 failures on AIX (user report) and Windows (mixing static and dynamic CRTs
 causes locale problems and more).
-


~~There's also a Modules/_decimal/windows/ directory to remove, and arguably Modules/_decimal/tests/.~~ Scratch that, we still use Modules/_decimal/windows/mpdecimal.h. Still on the fence about tests/, though.

zware · 2026-05-12T17:16:27Z

+* Remove the bundled copy of the libmpdec_ decimal library from the CPython source tree
+  to simplify maintenence and updates. The :mod:`decimal` module will now
+  unconditionally use the system's libmpdec decimal library. Also remove the
+  now unused :option:`!--with-system-libmpdec` :program:`configure` flag.


This could still use a note to the following effect:

Suggested change

now unused :option:`!--with-system-libmpdec` :program:`configure` flag.

now unused :option:`!--with-system-libmpdec` :program:`configure` flag.

This change has no impact on binary releases of Python, which have been

built against a separate copy of libmpdec for the past several releases.

zware · 2026-05-12T17:20:26Z

+Previously, the :mod:`!_decimal` C extension underlying the :mod:`decimal` module
+was built using an included copy of the libmpdec
 library unless the build is configured ``--with-system-libmpdec``::


Wouldn't hurt to fix the libffi section as well. Suggested wording here, though:

Suggested change

Previously, the :mod:`!_decimal` C extension underlying the :mod:`decimal` module

was built using an included copy of the libmpdec

library unless the build is configured ``--with-system-libmpdec``::

The :mod:`!_decimal` C extension underlying the :mod:`decimal` module

is linked to the libmpdec C library::

On the other hand, we could just leave this file alone for now and get someone more knowledgeable about such things to tell us what we should do in a separate issue :)

zware · 2026-05-12T17:23:27Z

+if test "$have_mpdec" = "no"
+then
+  AS_VAR_SET([py_cv_module_]_decimal, [missing])
+fi


Not quite the change I meant; this whole check should be completely unnecessary:

Suggested change

if test "$have_mpdec" = "no"

then

AS_VAR_SET([py_cv_module_]_decimal, [missing])

fi

The PY_STDLIB_MOD macro below just does what's needed.

zware · 2026-05-12T17:58:20Z

 # Disable forced inlining in debug builds, see GH-94847
 AS_VAR_IF(
  [with_pydebug], [yes],
  [AS_VAR_APPEND([LIBMPDEC_CFLAGS], [" -DTEST_COVERAGE"])])


This is only relevant for building libmpdec itself, isn't it?

There is a usage of TEST_COVERAGE in Modules/_decimal/_decimal.c, but I don't see anything ever defining it for that module.

zware · 2026-05-12T17:59:16Z

  [with_pydebug], [yes],
  [AS_VAR_APPEND([LIBMPDEC_CFLAGS], [" -DTEST_COVERAGE"])])

+AC_SUBST([LIBMPDEC_CFLAGS])


Why was this added?

zware · 2026-05-12T18:03:18Z

 decimal.so is not built from a static libmpdec.a since doing so led to
 failures on AIX (user report) and Windows (mixing static and dynamic CRTs
 causes locale problems and more).
-


As far as I can tell, these tests are not run by regrtest and are mostly relevant to libmpdec rather than _decimal. I mostly just don't see why they're here. If they're important enough to keep, they're important enough to wire into regrtest.

pythongh-115119: Removed bundled copy of the libmpdec

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bedevere-app Bot mentioned this pull request May 13, 2025

Unvendor libmpdec sources #115119

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skirpichev added 2 commits May 13, 2025 13:30

+ configure

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+1

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skirpichev force-pushed the remove-bundled-mpdecimal/115119 branch from 7cba56a to 9d6c9b5 Compare May 13, 2025 15:37

AA-Turner reviewed May 13, 2025

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Comment thread Doc/using/configure.rst

Comment thread Doc/whatsnew/3.13.rst Outdated

Comment thread Doc/whatsnew/3.15.rst Outdated

Comment thread Misc/sbom.spdx.json Outdated

Comment thread Modules/Setup.stdlib.in Outdated

skirpichev and others added 2 commits May 13, 2025 19:21

Apply suggestions from code review

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Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>

+ configure

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zware previously requested changes May 13, 2025

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AA-Turner and others added 6 commits May 13, 2025 19:52

autoconf

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revert

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makesetup

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+ lint and macos

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Windows: SBOM

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lint

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skirpichev added 2 commits May 14, 2025 06:20

fix docs

2fca0e5

XXX disable _decimal if no libmpdec is available

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bedevere-app Bot requested a review from zware May 14, 2025 03:41

+1

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skirpichev added 2 commits May 14, 2025 07:51

fix json

bd03679

reorder

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skirpichev force-pushed the remove-bundled-mpdecimal/115119 branch from 0a33f9f to 2ba6b8b Compare May 14, 2025 05:49

skirpichev marked this pull request as ready for review May 8, 2026 05:43

skirpichev requested review from a team, StanFromIreland, corona10, emmatyping, ericsnowcurrently, erlend-aasland, ezio-melotti, hugovk, sethmlarson and webknjaz as code owners May 8, 2026 05:43

ned-deily reviewed May 9, 2026

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zware reviewed May 9, 2026

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skirpichev and others added 4 commits May 9, 2026 06:33

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Co-authored-by: Zachary Ware <zachary.ware@gmail.com>

address review: restore license.rst

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address review: revert unrelated change

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address review: cleanup configure.ac

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skirpichev requested a review from zware May 9, 2026 05:59

sethmlarson approved these changes May 11, 2026

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bedevere-app Bot added awaiting core review and removed awaiting review labels May 11, 2026

Merge branch 'main' into remove-bundled-mpdecimal/115119

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zware reviewed May 11, 2026

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skirpichev added 5 commits May 12, 2026 07:06

Merge branch 'master' into remove-bundled-mpdecimal/115119

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address review: move news

0cab465

address review: trying to reword license.rst

f4a1b18

address review: cleanup Modules/_decimal/README.txt

11867cd

address review: set module missing, if have_mpdec==no

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skirpichev requested a review from zware May 12, 2026 04:07

zware reviewed May 12, 2026

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skirpichev commented May 13, 2025 •

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