Only pure Python decimal module is used in Python 3.15 on Ubuntu

[serhiy-storchaka]

Bug report

There is no system libmpdec on Ubuntu 24.04 LTS. So Python 3.15 falls back to pure-Python version for the decimal module.

[skirpichev]

I don't think this fits meaning of "regression".

Removal of the bundled libmpdec was previously announced and we already shifted schedule a little (to 3.16). The v3.15 was released only recently and only here silent fallback to bundled library was removed. It's in news, etc. It's not silent on the configure side. And nothing (modulo docs) was changed for previous versions of the CPython.

The lack of the libmpdec was a known issue for Ubuntu 24.04 (and Debian version, on which it based). (This is something was happened for me by accident, see #119299). And it has a known workaround (see our CI tests). We could document this in user docs (as we did for developers). Does this make sense for you?

Alternative - full reversion of #134078. I don't think it's a good idea - that means we shift our schedule again!

And what's special in this Ubuntu version? I beat there are lot of systems without libmpdec, available as a native package.

[serhiy-storchaka]

Python should fall back to the bundled version instead of pure Python implementation if system libmpdec is not found and --with-system-libmpdec is not explicitly used.

[serhiy-storchaka]

Yes, this means rollback of #134078.

[skirpichev]

Python should fall back to the bundled version instead of pure Python implementation if system libmpdec is not found and --with-system-libmpdec is not explicitly used.

Sorry, but that means we can't remove bundled libmpdec forever.

Currently we have following behavior (with --with-system-libmpdec is our default, since 3.13): #134078 (comment)

Yes, this means rollback of #134078.

What you suggest instead as a first step, before removal of the bundled copy, as scheduled in #115119?

[AA-Turner]

I think this is an Ubuntu bug rather than CPython. Debian/Ubuntu need to change how they build Python to include libmpdec, probably meaning using the bundled version now and libmpdec for the newer OS versions that it now exists in.

cc @doko42 @mitya57

A

[edmorley]

It looks like libmpdec-dev + libmpdec* is available for Ubuntu 22.04 and older, and also 25.04 and newer, and is just missing for the releases in-between (such as 24.04)?
https://packages.ubuntu.com/search?suite=all&section=all&arch=any&keywords=libmpdec-dev&searchon=names

I'm presuming this is a side-effect of the package's removal from Debian and then reintroduction under a new maintainer later? xref:

• Unvendor libmpdec sources #115119 (comment)
• Unvendor libmpdec sources #115119 (comment)

Seeing that the package is available in newer Ubuntu releases again, would the Ubuntu libmpdec maintainers be open to backporting it to 24.04 too?

Whilst Ubuntu 24.04 will never ship with Python 3.15/3.16 itself, there are many people who need to build newer Python versions on all currently supported Ubuntu LTS releases.

[skirpichev]

Debian/Ubuntu need to change how they build Python to include libmpdec

No, they shouldn't change anything. Never versions of the Debian/Ubuntu - will provide package for libmpdec, as before, see #115119 (comment).

IIUC, Serhiy's issue is related to custom builds (e.g. during development) and change of meaning ./configure --with-system-libmpdec (i.e. default), introduced in v3.15.

We have:

1. <= 3.14: --with-system-libmpdec with a silent fallback to the bundled copy of libmpdec
2. ???
3. >= 3.16: --with-system-libmpdec - removed, together with bundled copy. If no system library is available - only pure-Python version will be present, no configure failure (only a warning, like for other modules). PROFIT!

How to make 1->3 transition better?