Re: public extension linked with libpython* vs. -Wl,-no-undefined

[Jakub Wilk <jwilk@debian.org>]

* Nico Schlömer <nico.schloemer@gmail.com>, 2014-03-17, 15:49:
> I'm building a package with Python support and would like to reduce the 
> number of warnings that dh_python2 gives me.
>
> One of them is
>
>   public extension linked with libpython2.7
>
> for a number of libraries. It is true that libpython2.7 is linked into 
> them,
>
> $ readelf -d /path/to/_ML.so
> [...]
> 0x0000000000000001 (NEEDED)             Shared library: [libpython2.7.so.1.0]
> [...]
>
> but when I don't, builds with "-Wl,-no-undefined" will fail.
>
> What is the reason for discouraging explicit links with libpython*?

The fundamental reason is that, except in unusual circumstances, this 
library won't be used. Python Policy §2.1 reads: “some distributions 
link extensions to libpython, but this is not the case in Debian as 
symbols might as well be resolved by ‘/usr/bin/python<X>.<Y>’ which is 
not linked to libpython.”

In the past there used to be also a strong practical reason against 
linking to libpython: every ELF dependency on libpython2.X was 
translated by dpkg-shlibdeps to package dependency on python2.X, which 
typically meant that your package ended up depending on multiple 
different Python versions. But theses days there's only one supported 
Python (2.X) version, so this is not such a big deal.

BTW, there's a greater chance to meet a Python expert on debian-python@ 
than here. :-P

--
Jakub Wilk