Re: [boost] Did Boost 1.59.0 change the meaning of "using python : 3.4 : ..." in user-config.jam?

Subject: Re: [boost] Did Boost 1.59.0 change the meaning of "using python : 3.4 : ..." in user-config.jam?
From: Jonathan Wakely (jwakely.boost_at_[hidden])
Date: 2015-08-24 04:41:23

On 23 August 2015 at 18:05, Vladimir Prus wrote:
> On 21-Aug-15 8:11 PM, Jonathan Wakely wrote:
>>
>> Hi,
>>
>> I'm trying to update the Fedora rawhide package to Boost 1.59.0 and
>> the build fails with the output below (ignore the MPI warnings).
>>
>> We build with the following in user-config.jam:
>>
>> # This _adds_ extra python version.  It doesn't replace whatever
>> # python 2.X is default on the system.
>> using python : 3.4 : /usr/bin/python3 : /usr/include/python3.4m : : : : m
>> ;
>>
>>
>> Is that comment no longer true?
>
>
> Jonathan,
>
> I've just tried with the following user-config.jam:
>
>         using python : 2.7 : C:/Python27 ;
>         using python : 3.4 : C:/Python34 ;
>
> and was able to do both "b2 python=2.7" and "b2 python=3.4" and these appear
> to pick different
> python version - at least the latter produces a different set of warnings.

Yes, if I add "using python : 2.7 : blahblah;" then it works, but
we've been building using Python 2.7 without that extra line since
2012, and something seems to have changed in Boost 1.59.0

> I'm not entirely sure what the comment mean regarding system python 2.X.
> There is code in
> libs/python/build/Jamfile.v2 that does "using python" if no previous python
> initialization
> is found. That code is executed after user-config.jam and all other config
> files. I don't
> believe there was ever code to add 2.X python implicitly, so if you have
> "using python : 3.4",
> you must have "using python : 2.7" somewhere too for that version to be
> used.

OK, I guess we've been doing it wrong but it just worked by chance.

> Maybe 2.7 used to be initialized in site-config.jam?

No, we don't use a site-config.jam, we just unpack the release
tarball, apply some patches and create a user-config.jam

> Does that help?

Yes, it confirms that adding the "using python : 2.7 : blahblah;" line
is correct, thanks.