On 13.02.2017 23:53, Rene Rivera via Boost-build wrote:
> Warning: This is only a quick reply after some investigation. I would
> have to delve deeper to figure out more of the why things are the way
> they are (for better or worse)..
>
> On Mon, Feb 13, 2017 at 9:35 AM, Stefan Seefeld via Boost-build
> <boost-build@lists.boost.org <mailto:boost-build@lists.boost.org>> wrote:
>
>     Hello,
>
>     having learned a bit more about Boost.Build and its internals, I'm
>     trying to apply my understanding.
>
>
>     My user-config.jam file contains
>
>       using python : 2.7 ;
>       using pyhon : 3.5 ;
>
>     Then I issue this command:
>
>       ./b2 --with-python python=2.7
>
>
>     Inside my Jamfile, I see that `feature.values python` reports "2.7
>     3.5",
>     independently on what I specify on the command-line.
>
>
> The "python" feature is composed of *all* the configured version of
> python.

I find that quite counter-intuitive.

>     And what does it mean to provide the `python=2.7` option on the
>     command-line ?
>
>
> It means to add the "<python>2.7" as a build request to match against
> the targets to build.

OK, makes sense, too. So let me paraphrase, to make sure I understand
the idea:

Having the two "using python" lines above means that those two python
configurations are made available for future reference (in Jamfiles, as
well as command-line). It does *not* imply that both are being used (as
feature values) automatically (i.e. not without some other code
elsewhere to specify that "all" is indeed what the Jamfile requests.)

>     How can I specify that I want to
>
>     * build with Python 2.7 only
>
>
> You can't [1].
>
>
>     * build with Python 3.5 only
>
>
> You can't [1]
>
>
>     * build both variants at the same time
>
>
> [1] The way the BPL build file is written by default it builds both a
> 2 and 3 version of the library if both a 2 and 3 python version are
> configured. Now.. Theoretically it would be possible that specifying
> the version on the command would limit it to just that version to
> build. But it seems that the way the python.jam module sets up python
> libraries it fails to trigger the b2 target requirements selection
> mechanics. Or simply put.. It's broken. And I don't know entirely why
> yet. I'll keep looking at it so see if I can resolve this better
> eventually.

OK, so can we agree that this is a bug (no matter where) ? After all I
have read about b2, it would seem logical that the above amounts to
declaring a "python" feature, with two possible values, such that a
command-line option of "python=2.7" would then pick one of the two.

However, this example raises another question (already asked by Tom): As
in the above the discriminating label is the version number, it isn't
clear how one can declare multiple python configurations that use the
same version. So if, as per Tom's example, I configure two Python 2.7
variants (one for 32-bit and one for 64-bit builds), how can I refer to
them unambiguously ? The version string clearly isn't enough. Is that
also a bug / limitation in the python.jam file ?