[CMake] Problems with FindPythonInterp

[Campbell Barton]

On Tue, Oct 11, 2011 at 10:50 PM, Yngve Inntjore Levinsen
<yngve.levinsen at gmail.com> wrote:
> Fredag 07 oktober 2011 skrev Yngve Inntjore Levinsen:
>> Dear all,
>>
>> I have some configuration scripts in python that needs to be run for my
>> project, so I use the FindPythonInterp package. This has nicely found the
>> version 2.7 on my system so far, but now it suddenly finds /usr/bin/python
>> which is a soft link to version 3.2. My script doesn't work with python 3.
>>
>> First: Is it possible to set a requirement that it should be python version
> 2?
>>
>> Second: This for some weird reason only happens on my desktop, even though I
>> have pretty much exactly the same setup on my laptop. On my laptop it finds
>> /usr/bin/python2.7 (as it always has). They both have the same python
> versions
>> installed (2.7 and 3.2), and they both have the same cmake version installed
>> (2.8.6). I am running Arch Linux.
>>
>> On my laptop:
>> -- Found PythonInterp: /usr/bin/python2.7
>> On my desktop:
>> -- Found PythonInterp: /usr/bin/python (found version "3.2.2")
>>
>> I'm sure there must be some differences in what's installed on the two
> systems,
>> but I cannot figure out what.
>>
>> I tried
>> find_package(PythonInterp 2.7 REQUIRED)
>> but that still found the 3.2 version and only commented that 2.7 was
> required.
>>
>> Let me know if I can provide further information.
>>
>> Cheers,
>> Yngve
>
> Hi,
>
> I did not receive any response to my mail. Here is the solution which was the
> best I could come up with:
>
> First, I made a copy of each of the python scripts and gave them new names
> with "_py3" at the end. I converted those to python 3 syntax (2to3 helps you
> quite a bit to get started). Then I added the following in my cmake script:
>
> if(PYTHON_VERSION_MAJOR)
>    if(${PYTHON_VERSION_MAJOR} EQUAL 3)
>        set(PYSCRPT_END "_py3")
>    endif()
> endif()
>
> Finally I added ${PYSCRPT_END} to all python script names where they occured
> in the cmake scripts. That way cmake chooses the correct scripts
> automatically, which is a good enough solution for me.
>
> I would say though, I'd recommend that you create a way to require python 2.
> It isn't as easy for everyone to rewrite their code, some needed libraries
> might be missing. It is stupid if cmake cannot be used on systems where both
> python 2 and python 3 are installed for this rather simple reason.
>
> Cheers,
> Yngve
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Proper python3 support (which allows selecting python2 also), is not
trivial I think because it needs to support ABI flags for 3.2+, and no
ABI flags for previous versions (cant recall which version exactly API
flags were added).

Since CMake currently only supports python2.x some quick fix not to
select "python" binary which is version 3.x should be pretty easy.

On a similar topic, I maintain a py3.x find module which supports ABI
flags, perhaps helpful for updating CMake's support for py3:
https://svn.blender.org/svnroot/bf-blender/trunk/blender/build_files/cmake/Modules/FindPythonLibsUnix.cmake

-- 
- Campbell