[CMake] Finding Python3

[Michael Wild]

On 23. Jul, 2010, at 6:29 , Michael Hertling wrote:

> On 07/22/2010 10:36 AM, Michael Wild wrote:
>> 
>> On 22. Jul, 2010, at 10:17 , Marcel Loose wrote:
>> [...]
>>> 
>>> Hi Michael and others,
>>> 
>>> I mostly agree with what your saying. However, IMHO, you refer to a
>>> "perfect world" situation, where all Find modules properly use VERSION
>>> to specify a version number and do not abuse NAMES for that.
>>> 
>>> I know that the current discussion focuses on FindPython; hence the
>>> subject ;-). However, in the "real world" quite a number of other Find
>>> scripts are shipped as part of the CMake distribution that don't follow
>>> this "perfect" scheme either.
>>> 
>>> So the real question should be, I guess: Should CMake be fixed by
>>> swapping the paths and names loops in the FindXXX() functions (issue
>>> 10718)? Or should all abusing Find scripts be fixed?
>>> 
>>> Best regards,
>>> Marcel Loose.
>> 
>> My question is more fundamental:
>> 
>> How do I find the most recent version? Because that is why NAMES is being "abused" in the first place.
> 
> That's a very good point. The simplest approach is probably to rely on
> the assumption that the unversioned executable is the most recent one,
> but this won't be true in any case, of course. If one doesn't want to
> hardcode the possible versions in the correct order after the NAMES
> option there has to be a way to teach the find module how to respect
> such auxiliary conditions like "take the highest acceptable version".
> 
> Earlier in this thread, I presented a function GET_PYTHON_VERSION()
> that looks for Python libraries without querying the interpreter and
> extracts the version from the first one found; this function can be
> modified to return the highest version instead, e.g. as follows:
> 
> FUNCTION(GET_PYTHON_VERSION RESULT)
>    SET(v)
>    FOREACH(i IN LISTS PYTHON_ROOT ARGN)
>        FILE(GLOB j "${i}/lib/python*")
>        FOREACH(k IN LISTS j)
>            IF(k MATCHES "python[0-9].*\$")
>                STRING(REGEX REPLACE
>                    "^.*python([0-9].*)\$" "\\1" l "${k}"
>                )
>                IF("${l}" VERSION_GREATER "${v}")
>                    SET(v "${l}")
>                ENDIF()
>            ENDIF()
>        ENDFOREACH()
>    ENDFOREACH()
>    SET("${RESULT}" "${v}" PARENT_SCOPE)
> ENDFUNCTION()
> 
> Of course, the crucial moment is to provide the directories to search.
> Here, the variable PYTHON_ROOT assists, and the directories examined by
> FIND_PROGRAM() are known, so it should be possible to visit each Python
> installation and figure out the version of the most recent amongst them.
> Further refinements may be performed with the globbing expression, e.g.
> FILE(GLOB j "${i}/lib/python2.*") to search for Python 2 installations
> only, which could in turn be fed by the Python_FIND_VERSION variables.
> Finally, the result of GET_PYTHON_VERSION() can be used in subsequent
> calls to FIND_PROGRAM() et al. to locate the Python installation with
> the most recent version.
> 
> Regards,
> 
> Michael

<gollum>Hateful, ugly function! Go away! It blinds us!</gollum>

Essentially, you are replacing find_library, which IMHO is not desirable. Particularly, because you are not searching all the paths find_library would (CMAKE_PREFIX_PATH, CMAKE_LIBRARY_PATH, CMAKE_FRAMEWORK_PATH, CMAKE_SYSTEM_PREFIX_PATH, CMAKE_SYSTEM_LIBRARY_PATH, CMAKE_SYSTEM_FRAMEWORK_PATH, CMAKE_FIND_ROOT_PATH). So the above function would need to be much more elaborate.

I'm just listing a few ideas, no idea whether they are any good:

* How about globbing/regex support in find_XXX?
* How about an ALL option for find_XXX telling them to find all matching candidates? Is this desirable, or will this slow things down to a crawl?
* Support for a version-regex argument à la REGEX REPLACE which allows find_XXX to sort the results and pick the newest? Peformance?
* An option to pass a callback function which takes two paths and returns -1, 0, 1 (again, allowing find_XXX to sort the results)? Performance???

Michael