Specifying the Python version option has no effect

[alkersan]

According to the answer, a PYTHON_VERSION option should be used to explicitly specify the python interpreter, but an incorrect behavior is observed on a system where the python3 interpreter is the default one (e.g. Arch). Despite the PYTHON_VERSION=2 is being set, the result of find_package(PythonInterp) (called here) is not /usr/bin/python2, but /usr/bin/python.

Versions:
python3 3.6.3 is the default one /usr/bin/python
python2 2.7.14 also present, but as /usr/bin/python2 only
cmake 3.8

Example output:

> catkin_make --cmake-args -DPYTHON_VERSION=2
Base path: /home/testuser/dev/rosproj
Source space: /home/testuser/dev/rosproj/src
Build space: /home/testuser/dev/rosproj/build
Devel space: /home/testuser/dev/rosproj/devel
Install space: /home/testuser/dev/rosproj/install
####
#### Running command: "cmake /home/testuser/dev/rosproj/src -DPYTHON_VERSION=2 -DCATKIN_DEVEL_PREFIX=/home/testuser/dev/rosproj/devel -DCMAKE_INSTALL_PREFIX=/home/testuser/dev/rosproj/install -G Unix Makefiles" in "/home/testuser/dev/rosproj/build"
####
-- The C compiler identification is GNU 7.2.0
-- The CXX compiler identification is GNU 7.2.0
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Using CATKIN_DEVEL_PREFIX: /home/testuser/dev/rosproj/devel
-- Using CMAKE_PREFIX_PATH: /opt/ros/kinetic
-- This workspace overlays: /opt/ros/kinetic
-- Found PythonInterp: /usr/bin/python (found suitable version "3.6.3", minimum required is "2") 
-- Using PYTHON_EXECUTABLE: /usr/bin/python
-- Using default Python package layout
-- Could NOT find PY_em (missing: PY_EM) 
CMake Error at /opt/ros/kinetic/share/catkin/cmake/empy.cmake:29 (message):
  Unable to find either executable 'empy' or Python module 'em'...  try
  installing the package 'python-empy'
Call Stack (most recent call first):
  /opt/ros/kinetic/share/catkin/cmake/all.cmake:147 (include)
  /opt/ros/kinetic/share/catkin/cmake/catkinConfig.cmake:20 (include)
  CMakeLists.txt:52 (find_package)


-- Configuring incomplete, errors occurred!
See also "/home/testuser/dev/rosproj/build/CMakeFiles/CMakeOutput.log".
Invoking "cmake" failed

I've looked at the usage of PYTHON_VERSION option and it seems that setting just the PythonInterp_FIND_VERSION is not enough for find_package command to do its job properly. From the CMake sources it's clear that the FindPython module also relies on the PythonInterp_FIND_VERSION_COUNT, PythonInterp_FIND_VERSION_MAJOR, etc. options. Probably these variables are propagated only if the find_package is called with the version in the arguments, e.g. find_package(PythonInterp "${PYTHON_VERSION}" REQUIRED), which actually resolves the issue.