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@Obdolbino: You don't need this, you just need to add alias catkin_make="catkin_make -DPYTHON_EXECUTABLE=/usr/bin/python3" to your .${SHELL}rc file. ROS has a built-in workaround as this issue affects mac users, too.

Hello! Finally figured out about this compilation error: execute_process(/home/*/.cache/pacaur/ros-melodic-catkin/src/build/catkin_generated/env_cached.sh "/usr/bin/python3" "/home//.local/lib/python3.7/site-packages/em" "--raw-errors"

Got same for python 2.7 and 3.8. Cmake is trying to use "em" util to color text output or something like this.
And folder site-packages/em contains some em python module. So first I thought something is wrong with this package. Tried to reinstall it from different sources etc. Finally I renamed site-packages/em directory to other name and made a link to a script /usr/lib/python3.8/site-packages/em -> /usr/bin/em

Finally installed ros-melodic-desktop-full with python3.8.

@Some packages still have signals as required in the CMakelists.txt. Since you said you used catkin_make manually, this should be easy patchable for you, signals is usually unecessary. If this occurs during the build of one of the packages, please open a issue on github.

I tried compiling a package with catkin_make and it gave me an error about boost saying Could not find 'signals'. Found boost = 1.69. I looked it up and found that signals was removed after 1.68 and the use of signals2 was preferred. But when I tried installing the 1.68 version, catkin_make threw errors for undefined references to libraries that were added in 1.69. Anyone has any clue what might be the problem?

We moved our Development to Github: https://github.com/ros-melodic-arch
Please open issues there instead of commenting.

Please apply https://github.com/ros/catkin/pull/975 to fix the issue mentioned by theoT, as was done for https://aur.archlinux.org/packages/ros-kinetic-catkin/ .

There is an issue with catkin and the current CMake version: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=915148 and https://github.com/ros/catkin/issues/856 Building e.g. ros-melodic-class-loader results in an error complaining about a missing library "lpthread".

I am also having this trouble; catkin is compiled (per the PKGBUILD) with -DPYTHON_EXECUTABLE=/usr/bin/python3, but catkin_make always finds /usr/bin/python2 as the PYTHON_EXECUTABLE. Has anyone fixed this issue?

My installation of catkin_make tries to find python2:

$ catkin_make
Base path: /tmp/ws
Source space: /tmp/ws/src
Build space: /tmp/ws/build
Devel space: /tmp/ws/devel
Install space: /tmp/ws/install
Creating symlink "/tmp/ws/src/CMakeLists.txt" pointing to "/opt/ros/melodic/share/catkin/cmake/toplevel.cm
ake"
####
#### Running command: "cmake /tmp/ws/src -DCATKIN_DEVEL_PREFIX=/tmp/ws/devel -DCMAKE_INSTALL_PREFIX=/tmp/w
s/install -G Unix Makefiles" in "/tmp/ws/build"
####
-- The C compiler identification is GNU 8.2.1
-- The CXX compiler identification is GNU 8.2.1
-- 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: /tmp/ws/devel
-- Using CMAKE_PREFIX_PATH: /opt/ros/melodic
-- This workspace overlays: /opt/ros/melodic
-- Found PythonInterp: /usr/bin/python2 (found suitable version "2.7.15", minimum required is "2")
-- Using PYTHON_EXECUTABLE: /usr/bin/python2
-- Using default Python package layout
-- Could NOT find PY_em (missing: PY_EM)
CMake Error at /opt/ros/melodic/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/melodic/share/catkin/cmake/all.cmake:159 (include)
  /opt/ros/melodic/share/catkin/cmake/catkinConfig.cmake:20 (include)
  CMakeLists.txt:52 (find_package)


-- Configuring incomplete, errors occurred!
See also "/tmp/ws/build/CMakeFiles/CMakeOutput.log".
Invoking "cmake" failed

If I force to use python3, then:

$ catkin_make --cmake-args -DPYTHON_VERSION=3
Base path: /tmp/ws
Source space: /tmp/ws/src
Build space: /tmp/ws/build
Devel space: /tmp/ws/devel
Install space: /tmp/ws/install
####
#### Running command: "cmake /tmp/ws/src -DPYTHON_VERSION=3 -DCATKIN_DEVEL_PREFIX=/tmp/ws/devel -DCMAKE_INSTALL_PREFIX=/tmp/ws/install -G Unix Makefiles" in "/tmp/ws/build"
####
-- Using CATKIN_DEVEL_PREFIX: /tmp/ws/devel
-- Using CMAKE_PREFIX_PATH: /opt/ros/melodic
-- This workspace overlays: /opt/ros/melodic
CMake Error at /usr/share/cmake-3.12/Modules/FindPackageHandleStandardArgs.cmake:137 (message):
  Could NOT find PythonInterp: Found unsuitable version "2.7.15", but
  required is at least "3" (found /usr/bin/python2)
Call Stack (most recent call first):
  /usr/share/cmake-3.12/Modules/FindPackageHandleStandardArgs.cmake:376 (_FPHSA_FAILURE_MESSAGE)
  /usr/share/cmake-3.12/Modules/FindPythonInterp.cmake:159 (FIND_PACKAGE_HANDLE_STANDARD_ARGS)
  /opt/ros/melodic/share/catkin/cmake/python.cmake:4 (find_package)
  /opt/ros/melodic/share/catkin/cmake/all.cmake:159 (include)
  /opt/ros/melodic/share/catkin/cmake/catkinConfig.cmake:20 (include)
  CMakeLists.txt:52 (find_package)


-- Configuring incomplete, errors occurred!
See also "/tmp/ws/build/CMakeFiles/CMakeOutput.log".
Invoking "cmake" failed