When finding Python3, use python3 executable as a hint (#650)

The existing find_package(Python3) call will locate the Python
interpreter with the latest version number. However, this may not be the
system's default Python interpreter for which our dependencies have been
installed.

If Python3_EXECUTABLE is not explicitly specified, use find_program to
locate the Python interpreter behind the "python3" command, which is
likely the system's default and the one that we want.

If no executable can be found by the name "python3", the find_program
call will silently fail and the existing behavior will manifest.

Signed-off-by: Scott K Logan <logans@cottsay.net>

tf2_geometry_msgs/CMakeLists.txt

@@ -15,6 +15,17 @@ if(WIN32 AND CMAKE_BUILD_TYPE STREQUAL "Debug")
   find_package(PythonExtra REQUIRED)
   # Force FindPython3 to use the debug interpretter where ROS 2 expects it
   set(Python3_EXECUTABLE "${PYTHON_EXECUTABLE_DEBUG}")
+elseif(NOT WIN32 AND NOT Python3_EXECUTABLE)
+  # We expect that Python dependencies are met for whatever Python interpreter
+  # is invoked by the "python3" executable, however this may not be the latest
+  # Python version installed on the system. The default behavior of
+  # find_package(Python3) is to use the latest version (e.x. python3.12), so we
+  # specifically look for a "python3" executable and if found, instruct
+  # find_package(Python3) to use that.
+  # On Windows, the find_package(Python3) logic is different and doesn't
+  # appear to prefer specific versions (e.x. python3.12) over plain
+  # "python.exe" so this extra logic is unnecessary there.
+  find_program(Python3_EXECUTABLE python3)
 endif()
 find_package(Python3 REQUIRED COMPONENTS Interpreter Development)
 

tf2_py/CMakeLists.txt

@@ -26,6 +26,17 @@ if(WIN32 AND CMAKE_BUILD_TYPE STREQUAL "Debug")
   find_package(PythonExtra REQUIRED)
   # Force FindPython3 to use the debug interpretter where ROS 2 expects it
   set(Python3_EXECUTABLE "${PYTHON_EXECUTABLE_DEBUG}")
+elseif(NOT WIN32 AND NOT Python3_EXECUTABLE)
+  # We expect that Python dependencies are met for whatever Python interpreter
+  # is invoked by the "python3" executable, however this may not be the latest
+  # Python version installed on the system. The default behavior of
+  # find_package(Python3) is to use the latest version (e.x. python3.12), so we
+  # specifically look for a "python3" executable and if found, instruct
+  # find_package(Python3) to use that.
+  # On Windows, the find_package(Python3) logic is different and doesn't
+  # appear to prefer specific versions (e.x. python3.12) over plain
+  # "python.exe" so this extra logic is unnecessary there.
+  find_program(Python3_EXECUTABLE python3)
 endif()
 find_package(Python3 REQUIRED COMPONENTS Interpreter Development)