Change to find_package(Python3), set Python3_EXECUTABLE to PYTHON_EXE…

…CUTABLE (TriBITSPub#610)

Change from calling deprecated find_package(PythonInterp) to call
find_package(Python3).  To maintain backward compatibility for external users,
set Python3_EXECUTABLE to PYTHON_EXECUTABLE if the former is unset and the
later is set.

The rest of TriBITS will need to be refactored to absorb this.

tribits/core/package_arch/TribitsFindPythonInterp.cmake

@@ -9,29 +9,20 @@
 
 # Find Python executable which is needed for dependency file building
 macro(tribits_find_python)
-  set(PythonInterp_FIND_VERSION_MIN "2.6")
-  if ("${PythonInterp_FIND_VERSION_DEFAULT}" STREQUAL "")
-    set(PythonInterp_FIND_VERSION_DEFAULT "${PythonInterp_FIND_VERSION_MIN}")
+  if ((NOT "${PYTHON_EXECUTABLE}" STREQUAL "") AND ("${Python3_EXECUTABLE}" STREQUAL ""))
+    tribits_deprecated("Python3_EXECUTABLE being set by default to PYTHON_EXECUTABLE = '${PYTHON_EXECUTABLE}' is deprecated!")
+    set(Python3_EXECUTABLE "${PYTHON_EXECUTABLE}" CACHE FILEPATH
+      "Set by default to PYTHON_EXECUTABLE!")
   endif()
-  advanced_set(PythonInterp_FIND_VERSION  ${PythonInterp_FIND_VERSION_DEFAULT}
-    CACHE  STRING
-    "Default version of Python to find (must be ${PythonInterp_FIND_VERSION_DEFAULT} or greater")
-  if (PythonInterp_FIND_VERSION  VERSION_LESS  "${PythonInterp_FIND_VERSION_MIN}")
-    message_wrapper(FATAL_ERROR  "Error,"
-      " PythonInterp_FIND_VERSION=${PythonInterp_FIND_VERSION} < ${PythonInterp_FIND_VERSION_MIN}"
-      " is not allowed!" )
-  endif()
-  advanced_set(PythonInterp_MUST_BE_FOUND FALSE CACHE BOOL "Require Python to be found or not.")
   if (${PROJECT_NAME}_REQUIRES_PYTHON)
-    set(PythonInterp_REQUIRED_ARG "REQUIRED")
+    set(Python3_REQUIRED_ARG "REQUIRED")
   else()
-    set(PythonInterp_REQUIRED_ARG "")
+    set(Python3_REQUIRED_ARG "")
   endif()
-  set(FIND_PythonInterp_ARGS PythonInterp ${PythonInterp_REQUIRED_ARG}) 
   if (TRIBITS_FIND_PYTHON_UNITTEST)
-    set(PYTHON_EXECUTABLE  ${PYTHON_EXECUTABLE_UNITTEST_VAL})
+    set(Python3_EXECUTABLE  ${PYTHON_EXECUTABLE_UNITTEST_VAL})
   else()
-    find_package(${FIND_PythonInterp_ARGS})
+    find_package(Python3 ${Python3_REQUIRED_ARG})
   endif()
 endmacro()
 
@@ -48,9 +39,9 @@ macro(tribits_find_python_interp)
   endif()
   if (${PROJECT_NAME}_USES_PYTHON)
     tribits_find_python()
-    print_var(PYTHON_EXECUTABLE)
-    if (${PROJECT_NAME}_REQUIRES_PYTHON  AND  PYTHON_EXECUTABLE  STREQUAL "")
-      message_wrapper(FATAL_ERROR "Error, PYTHON_EXECUTABLE='' but"
+    print_var(Python3_EXECUTABLE)
+    if (${PROJECT_NAME}_REQUIRES_PYTHON  AND  Python3_EXECUTABLE  STREQUAL "")
+      message_wrapper(FATAL_ERROR "Error, Python3_EXECUTABLE='' but"
         " ${PROJECT_NAME}_REQUIRES_PYTHON=${${PROJECT_NAME}_REQUIRES_PYTHON}!" )
     endif()
   else()