COMP: Update to find_package(Python3)

The FindPythonInterp and FindPythonLibs modules are removed.

These modules have been deprecated since CMake 3.12.

CMake 3.27 and above prefer to not provide the modules. This policy provides
compatibility for projects that have not been ported away from them.

Projects using the FindPythonInterp and/or FindPythonLibs modules should
be updated to use one of their replacements:
    FindPython3
    FindPython2
    FindPython

Author: hjmjohnson

CMake/ctkBlockCheckDependencies.cmake

@@ -90,7 +90,7 @@ set(${Log4Qt_enabling_variable}_INCLUDE_DIRS Log4Qt_INCLUDE_DIRS)
 set(${Log4Qt_enabling_variable}_FIND_PACKAGE_CMD Log4Qt)
 
 set(PythonQt_enabling_variable PYTHONQT_LIBRARIES)
-set(${PythonQt_enabling_variable}_INCLUDE_DIRS PYTHONQT_INCLUDE_DIR PYTHON_INCLUDE_DIRS)
+set(${PythonQt_enabling_variable}_INCLUDE_DIRS PYTHONQT_INCLUDE_DIR Python3_INCLUDE_DIRS)
 set(${PythonQt_enabling_variable}_FIND_PACKAGE_CMD PythonQt)
 
 set(QtSOAP_enabling_variable QtSOAP_LIBRARIES)

CMake/ctkMacroBuildLibWrapper.cmake

@@ -95,8 +95,8 @@ macro(ctkMacroBuildLibWrapper)
   # Since the PythonQt decorator depends on PythonQt, Python and VTK, let's link against
   # these ones to avoid complaints of MSVC
   # Note: "LINK_DIRECTORIES" has to be invoked before "ADD_LIBRARY"
-  ctkFunctionExtractOptimizedLibrary(PYTHON_LIBRARIES PYTHON_LIBRARY)
-  set(my_EXTRA_PYTHON_LIBRARIES ${PYTHON_LIBRARY} ${PYTHONQT_LIBRARIES})
+  ctkFunctionExtractOptimizedLibrary(Python3_LIBRARIES PYTHON_LIBRARY)
+  set(my_EXTRA_Python3_LIBRARIES ${PYTHON_LIBRARY} ${PYTHONQT_LIBRARIES})
 
   # Does a header having the expected filename exists ?
   string(REGEX REPLACE "^CTK" "ctk" lib_name_lc_ctk ${lib_name})
@@ -122,7 +122,7 @@ macro(ctkMacroBuildLibWrapper)
     list(APPEND KIT_PYTHONQT_SRCS ${DECORATOR_HEADER})
   endif()
   add_library(${lib_name}PythonQt ${MY_WRAPPER_LIBRARY_TYPE} ${KIT_PYTHONQT_SRCS})
-  target_link_libraries(${lib_name}PythonQt ${lib_name} ${my_EXTRA_PYTHON_LIBRARIES})
+  target_link_libraries(${lib_name}PythonQt ${lib_name} ${my_EXTRA_Python3_LIBRARIES})
   if(MY_WRAPPER_LIBRARY_TYPE STREQUAL "STATIC")
     if(CMAKE_SIZEOF_VOID_P EQUAL 8) # 64-bit
       set_target_properties(${lib_name}PythonQt PROPERTIES COMPILE_FLAGS "-fPIC")

CMake/ctkMacroCompilePythonScript.cmake

@@ -164,15 +164,15 @@ function(_ctk_add_compile_python_directories_target target keep_only_pyc)
       endif()
     endforeach()
 
-    if(NOT PYTHONINTERP_FOUND)
-      find_package(PythonInterp REQUIRED)
+    if(NOT Python3_Interpreter_FOUND)
+      find_package(Python3 COMPONENTS Interpreter REQUIRED)
     endif()
-    if(NOT PYTHONLIBS_FOUND)
-      find_package(PythonLibs REQUIRED)
+    if(NOT Python3_Development_FOUND)
+      find_package(Python3 COMPONENTS Development REQUIRED)
     endif()
 
     # Extract python lib path
-    ctkFunctionExtractOptimizedLibrary(PYTHON_LIBRARIES PYTHON_LIBRARY)
+    ctkFunctionExtractOptimizedLibrary(Python3_LIBRARIES PYTHON_LIBRARY)
     get_filename_component(PYTHON_LIBRARY_PATH "${PYTHON_LIBRARY}" PATH)
 
     # Configure cmake script associated with the custom command

CMake/ctkMacroWrapPythonQt.cmake

@@ -62,10 +62,10 @@ macro(ctkMacroWrapPythonQt WRAPPING_NAMESPACE TARGET SRCS_LIST_NAME SOURCES IS_W
   # TODO: this find package seems not to work when called form a superbuild, but the call is needed
   # in general to find the python interpreter.  In CTK, the toplevel CMakeLists.txt does the find
   # package so this is a no-op.  Other uses of this file may need to have this call so it is still enabled.
-  if(NOT PYTHONINTERP_FOUND)
-    find_package(PythonInterp)
-    if(NOT PYTHONINTERP_FOUND)
-      message(FATAL_ERROR "PYTHON_EXECUTABLE not specified or inexistent when calling ctkMacroWrapPythonQt")
+  if(NOT Python3_Interpreter_FOUND)
+    find_package(Python3 COMPONENTS Interpreter)
+    if(NOT Python3_Interpreter_FOUND)
+      message(FATAL_ERROR "Python3_EXECUTABLE not specified or inexistent when calling ctkMacroWrapPythonQt")
     endif()
   endif()
 
@@ -157,7 +157,7 @@ macro(ctkMacroWrapPythonQt WRAPPING_NAMESPACE TARGET SRCS_LIST_NAME SOURCES IS_W
     DEPENDS
       ${SOURCES_TO_WRAP}
       ${CTK_CMAKE_DIR}/ctkWrapPythonQt.py
-    COMMAND ${PYTHON_EXECUTABLE} ${CTK_CMAKE_DIR}/ctkWrapPythonQt.py
+    COMMAND ${Python3_EXECUTABLE} ${CTK_CMAKE_DIR}/ctkWrapPythonQt.py
       --target=${TARGET}
       --namespace=${WRAPPING_NAMESPACE}
       --output-dir=${CMAKE_CURRENT_BINARY_DIR}/${wrap_int_dir} ${extra_args}
@@ -188,6 +188,6 @@ macro(ctkMacroWrapPythonQt WRAPPING_NAMESPACE TARGET SRCS_LIST_NAME SOURCES IS_W
   if(NOT PYTHONQT_FOUND)
     message(FATAL_ERROR "error: PythonQt package is required to build ${TARGET}PythonQt")
   endif()
-  include_directories(${PYTHON_INCLUDE_DIRS} ${PYTHONQT_INCLUDE_DIR})
+  include_directories(${Python3_INCLUDE_DIRS} ${PYTHONQT_INCLUDE_DIR})
 
 endmacro()

CMake/ctk_compile_python_scripts.cmake.in

@@ -95,7 +95,7 @@ else()
 endif()
 
 execute_process(
-  COMMAND "@PYTHON_EXECUTABLE@" "@compile_all_script@"
+  COMMAND "@Python3_EXECUTABLE@" "@compile_all_script@"
   RESULT_VARIABLE result_var
   )
 if(NOT result_var STREQUAL 0)

CMakeExternals/PythonQt.cmake

@@ -74,22 +74,16 @@ if(NOT DEFINED PYTHONQT_INSTALL_DIR)
     endforeach()
   endif()
 
-  # Python is required
-  if(NOT PYTHONLIBS_FOUND)
-    find_package(PythonLibs)
-    if(NOT PYTHONLIBS_FOUND)
-      message(FATAL_ERROR "error: Python is required to build ${PROJECT_NAME}")
-    endif()
-  endif()
-
-  # Variable expected by FindPython3 CMake module
-  set(Python3_INCLUDE_DIR ${PYTHON_INCLUDE_DIR})
-  set(Python3_LIBRARY ${PYTHON_LIBRARY})
-  set(Python3_LIBRARY_DEBUG ${PYTHON_LIBRARY})
-  set(Python3_LIBRARY_RELEASE ${PYTHON_LIBRARY})
-  find_package(Python3 COMPONENTS Development REQUIRED)
+  # CMakeLists.txt already required Python3 with Interpreter and Development
+  # before reaching this block. A subsequent dependency (e.g. system VTK) may
+  # call find_package(Python3) with a narrower component set, which resets
+  # Python3_Development_FOUND. Force a fresh REQUIRED detection so the cache
+  # carries Development.Module + Development.Embed alongside Interpreter.
+  find_package(Python3 COMPONENTS Interpreter Development REQUIRED)
 
-  ctkFunctionExtractOptimizedLibrary(PYTHON_LIBRARIES PYTHON_LIBRARY)
+  # Bridge to legacy PYTHON_* names that downstream CTK CMake code still reads.
+  set(PYTHON_INCLUDE_DIR "${Python3_INCLUDE_DIRS}")
+  set(PYTHON_LIBRARY "${Python3_LIBRARY}")
 
   set(revision_tag 7ef4c5ee066b8ef1e7dc609b14071d504f59d4ba) # patched-v4.0.1-2026-03-16-5cd9b581f
   if(${proj}_REVISION_TAG)
@@ -299,7 +293,7 @@ set(PythonQt_DIR ${PYTHONQT_INSTALL_DIR})
 mark_as_superbuild(
   VARS
     PYTHONQT_INSTALL_DIR:PATH
-    PYTHON_EXECUTABLE:FILEPATH # FindPythonInterp expects PYTHON_EXECUTABLE variable to be defined
+    Python3_EXECUTABLE:FILEPATH # FindPythonInterp expects Python3_EXECUTABLE variable to be defined
     PYTHON_INCLUDE_DIR:PATH # FindPythonQt expects PYTHON_INCLUDE_DIR variable to be defined
     PYTHON_INCLUDE_DIR2:PATH
     PYTHON_LIBRARY:FILEPATH # FindPythonQt expects PYTHON_LIBRARY variable to be defined

CMakeExternals/VTK.cmake

@@ -62,17 +62,17 @@ if(NOT DEFINED VTK_DIR AND NOT ${CMAKE_PROJECT_NAME}_USE_SYSTEM_${proj})
 
   if(CTK_LIB_Scripting/Python/Core_PYTHONQT_USE_VTK)
 
-    set(Python3_EXECUTABLE ${PYTHON_EXECUTABLE})
+    set(Python3_EXECUTABLE ${Python3_EXECUTABLE})
     set(Python3_INCLUDE_DIR ${PYTHON_INCLUDE_DIR})
     set(Python3_LIBRARY ${PYTHON_LIBRARY})
     set(Python3_LIBRARY_DEBUG ${PYTHON_LIBRARY})
     set(Python3_LIBRARY_RELEASE ${PYTHON_LIBRARY})
     find_package(Python3 COMPONENTS Interpreter Development)
 
-    ctkFunctionExtractOptimizedLibrary(PYTHON_LIBRARIES PYTHON_LIBRARY)
+    ctkFunctionExtractOptimizedLibrary(Python3_LIBRARIES PYTHON_LIBRARY)
     list(APPEND additional_vtk_cmakevars
       # FindPythonInterp, FindPythonLibs
-      -DPYTHON_EXECUTABLE:PATH=${PYTHON_EXECUTABLE}
+      -DPython3_EXECUTABLE:PATH=${Python3_EXECUTABLE}
       -DPYTHON_INCLUDE_DIR:PATH=${PYTHON_INCLUDE_DIR}
       -DPYTHON_LIBRARY:FILEPATH=${PYTHON_LIBRARY}
       -DPYTHON_DEBUG_LIBRARIES:FILEPATH=${PYTHON_DEBUG_LIBRARIES}

CMakeLists.txt

@@ -965,16 +965,16 @@ endif()
 
 # Check if dependencies are satisfied
 if(CTK_LIB_Scripting/Python/Core)
-  if(NOT PYTHONINTERP_FOUND)
-    find_package(PythonInterp)
-    if(NOT PYTHONINTERP_FOUND)
-      message(FATAL_ERROR "PYTHON_EXECUTABLE variable should be set to build CTK_LIB_Scripting/Python")
+  if(NOT Python3_Interpreter_FOUND)
+    find_package(Python3 COMPONENTS Interpreter Development REQUIRED)
+    if(NOT Python3_Interpreter_FOUND)
+      message(FATAL_ERROR "Python3_EXECUTABLE variable should be set to build CTK_LIB_Scripting/Python")
     endif()
   endif()
-  if(NOT PYTHONLIBS_FOUND)
-    find_package(PythonLibs)
-    if(NOT PYTHONLIBS_FOUND)
-      message(FATAL_ERROR "PYTHON_LIBRARIES and PYTHON_INCLUDE_DIRS should be set to build CTK_LIB_Scripting/Python")
+  if(NOT Python3_Development_FOUND)
+    find_package(Python3 COMPONENTS Interpreter Development REQUIRED)
+    if(NOT Python3_Development_FOUND)
+      message(FATAL_ERROR "Python3_LIBRARIES and Python3_INCLUDE_DIRS should be set to build CTK_LIB_Scripting/Python")
     endif()
   endif()
 endif()

Libs/Scripting/Python/Core/CMakeLists.txt

@@ -36,7 +36,7 @@ configure_file(
 
 # Target libraries - See CMake/ctkFunctionGetTargetLibraries.cmake
 # The following macro will read the target libraries from the file 'target_libraries.cmake'
-ctkFunctionExtractOptimizedLibrary(PYTHON_LIBRARIES PYTHON_LIBRARY)
+ctkFunctionExtractOptimizedLibrary(Python3_LIBRARIES PYTHON_LIBRARY)
 ctkFunctionGetTargetLibraries(KIT_target_libraries)
 
 ctkMacroBuildLib(