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

Both the Interpreter and Development checks called find_package(Python3
COMPONENTS Interpreter Development REQUIRED) independently. Combine
into a single call since find_package(Python3) should request all
needed components at once to ensure version consistency.

The old bridging code set Python3_ hints from PYTHON_ variables that
are no longer populated after the FindPython3 migration. The VTK8
path was also passing Python3_EXECUTABLE under the wrong variable name.

Fix by using find_package(Python3) results directly:
- VTK8 path: pass Python3_EXECUTABLE, Python3_INCLUDE_DIRS, and the
  extracted PYTHON_LIBRARY under the old PYTHON_ variable names
- VTK9 path: pass the same values under Python3_ hint names

The superbuild export was missing Python3_INCLUDE_DIRS, Python3_LIBRARIES,
and PYTHON_EXECUTABLE. Add the modern Python3_ variables so downstream
superbuild projects using FindPython3 receive them, and re-add
PYTHON_EXECUTABLE for legacy project compatibility.

The default FindPython3 strategy (VERSION) searches all locations and
picks the newest Python3, which can find the wrong Python in
multi-version environments or when a virtualenv is active. Set
LOCATION strategy to stop at the first version found, respecting
PATH order and virtual environments.

Author: hjmjohnson

CMake/ctkBlockCheckDependencies.cmake

@@ -75,7 +75,7 @@ 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

@@ -95,7 +95,7 @@ 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)
+  ctkFunctionExtractOptimizedLibrary(Python3_LIBRARIES PYTHON_LIBRARY)
   set(my_EXTRA_PYTHON_LIBRARIES ${PYTHON_LIBRARY} ${PYTHONQT_LIBRARIES})
 
   # Does a header having the expected filename exists ?

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

@@ -75,21 +75,19 @@ if(NOT DEFINED PYTHONQT_INSTALL_DIR)
   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()
+  if(NOT Python3_Development_FOUND)
+    find_package(Python3 COMPONENTS Interpreter Development REQUIRED)
   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)
+  ctkFunctionExtractOptimizedLibrary(Python3_LIBRARIES PYTHON_LIBRARY)
 
-  ctkFunctionExtractOptimizedLibrary(PYTHON_LIBRARIES PYTHON_LIBRARY)
+  # Bridge Python3_ results to legacy PYTHON_ variables needed by
+  # FindPythonQt and downstream superbuild projects
+  set(PYTHON_EXECUTABLE ${Python3_EXECUTABLE})
+  set(PYTHON_INCLUDE_DIR ${Python3_INCLUDE_DIRS})
+  set(PYTHON_INCLUDE_DIRS ${Python3_INCLUDE_DIRS})
+  set(PYTHON_LIBRARY ${PYTHON_LIBRARY})  # already extracted above
+  set(PYTHON_LIBRARIES ${Python3_LIBRARIES})
 
   set(revision_tag ec7e1416e17fae645f3b99a8df4533719e17cbf6) # patched-v3.6.1-2025-12-22-469f01f6a
   if(${proj}_REVISION_TAG)
@@ -299,10 +297,15 @@ 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
-    PYTHON_INCLUDE_DIR:PATH # FindPythonQt expects PYTHON_INCLUDE_DIR variable to be defined
+    # FindPython3 variables (modern)
+    Python3_EXECUTABLE:FILEPATH
+    Python3_INCLUDE_DIRS:PATH
+    Python3_LIBRARIES:FILEPATH
+    # Legacy variables needed by FindPythonQt and older projects
+    PYTHON_EXECUTABLE:FILEPATH
+    PYTHON_INCLUDE_DIR:PATH
     PYTHON_INCLUDE_DIR2:PATH
-    PYTHON_LIBRARY:FILEPATH # FindPythonQt expects PYTHON_LIBRARY variable to be defined
+    PYTHON_LIBRARY:FILEPATH
   LABELS "FIND_PACKAGE_VARS"
   )
 mark_as_superbuild(

CMakeExternals/VTK.cmake

@@ -67,29 +67,23 @@ 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_INCLUDE_DIR ${PYTHON_INCLUDE_DIR})
-    set(Python3_LIBRARY ${PYTHON_LIBRARY})
-    set(Python3_LIBRARY_DEBUG ${PYTHON_LIBRARY})
-    set(Python3_LIBRARY_RELEASE ${PYTHON_LIBRARY})
-    find_package(Python3 COMPONENTS Interpreter Development)
-
-    ctkFunctionExtractOptimizedLibrary(PYTHON_LIBRARIES PYTHON_LIBRARY)
+    if(NOT Python3_Development_FOUND)
+      find_package(Python3 COMPONENTS Interpreter Development REQUIRED)
+    endif()
+
+    ctkFunctionExtractOptimizedLibrary(Python3_LIBRARIES PYTHON_LIBRARY)
+
+    # VTK8 and earlier use the old FindPythonInterp/FindPythonLibs variable names
     list(APPEND additional_vtk_cmakevars
-      # FindPythonInterp, FindPythonLibs
-      -DPYTHON_EXECUTABLE:PATH=${PYTHON_EXECUTABLE}
-      -DPYTHON_INCLUDE_DIR:PATH=${PYTHON_INCLUDE_DIR}
+      -DPYTHON_EXECUTABLE:PATH=${Python3_EXECUTABLE}
+      -DPYTHON_INCLUDE_DIR:PATH=${Python3_INCLUDE_DIRS}
       -DPYTHON_LIBRARY:FILEPATH=${PYTHON_LIBRARY}
-      -DPYTHON_DEBUG_LIBRARIES:FILEPATH=${PYTHON_DEBUG_LIBRARIES}
       )
-    # VTK9
+    # VTK9+ uses FindPython3 hint variables
     list(APPEND additional_vtk9_cmakevars
-      # FindPython3
-      -DPython3_INCLUDE_DIR:PATH=${Python3_INCLUDE_DIR}
-      -DPython3_LIBRARY:FILEPATH=${Python3_LIBRARY}
-      -DPython3_LIBRARY_DEBUG:FILEPATH=${Python3_LIBRARY}
-      -DPython3_LIBRARY_RELEASE:FILEPATH=${Python3_LIBRARY}
       -DPython3_EXECUTABLE:FILEPATH=${Python3_EXECUTABLE}
+      -DPython3_INCLUDE_DIR:PATH=${Python3_INCLUDE_DIRS}
+      -DPython3_LIBRARY:FILEPATH=${PYTHON_LIBRARY}
       )
   endif()
 

CMakeLists.txt

@@ -944,17 +944,11 @@ 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")
-    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")
-    endif()
+  if(NOT Python3_Interpreter_FOUND OR NOT Python3_Development_FOUND)
+    # Use LOCATION strategy to respect PATH order and virtual environments
+    # rather than VERSION which picks the newest Python found anywhere.
+    set(Python3_FIND_STRATEGY LOCATION)
+    find_package(Python3 COMPONENTS Interpreter Development REQUIRED)
   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(

Libs/Visualization/VTK/Core/CMakeLists.txt

@@ -98,7 +98,7 @@ endif()
 
 if(CTK_LIB_Scripting/Python/Core AND CTK_LIB_Scripting/Python/Core_PYTHONQT_USE_VTK)
   include_directories(
-    ${PYTHON_INCLUDE_DIRS}
+    ${Python3_INCLUDE_DIRS}
     ${PYTHONQT_INCLUDE_DIR}
     )
 endif()
@@ -192,7 +192,7 @@ endif()
 if(${VTK_VERSION} VERSION_LESS "8.90")
 
 if(CTK_LIB_Scripting/Python/Core AND CTK_LIB_Scripting/Python/Core_PYTHONQT_USE_VTK)
-  ctkFunctionExtractOptimizedLibrary(PYTHON_LIBRARIES PYTHON_LIBRARY)
+  ctkFunctionExtractOptimizedLibrary(Python3_LIBRARIES PYTHON_LIBRARY)
   list(APPEND VTK_LIBRARIES ${PYTHON_LIBRARY} ${PYTHONQT_LIBRARIES})
   if(${VTK_VERSION_MAJOR} GREATER 5)
     list(APPEND VTK_LIBRARIES vtkWrappingPythonCore)