python: update CMake install rules

modules/python/common.cmake

@@ -135,10 +135,14 @@ if(NOT OPENCV_SKIP_PYTHON_LOADER AND DEFINED OPENCV_PYTHON_INSTALL_PATH)
   set(OPENCV_PYTHON_INSTALL_PATH_SETUPVARS "${OPENCV_PYTHON_INSTALL_PATH}" CACHE INTERNAL "")
 endif()
 
-if(NOT " ${PYTHON}" STREQUAL " PYTHON" AND DEFINED OPENCV_${PYTHON}_INSTALL_PATH)
-  set(__python_binary_install_path "${OPENCV_${PYTHON}_INSTALL_PATH}")
-elseif(OPENCV_SKIP_PYTHON_LOADER AND DEFINED ${PYTHON}_PACKAGES_PATH)
-  set(__python_binary_install_path "${${PYTHON}_PACKAGES_PATH}")
+if(OPENCV_SKIP_PYTHON_LOADER)
+  if(DEFINED OPENCV_${PYTHON}_INSTALL_PATH)
+    set(__python_binary_install_path "${OPENCV_${PYTHON}_INSTALL_PATH}")
+  elseif(DEFINED ${PYTHON}_PACKAGES_PATH)
+    set(__python_binary_install_path "${${PYTHON}_PACKAGES_PATH}")
+  else()
+    message(FATAL_ERROR "Specify 'OPENCV_${PYTHON}_INSTALL_PATH' variable")
+  endif()
 else()
   ocv_assert(DEFINED OPENCV_PYTHON_INSTALL_PATH)
   set(__python_binary_install_path "${OPENCV_PYTHON_INSTALL_PATH}/${__python_loader_subdir}python-${${PYTHON}_VERSION_MAJOR}.${${PYTHON}_VERSION_MINOR}")

modules/python/standalone.cmake

@@ -5,11 +5,10 @@ include("${OpenCV_BINARY_DIR}/opencv_python_config.cmake")
 if(NOT DEFINED OpenCV_SOURCE_DIR)
   message(FATAL_ERROR "Missing OpenCV_SOURCE_DIR")
 endif()
-if(NOT OPENCV_PYTHON_INSTALL_PATH)
-  if(NOT DEFINED OPENCV_PYTHON_STANDALONE_INSTALL_PATH)
-    message(FATAL_ERROR "Missing OPENCV_PYTHON_STANDALONE_INSTALL_PATH / OPENCV_PYTHON_INSTALL_PATH")
-  endif()
+if(DEFINED OPENCV_PYTHON_STANDALONE_INSTALL_PATH)
   set(OPENCV_PYTHON_INSTALL_PATH "${OPENCV_PYTHON_STANDALONE_INSTALL_PATH}")
+elseif(NOT OPENCV_PYTHON_INSTALL_PATH)
+  message(FATAL_ERROR "Missing OPENCV_PYTHON_STANDALONE_INSTALL_PATH / OPENCV_PYTHON_INSTALL_PATH")
 endif()
 
 include("${OpenCV_SOURCE_DIR}/cmake/OpenCVUtils.cmake")