Merge pull request #12793 from alalek:cmake_fix_python_standalone

cmake/OpenCVDetectPython.cmake

@@ -59,15 +59,21 @@ if(NOT ${found})
     if(NOT PYTHONINTERP_FOUND)
       message(STATUS "Python is not found: ${preferred_version} EXACT")
     endif()
-  else()
+  elseif(min_version)
     set(__python_package_version "${min_version}")
     find_host_package(PythonInterp "${min_version}")
+  else()
+    set(__python_package_version "")
+    find_host_package(PythonInterp)
   endif()
 
   string(REGEX MATCH "^[0-9]+" _python_version_major "${min_version}")
 
   if(PYTHONINTERP_FOUND)
     # Check if python major version is correct
+    if(" ${_python_version_major}" STREQUAL " ")
+      set(_python_version_major "${PYTHON_VERSION_MAJOR}")
+    endif()
     if(NOT "${_python_version_major}" STREQUAL "${PYTHON_VERSION_MAJOR}"
         AND NOT DEFINED ${executable}
     )

modules/python/standalone.cmake

@@ -10,7 +10,7 @@ include("${OpenCV_SOURCE_DIR}/cmake/OpenCVUtils.cmake")
 
 set(OPENCV_PYTHON_SKIP_DETECTION ON)
 include("${OpenCV_SOURCE_DIR}/cmake/OpenCVDetectPython.cmake")
-find_python("" "2.7" PYTHON_LIBRARY PYTHON_INCLUDE_DIR
+find_python("${OPENCV_PYTHON_VERSION}" "${OPENCV_PYTHON_VERSION}" PYTHON_LIBRARY PYTHON_INCLUDE_DIR
     PYTHONINTERP_FOUND PYTHON_EXECUTABLE PYTHON_VERSION_STRING
     PYTHON_VERSION_MAJOR PYTHON_VERSION_MINOR PYTHONLIBS_FOUND
     PYTHONLIBS_VERSION_STRING PYTHON_LIBRARIES PYTHON_LIBRARY