diff --git a/PyIlmBase/CMakeLists.txt b/PyIlmBase/CMakeLists.txt
index 0c6c7f974..71a130138 100644
--- a/PyIlmBase/CMakeLists.txt
+++ b/PyIlmBase/CMakeLists.txt
@@ -55,48 +55,39 @@ endif()
 # for our other dependency - boost.
 # Boost Python has some .. annoyances in that the python module
 # has version names attached to it
+function(PYILMBASE_EXTRACT_REL_SITEARCH varname pyver pyexe pysitearch)
+  get_filename_component(_exedir ${pyexe} DIRECTORY)
+  string(FIND ${pysitearch} ${_exedir} _findloc)
+  string(LENGTH ${_exedir} _elen)
+  while(_findloc EQUAL -1 AND _elen GREATER 0)
+    get_filename_component(_nexedir ${_exedir} DIRECTORY)
+    string(FIND ${pysitearch} ${_nexedir} _findloc)
+    if (_nexedir STREQUAL _exedir)
+        message(FATAL_ERROR "Unable to get parent directory for ${_exedir}")
+    else()
+        set(_exedir ${_nexedir})
+    endif()
+    string(LENGTH ${_exedir} _elen)
+  endwhile()
+  math(EXPR _elen "${_elen}+1")
+  string(SUBSTRING ${pysitearch} ${_elen} -1 _reldir)
+  set(${varname} ${_reldir} CACHE STRING "Destination sub-folder (relative) for the python ${pyver} modules")
+endfunction()
+
 if (TARGET Python2::Python)
   set(PYILMBASE_BOOST_PY2_COMPONENT "python${Python2_VERSION_MAJOR}${Python2_VERSION_MINOR}")
   message(STATUS "Found Python2 libraries: ${Python2_VERSION_MAJOR}${Python2_VERSION_MINOR}")
   # we can't just use the Python2_SITEARCH variable as that then will
   # ignore CMAKE_INSTALL_PREFIX. Could extract this to a function somewhere
   # if it is generally useful
-  get_filename_component(_exedir ${Python2_EXECUTABLE} DIRECTORY)
-  string(FIND ${Python2_SITEARCH} ${_exedir} _findloc)
-  string(LENGTH ${_exedir} _elen)
-  while(_findloc EQUAL -1 AND _elen GREATER 0)
-    get_filename_component(_exedir ${_exedir} DIRECTORY)
-    string(FIND ${Python2_SITEARCH} ${_exedir} _findloc)
-    string(LENGTH ${_exedir} _elen)
-  endwhile()
-  math(EXPR _elen "${_elen}+1")
-  string(SUBSTRING ${Python2_SITEARCH} ${_elen} -1 _reldir)
-  set(PyIlmBase_Python2_SITEARCH_REL ${_reldir} CACHE STRING "Destination sub-folder (relative) for the python 2 modules")
-  unset(_elen)
-  unset(_exedir)
-  unset(_findloc)
-  unset(_reldir)
+  pyilmbase_extract_rel_sitearch(PyIlmBase_Python2_SITEARCH_REL 2 ${Python2_EXECUTABLE} ${Python2_SITEARCH})
   message(STATUS "  -> Installing to: \${CMAKE_INSTALL_PREFIX}/${PyIlmBase_Python2_SITEARCH_REL}")
 endif()
 if (TARGET Python3::Python)
   set(PYILMBASE_BOOST_PY3_COMPONENT "python${Python3_VERSION_MAJOR}${Python3_VERSION_MINOR}")
   message(STATUS "Found Python3 libraries: ${Python3_VERSION_MAJOR}${Python3_VERSION_MINOR}")
   # and figure out the install root here
-  get_filename_component(_exedir ${Python3_EXECUTABLE} DIRECTORY)
-  string(FIND ${Python3_SITEARCH} ${_exedir} _findloc)
-  string(LENGTH ${_exedir} _elen)
-  while(_findloc EQUAL -1 AND _elen GREATER 0)
-    get_filename_component(_exedir ${_exedir} DIRECTORY)
-    string(FIND ${Python3_SITEARCH} ${_exedir} _findloc)
-    string(LENGTH ${_exedir} _elen)
-  endwhile()
-  math(EXPR _elen "${_elen}+1")
-  string(SUBSTRING ${Python3_SITEARCH} ${_elen} -1 _reldir)
-  set(PyIlmBase_Python3_SITEARCH_REL ${_reldir} CACHE STRING "Destination sub-folder (relative) for the python 3 modules")
-  unset(_elen)
-  unset(_exedir)
-  unset(_findloc)
-  unset(_reldir)
+  pyilmbase_extract_rel_sitearch(PyIlmBase_Python3_SITEARCH_REL 3 ${Python3_EXECUTABLE} ${Python3_SITEARCH})
   message(STATUS "  -> Installing to: \${CMAKE_INSTALL_PREFIX}/${PyIlmBase_Python3_SITEARCH_REL}")
 endif()
 # different flavors of O.S. have multiple versions of python