python: add option INSTALL_TO_SITEPACKAGES

If INSTALL_TO_SITEPACKAGES is enabled, the freecad-namespace package (freecad/__init__.py) is installed in the python-sitepackage-dir. Further, the library install path (CMAKE_INSTALL_LIBDIR)
is used to find the FreeCAD shared library. If, for some reason, one wants to import another installed FreeCAD version it's possible to set the environment variable "PATH_TO_FREECAD_LIBDIR" to
point to the wanted FreeCAD-library (FreeCAD.so / FreeCAD.dll)

cMake/FreeCAD_Helpers/InitializeFreeCADBuildOptions.cmake

@@ -10,8 +10,9 @@ macro(InitializeFreeCADBuildOptions)
     option(FREECAD_USE_EXTERNAL_KDL "Use system installed orocos-kdl instead of the bundled." OFF)
     option(FREECAD_USE_FREETYPE "Builds the features using FreeType libs" ON)
     option(FREECAD_BUILD_DEBIAN "Prepare for a build of a Debian package" OFF)
-    option(BUILD_WITH_CONDA "Set ON if you build freecad with conda" OFF)
+    option(BUILD_WITH_CONDA "Set ON if you build FreeCAD with conda" OFF)
     option(BUILD_DYNAMIC_LINK_PYTHON "If OFF extension-modules do not link against python-libraries" ON)
+    option(INSTALL_TO_SITEPACKAGES "If ON the freecad root namespace (python) is installed into python's site-packages" OFF)
     option(OCCT_CMAKE_FALLBACK "disable usage of occt-config files" OFF)
     if (WIN32 OR APPLE)
         option(FREECAD_USE_QT_FILEDIALOG "Use Qt's file dialog instead of the native one." OFF)

src/Ext/freecad/CMakeLists.txt

@@ -3,18 +3,18 @@ EXECUTE_PROCESS(COMMAND ${PYTHON_EXECUTABLE} -c
 OUTPUT_VARIABLE python_libs OUTPUT_STRIP_TRAILING_WHITESPACE )
 SET(PYTHON_MAIN_DIR ${python_libs})
 
-SET(EXT_SRCS
-    __init__.py
-)
+set(NAMESPACE_INIT "${CMAKE_BINARY_DIR}/Ext/freecad/__init__.py")
+configure_file(__init__.py.template ${NAMESPACE_INIT})
 
-ADD_CUSTOM_TARGET(freecad_COPY_SOURCE ALL
-    SOURCES ${EXT_SRCS})
-
-fc_copy_sources(freecad_COPY_SOURCE "${CMAKE_BINARY_DIR}/Ext/freecad" ${EXT_SRCS})
+if (INSTALL_TO_SITEPACKAGES)
+    SET(SITE_PACKAGE_DIR ${PYTHON_MAIN_DIR}/freecad)
+else ()
+    SET(SITE_PACKAGE_DIR Ext/freecad)
+endif()
 
 INSTALL(
     FILES
-        __init__.py
+        ${NAMESPACE_INIT}
     DESTINATION
-        Ext/freecad
-)
+        ${SITE_PACKAGE_DIR}
+)

src/Ext/freecad/__init__.py

@@ -1,4 +0,0 @@
-import pkgutil
-__path__ = pkgutil.extend_path(__path__, __name__)
-
-import FreeCAD as app

src/Ext/freecad/__init__.py.template

@@ -0,0 +1,30 @@
+try:
+    import FreeCAD as app
+except ModuleNotFoundError:
+    # sys.path modification has not happened yet
+
+    import sys as _sys
+    import os as _os
+
+    # 1. we check if an env-variable "PATH_TO_FREECAD_LIB" is already set
+    try:
+        _path_to_freecad_libdir = _os.environ["PATH_TO_FREECAD_LIBDIR"]
+    except KeyError:
+
+        # 2. we use the default freecad defined for this package
+        _path_to_freecad_libdir = "${CMAKE_INSTALL_LIBDIR}"
+        print("PATH_TO_FREECAD_LIBDIR not specified, using default \
+FreeCAD version in {}".format( "${CMAKE_INSTALL_LIBDIR}"))
+
+    _sys.path.append(_path_to_freecad_libdir) # this is the default version
+    import FreeCAD as app
+
+# as this is a namespace-package we need to extend the path
+
+#!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
+# TO NOT OVERWRITE THIS FILE, NO OTHER MODULE IS ALLOWED TO      !
+# PROVIDE A freecad/__init__.py FILE                             !
+#!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
+
+from pkgutil import extend_path as _extend_path
+__path__ = _extend_path(__path__, __name__)