Fix loading of Python bindings on Windows when installed in arbitrary…

… directory

bindings/CMakeLists.txt

@@ -139,6 +139,26 @@ if (IDYNTREE_USES_PYTHON_PYBIND11 OR IDYNTREE_USES_PYTHON)
     # Install main __init__.py
     # Clear the file first if it exists.
     file(WRITE ${CMAKE_CURRENT_BINARY_DIR}/__init__.py "")
+
+    # If we are on Windows and BUILD_SHARED_LIBS is ON, handle the fact that
+    # the Python interpreter does not look into PATH to find dll (see https://docs.python.org/3.8/library/os.html#os.add_dll_directory)
+    if(WIN32 AND BUILD_SHARED_LIBS)
+        if(IS_ABSOLUTE PYTHON_INSTDIR)
+            set(PYTHON_FULL_INSTDIR "${PYTHON_INSTDIR}")
+        else()
+            set(PYTHON_FULL_INSTDIR "${CMAKE_INSTALL_PREFIX}/${PYTHON_INSTDIR}")
+        endif()
+        file(RELATIVE_PATH RELATIVE_PATH_BETWEEN_INIT_PY_AND_DLL_DIRECTORY ${PYTHON_FULL_INSTDIR} ${CMAKE_INSTALL_FULL_BINDIR})
+        file(APPEND ${CMAKE_CURRENT_BINARY_DIR}/__init__.py "import os${NEW_LINE}")
+        file(APPEND ${CMAKE_CURRENT_BINARY_DIR}/__init__.py "library_dll_path = os.path.join(os.path.dirname(__file__),'${RELATIVE_PATH_BETWEEN_INIT_PY_AND_DLL_DIRECTORY}')${NEW_LINE}")
+        file(APPEND ${CMAKE_CURRENT_BINARY_DIR}/__init__.py "# Avoid to call add_dll_directory if not necessary,${NEW_LINE}")
+        file(APPEND ${CMAKE_CURRENT_BINARY_DIR}/__init__.py "# for example if the library to find are already found in the proper location in a conda${NEW_LINE}")
+        file(APPEND ${CMAKE_CURRENT_BINARY_DIR}/__init__.py "if(library_dll_path != os.path.join(os.environ['CONDA_PREFIX'],'Library','bin') and library_dll_path != os.path.join(os.environ['CONDA_PREFIX'],'bin')):${NEW_LINE}")
+        file(APPEND ${CMAKE_CURRENT_BINARY_DIR}/__init__.py "    if(os.path.exists(library_dll_path)):${NEW_LINE}")
+        file(APPEND ${CMAKE_CURRENT_BINARY_DIR}/__init__.py "        os.add_dll_directory(library_dll_path)${NEW_LINE}")
+    endif()
+
+
     # If pybind is enabled, add the corresponding import.
     if (${IDYNTREE_USES_PYTHON_PYBIND11})
         file(APPEND ${CMAKE_CURRENT_BINARY_DIR}/__init__.py "from . import pybind${NEW_LINE}")