Accelerate IO operations with Excel

[alixdamman]

Code below should be tested in some way:

    def _create_instance(app_id):
        def __create_instance(app_id):
            import pythoncom
            from win32com.client import gencache
            # EnsureDispatch use early binding but will reuse an existing instance,
            # while DispatchEx creates an new application but uses late binding.
            # We want both a new instance and use early binding.
            app_id = pythoncom.CoCreateInstanceEx(app_id, None, pythoncom.CLSCTX_SERVER, None,
                                                  (pythoncom.IID_IDispatch,))[0]
            if gencache.is_readonly:
                # needed for "frozen" apps
                gencache.is_readonly = False
                gencache.Rebuild()
            return gencache.EnsureDispatch(app_id)

        # open an excel instance via win32com "EnsureDispatch" so that we flag Excel as using early binding.
        # it is much faster than late binding BUT sometimes (I don't know the
        # precise circumstances yet -- when Excel is updated?) it gets corrupt and nothing works until you re-generate them.
        # Note that this only needs to be done _once_ per machine.

        # This can also be done explicitly on the command line in two different ways:
        # * to generate one big file with bindings for all Excel objects:
        #   python -m win32com.client.makepy Excel.Application
        # * to flag "Excel" as using early binding. Bindings will be generated as many small files as needed:
        #   python -c "from win32com.client import gencache;gencache.EnsureDispatch('Excel.Application')"
        # In case something goes wrong, one can go back to late binding by removing generated bindings from:
        #   'C:\\Users\\<user>\\AppData\\Local\\Temp\\gen_py\\3.6'
        try:
            return __create_instance(app_id)
        except AttributeError:
            from win32com import __gen_path__
            from shutil import rmtree
            rmtree(__gen_path__)
            return __create_instance(app_id)