'Return the Folder() object for a foldername.
Caller must call closefiles() on the folder when done.'

def getfolder(self, foldername):

    if (foldername in self._folders):
        return self._folders[foldername]
    folder = self._instanciatefolder(foldername)
    if folder.isnewfolder():
        self.import_other_backend(folder)
    self._folders[foldername] = folder
    return folder
