'Get the file associated with this session key.'

def _key_to_file(self, session_key=None):

    if (session_key is None):
        session_key = self.session_key
    if (not set(session_key).issubset(self.VALID_KEY_CHARS)):
        raise SuspiciousOperation('Invalid    characters    in    session    key')
    return os.path.join(self.storage_path, (self.file_prefix + session_key))
