def append(self, data: str) -> None:
        """ append data to file. for the moment, only non-encrypted file"""
        assert not self.is_encrypted()
        with open(self.path, "rb+") as f:
            pos = f.seek(0, os.SEEK_END)
            if pos != self.pos:
                raise StorageOnDiskUnexpectedlyChanged(f"expected size {self.pos}, found {pos}")
            f.write(data.encode("utf-8"))
            self.pos = f.seek(0, os.SEEK_END)
            f.flush()
            os.fsync(f.fileno())
 append data to file. for the moment, only non-encrypted file
electrum/storage.py