@property
    def size(self):
        """
        Returns the size (in bytes) of the HDU's data part.
        """

        if self._buffer is not None:
            return self._data_offset - len(self._buffer)

        return self._file.size - self._data_offset
dataset/ETHPy150Open spacetelescope/PyFITS/pyfits/hdu/base.py/_NonstandardHDU.size/swapped operands of `-`