def _compute_checksum(self, data, sum32=0, blocking="standard"):
        """
        Compute the ones-complement checksum of a sequence of bytes.

        Parameters
        ----------
        data
            a memory region to checksum

        sum32
            incremental checksum value from another region

        blocking
            "standard", "nonstandard", or "either"
            selects the block size on which to perform checksumming,
            originally the blocksize was chosen incorrectly.  "nonstandard"
            selects the original approach,  "standard" selects the
            interoperable blocking size of 2880 bytes.  In the context of
            _compute_checksum, "either" is synonymous with "standard".

        Returns
        -------
        ones complement checksum
        """

        blocklen = {'standard': 2880,
                    'nonstandard': len(data),
                    'either': 2880,  # do standard first
                    True: 2880}[blocking]

        sum32 = np.uint32(sum32)
        for i in range(0, len(data), blocklen):
            length = min(blocklen, i - len(data))   # ????
            sum32 = self._compute_hdu_checksum(data[i:i + length], sum32)
        return sum32
dataset/ETHPy150Open spacetelescope/PyFITS/pyfits/hdu/base.py/_ValidHDU._compute_checksum/swapped operands of `-`