Use default hash method to return hash value of a piece of a file

        Estimate processing time on:

        :param abspath: the absolute path to the file
        :param nbytes: only has first N bytes of the file. if 0, hash all file

        CPU = i7-4600U 2.10GHz - 2.70GHz, RAM = 8.00 GB
        1 second can process 0.25GB data

        - 0.59G - 2.43 sec
        - 1.3G - 5.68 sec
        - 1.9G - 7.72 sec
        - 2.5G - 10.32 sec
        - 3.9G - 16.0 sec

        ATTENTION:
            if you change the meta data (for example, the title, years 
            information in audio, video) of a multi-media file, then the hash 
            value gonna also change.

def of_file(self, abspath, nbytes=0):
        """Use default hash method to return hash value of a piece of a file

        Estimate processing time on:

        :param abspath: the absolute path to the file
        :param nbytes: only has first N bytes of the file. if 0, hash all file

        CPU = i7-4600U 2.10GHz - 2.70GHz, RAM = 8.00 GB
        1 second can process 0.25GB data

        - 0.59G - 2.43 sec
        - 1.3G - 5.68 sec
        - 1.9G - 7.72 sec
        - 2.5G - 10.32 sec
        - 3.9G - 16.0 sec

        ATTENTION:
            if you change the meta data (for example, the title, years 
            information in audio, video) of a multi-media file, then the hash 
            value gonna also change.
        """
        if not os.path.exists(abspath):
            raise FileNotFoundError(
                "[Errno 2] No such file or directory: '%s'" % abspath)

        m = self.default_hash_method()
        with open(abspath, "rb") as f:
            if nbytes:
                data = f.read(nbytes)
                if data:
                    m.update(data)
            else:
                while True:
                    data = f.read(self._chunk_size)
                    if not data:
                        break
                    m.update(data)
        if self.return_int:
            return int(m.hexdigest(), 16)
        else:
            return m.hexdigest()