MNIST文件读取，谁来讲讲呗

 def _parse_dataset(self, buffer_size=100):
        self.images = []
        self.labels = []
        with gzip.GzipFile(self.image_path, 'rb') as image_file:
            img_buf = image_file.read()
            with gzip.GzipFile(self.label_path, 'rb') as label_file:
                lab_buf = label_file.read()

                step_label = 0
                offset_img = 0
                # read from Big-endian
                # get file info from magic byte
                # image file : 16B
                magic_byte_img = '>IIII'
                magic_img, image_num, rows, cols = struct.unpack_from(
                    magic_byte_img, img_buf, offset_img)
                offset_img += struct.calcsize(magic_byte_img)

                offset_lab = 0
                # label file : 8B
                magic_byte_lab = '>II'
                magic_lab, label_num = struct.unpack_from(magic_byte_lab,
                                                          lab_buf, offset_lab)
                offset_lab += struct.calcsize(magic_byte_lab)

                while True:
                    if step_label >= label_num:
                        break
                    fmt_label = '>' + str(buffer_size) + 'B'
                    labels = struct.unpack_from(fmt_label, lab_buf, offset_lab)
                    offset_lab += struct.calcsize(fmt_label)
                    step_label += buffer_size

                    fmt_images = '>' + str(buffer_size * rows * cols) + 'B'
                    images_temp = struct.unpack_from(fmt_images, img_buf,
                                                     offset_img)
                    images = np.reshape(images_temp, (buffer_size, rows *
                                                      cols)).astype('float32')
                    offset_img += struct.calcsize(fmt_images)

                    for i in range(buffer_size):
                        self.images.append(images[i, :])
                        self.labels.append(
                            np.array([labels[i]]).astype('int64'))