函数挺有意思的，可能将来写项目会用到。

import tensorflow as tf
import os
import sys
from six.moves import urllib
import tarfile

FLAGS = tf.app.flags.FLAGS
FLAGS.data_dir = 'cifar10_data/'

DATA_URL = 'http://www.cs.toronto.edu/~kriz/cifar-10-binary.tar.gz'


# Download and extract the tarball from Alex's website.
def maybe_download_and_extract():
    dest_directory = FLAGS.data_dir
    if not os.path.exists(dest_directory):
        os.makedirs(dest_directory)

    filename = DATA_URL.split('/')[-1]  # 文件名
    filepath = os.path.join(dest_directory, filename)

    if not os.path.exists(filepath):
        # 文件下载函数
        def _progress(count, block_size, total_size):
            # %.1f%% -> 实数后面输出1个 %
            sys.stdout.write('\r>> Downloading %s %.1f%%' % (filename,
                                                             float(count * block_size) / float(total_size) * 100.0))
            sys.stdout.flush()  # 更新stdout

        filepath, _ = urllib.request.urlretrieve(DATA_URL, filepath, _progress)
        print()
        statinfo = os.stat(filepath)
        print('Successfully downloaded', filename, statinfo.st_size, 'bytes.')

    # 提取 bin 文件路径
    extracted_dir_path = os.path.join(dest_directory, 'cifar-10-batches-bin')

    if not os.path.exists(extracted_dir_path):
        tarfile.open(filepath, 'r:gz').extractall(dest_directory)  # Read from and write to tar format archives


if __name__ == '__main__':
    maybe_download_and_extract()