在用TensorFlow的mnist資料集做手寫數字識別任務時，使用tensorflow自帶的模組（如下所示）下載和匯入資料集會報錯，原因是該模組爬取的資料集網站不能訪問。。因為該模組是用python內建urllib模組來下載資料的，需要提供有效的資料集網站地址。

import tensorflow as tf
from tensorflow.examples.tutorials.mnist import input_data
mnist = input_data.read_data_sets(data_dir, one_hot=True)

首先我看看read_data_sets()函式原始碼：

def read_data_sets(train_dir,
                   fake_data=False,
                   one_hot=False,
                   dtype=dtypes.float32,
                   reshape=True,
                   validation_size=5000,
                   seed=None,
                   source_url=DEFAULT_SOURCE_URL):
  if
 
 fake_data:

    def fake():
      return DataSet(
          [], [], fake_data=True, one_hot=one_hot, dtype=dtype, seed=seed)

    train = fake()
    validation = fake()
    test = fake()
    return base.Datasets(train=train, validation=validation, test=test)

  if not source_url:  # empty string check
 

    source_url = DEFAULT_SOURCE_URL

  TRAIN_IMAGES = 'train-images-idx3-ubyte.gz'
  TRAIN_LABELS = 'train-labels-idx1-ubyte.gz'
  TEST_IMAGES = 't10k-images-idx3-ubyte.gz'
  TEST_LABELS = 't10k-labels-idx1-ubyte.gz'

  local_file = base.maybe_download(TRAIN_IMAGES, train_dir,
                                   source_url + TRAIN_IMAGES)
  with gfile.Open(local_file, 'rb') as f:
    train_images = extract_images(f)

  local_file = base.maybe_download(TRAIN_LABELS, train_dir,
                                   source_url + TRAIN_LABELS)
  with gfile.Open(local_file, 'rb') as f:
    train_labels = extract_labels(f, one_hot=one_hot)

  local_file = base.maybe_download(TEST_IMAGES, train_dir,
                                   source_url + TEST_IMAGES)
  with gfile.Open(local_file, 'rb') as f:
    test_images = extract_images(f)

  local_file = base.maybe_download(TEST_LABELS, train_dir,
                                   source_url + TEST_LABELS)
  with gfile.Open(local_file, 'rb') as f:
    test_labels = extract_labels(f, one_hot=one_hot)

  if not 0 <= validation_size <= len(train_images):
    raise ValueError('Validation size should be between 0 and {}. Received: {}.'
                     .format(len(train_images), validation_size))

  validation_images = train_images[:validation_size]
  validation_labels = train_labels[:validation_size]
  train_images = train_images[validation_size:]
  train_labels = train_labels[validation_size:]

  options = dict(dtype=dtype, reshape=reshape, seed=seed)

  train = DataSet(train_images, train_labels, **options)
  validation = DataSet(validation_images, validation_labels, **options)
  test = DataSet(test_images, test_labels, **options)

  return base.Datasets(train=train, validation=validation, test=test)

在呼叫這個方法的時候是設定source_url引數的，如果沒有設定，就會使用預設的source_url = DEFAULT_SOURCE_URL，預設的DEFAULT_SOURCE_URL在原始碼中也可以找到：

# CVDF mirror of http://yann.lecun.com/exdb/mnist/
DEFAULT_SOURCE_URL = 'https://storage.googleapis.com/cvdf-datasets/mnist/'

有時這個url地址是能用的，報錯的話可以將註釋掉的url地址替換掉下邊的地址，就可以用了。也可以在呼叫read_data_sets函式的時候指定source_url引數為下載的資料集網址。

原始碼中下載資料集呼叫的是base.maybe_download函式，它的原始碼為：

def maybe_download(filename, work_directory, source_url):
  """Download the data from source url, unless it's already here.

  Args:
      filename: string, name of the file in the directory.
      work_directory: string, path to working directory.
      source_url: url to download from if file doesn't exist.

  Returns:
      Path to resulting file.
  """
  if not gfile.Exists(work_directory):
    gfile.MakeDirs(work_directory)
  filepath = os.path.join(work_directory, filename)
  if not gfile.Exists(filepath):
    temp_file_name, _ = urlretrieve_with_retry(source_url)
    gfile.Copy(temp_file_name, filepath)
    with gfile.GFile(filepath) as f:
      size = f.size()
    print('Successfully downloaded', filename, size, 'bytes.')
  return filepath

下載資料集又呼叫了urlretrieve_with_retry函式，原始碼為：

def urlretrieve_with_retry(url, filename=None):
  return urllib.request.urlretrieve(url, filename)

通過上述對原始碼的一步步追蹤，最後看到，tensorflow下載資料集使用的python內建的urllib模組