//  [python]
def train_and_test(dataset_url, training_iterations, batch_size, evaluation_interval, start):
  """
  Train a model for training iterations with a batch size batch_size, printing accuracy every log_interval.
  :param dataset_url: The MNIST dataset url.
  :param training_iterations: The training iterations to train for.
  :param batch_size: The batch size for training.
  :param evaluation_interval: The interval used to print the accuracy.
  :return:
  """
  with make_reader(os.path.join(dataset_url, 'train'), num_epochs=None, is_batch=False) as train_reader:
    with make_reader(os.path.join(dataset_url, 'test'), num_epochs=None, is_batch=False) as test_reader:
      train_readout = tf_tensors(train_reader)
      train_image = tf.cast(tf.reshape(train_readout.image, [784]), tf.float32)
      train_label = train_readout.digit
      batch_image, batch_label = tf.train.batch(
        [train_image, train_label], batch_size=batch_size
      )

      W = tf.Variable(tf.zeros([784, 10]))
      b = tf.Variable(tf.zeros([10]))
      y = tf.matmul(batch_image, W) + b

      # The raw formulation of cross-entropy,
      #
      #   tf.reduce_mean(-tf.reduce_sum(y_ * tf.log(tf.nn.softmax(y)),
      #                                 reduction_indices=[1]))
      #
      # can be numerically unstable.
      #
      # So here we use tf.losses.sparse_softmax_cross_entropy on the raw
      # outputs of 'y', and then average across the batch.
      cross_entropy = tf.losses.sparse_softmax_cross_entropy(labels=batch_label, logits=y)
      train_step = tf.train.GradientDescentOptimizer(0.5).minimize(cross_entropy)

      correct_prediction = tf.equal(tf.argmax(y, 1), batch_label)
      accuracy = tf.reduce_mean(tf.cast(correct_prediction, tf.float32))

      test_readout = tf_tensors(test_reader)
      test_image = tf.cast(tf.reshape(test_readout.image, [784]), tf.float32)
      test_label = test_readout.digit
      test_batch_image, test_batch_label = tf.train.batch(
        [test_image, test_label], batch_size=batch_size
      )

      end = time.time()
      print("before train time: " + str(end - start))

      # Train
      print('Training model for {0} training iterations with batch size {1} and evaluation interval {2}'.format(
        training_iterations, batch_size, evaluation_interval
      ))
      with tf.Session() as sess:
        sess.run([
          tf.local_variables_initializer(),
          tf.global_variables_initializer(),
        ])
        coord = tf.train.Coordinator()
        threads = tf.train.start_queue_runners(sess=sess, coord=coord)
        try:
          for i in range(training_iterations):
            if coord.should_stop():
              break

            sess.run(train_step)

            if (i % evaluation_interval) == 0 or i == (training_iterations - 1):
              feed_batch_image, feed_batch_label = sess.run([test_batch_image, test_batch_label])
              print('After {0} training iterations, the accuracy of the model is: {1:.2f}'.format(
                i,
                sess.run(accuracy, feed_dict={
                  batch_image: feed_batch_image, batch_label: feed_batch_label
                })))
        finally:
          coord.request_stop()
          coord.join(threads)