“sess.graph”和“tf.get_default_graph（）”之间的区别？

Question

sess.graph和tf.get_default_graph()在tensorboard中给出了相同的结果。根据说明书，我不太清楚它们之间有什么区别。有人能解释一下区别吗？任何人都可以提供一个例子，说明sess.graph和tf.get_default_graph()不能互换使用吗？

#!/usr/bin/env python
# vim: set noexpandtab tabstop=2 shiftwidth=2 softtabstop=-1 fileencoding=utf-8:

import tensorflow as tf
import sys

x = tf.Variable(1.0)
with tf.name_scope('ns_loss'):
    loss = tf.square(x)
with tf.name_scope('ns_adam'):
    train_op = tf.train.AdamOptimizer().minimize(loss)

init = tf.global_variables_initializer()

tf.summary.scalar('scalar_loss', loss)
merged_summary_op = tf.summary.merge_all()

with tf.Session() as sess:
    sess.run(init)
    summary_writer = tf.summary.FileWriter(logdir=sys.argv[1])
    summary_writer.add_graph(graph=tf.get_default_graph())
    #summary_writer.add_graph(graph=sess.graph)

    for i in xrange(100):
        sess.run(train_op)
        summary_writer.add_summary(
                summary = sess.run(merged_summary_op)
                , global_step = i
                )