tf.assign，tf.assign_add，tf.assign_sub

a = tf.Variable(0.0,dtype=tf.float32)
with tf.Session() as sess:
    sess.run(tf.global_variables_initializer())
    print(sess.run(a))
    a = tf.assign(a,10)
    print(sess.run(a))
    a = tf.assign(a,20)
    print(sess.run(a))

a = tf.Variable(1,dtype=tf.float32)
with tf.Session() as sess:
    sess.run(tf.global_variables_initializer())
    print(sess.run(a))
    a = tf.assign_add(a,10)
    print(sess.run(a))
    a = tf.assign_sub(a,20)
    print(sess.run(a))