获取tf中占位符的维度报错

Question

import tensorflow as tf

feed_x=[[[1,2,3],[2,3,4]],[[5,6,7],[7,8,9]],[[2,3,4],[4,5,6]]]

x=tf.placeholder('int32',[None,2,3])

len=x.get_shape()

with tf.Session() as sess:

sess.run(tf.global_variables_initializer())

print(sess.run(len,feed_dict={x:feed_x}))