BUG InvalidArgumentError (see above for traceback): You must feed a value for placeholder tensor 'Pl

为了想看一段代买的Tensor，加入了

                sess = tf.Session()
                print(sess.run(self.embedded_chars))

然后就报错：

InvalidArgumentError (see above for traceback): You must feed a value for placeholder tensor 'Placeholder_1' with dtype float
     [[Node: Placeholder_1 = Placeholder[dtype=DT_FLOAT, shape=<unknown>, _device="/job:localhost/replica:0/task:0/device:CPU:0"]()]]

刚开始没找到错误，后来明白了。embedded_chars由两个量算得，但是都只定义了一个placeholder，没有赋值，所以得不到embedded_chars的值。例如（from https://github.com/tensorflow/tensorflow/issues/10632）：

import tensorflow as tf

a = tf.placeholder(tf.float32)
b = tf.placeholder(tf.float32)
c = a + b

with tf.Session() as sess:
    print(c.eval(feed_dict={a:1.0})) # I did not feed any value to
                                     # `b` which is required for
                                     # evaluating `c`

里面只定义了a，没有定义b的量，会产生同样的报错。