Tensorflow error using while_loop: "List of Tensors when single Tensor expected"

Question

I'm getting a TypeError("List of Tensors when single Tensor expected") when I run a Tensorflow while_loop. The error is from the third parameter, which should be a list of Tensors, according to the documentation. x, W, Win, Y, temp, and Wout are all previously declared as floats and arrays of floats. cond2 and test2 are functions I've written to be the condition and body. I use an almost identical call earlier in the program with no issues.

t=0
t,x,W,Win,Y,temp,Wout = sess.run(tf.while_loop(cond2, test2,
                                 [t, tf.Variable(x), tf.constant(W),
                                  tf.constant(Win), tf.Variable(Y),
                                  tf.Variable(temp), tf.constant(Wout)],
                                 shape_invariants=[tf.TensorShape(None),
                                                   tf.TensorShape(None),
                                                   tf.TensorShape(None),
                                                   tf.TensorShape(None),
                                                   tf.TensorShape(None),
                                                   tf.TensorShape(None),
                                                   tf.TensorShape(None)]))

Linda W · Accepted Answer · 2017-03-22 18:32:41Z

2

I fixed the error by removing the tf.constant() for Wout, since Wout was already declared as a tensor.

answered Mar 22, 2017 at 18:32

Linda W

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Brandon McKinzie · Accepted Answer · 2017-03-20 22:03:41Z

0

This would be easier to diagnose with (a) your definitions for condition and body, and (b) the full error output from TensorFlow (it usually also outputs a full dump of the input tensors when issuing these errors.)

With that said, the source of the problem seems to be that TensorFlow is viewing your loop_vars list as a single Tensor, and/or your cond2 and test2 functions only accept a single argument each. If neither of these is true, then providing more detail would help answer the question (specifically the full error message and the definition for every value/tensor/function you're passing to tf.while_loop. I've found that the majority of while_loop errors can be fixed by paying attention to the tensors in the error output.

The while_loop can throw pretty confusing errors at times so I'd like to help; I'll check back and update/edit my answer if more info is provided.

answered Mar 20, 2017 at 22:03

Brandon McKinzie

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1 Comment

Linda W Over a year ago

Thanks for your response! I resolved the issue as described in my answer. As you suspected, the full error output told me the problem was with Wout, and I had just been confused by the error because what it said wasn't really the problem.

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