How to print the value of a Tensor object in TensorFlow?

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How to print the value of a Tensor object in TensorFlow?

I have been using the introductory example of matrix multiplication in TensorFlow.

``````matrix1 = tf.constant([[3., 3.]])
matrix2 = tf.constant([[2.],[2.]])
product = tf.matmul(matrix1, matrix2)
``````

When I print the product, it is displaying it as a `Tensor` object:

``````<tensorflow.python.framework.ops.Tensor object at 0x10470fcd0>
``````

But how do I know the value of `product`?

The following doesn’t help:

``````print product
Tensor("MatMul:0", shape=TensorShape([Dimension(1), Dimension(1)]), dtype=float32)
``````

I know that graphs run on `Sessions`, but isn’t there any way I can check the output of a `Tensor` object without running the graph in a `session`?

The easiest[A] way to evaluate the actual value of a `Tensor` object is to pass it to the `Session.run()` method, or call `Tensor.eval()` when you have a default session (i.e. in a `with tf.Session():` block, or see below). In general[B], you cannot print the value of a tensor without running some code in a session.

If you are experimenting with the programming model, and want an easy way to evaluate tensors, the `tf.InteractiveSession` lets you open a session at the start of your program, and then use that session for all `Tensor.eval()` (and `Operation.run()`) calls. This can be easier in an interactive setting, such as the shell or an IPython notebook, when it’s tedious to pass around a `Session` object everywhere. For example, the following works in a Jupyter notebook:

``````with tf.Session() as sess:  print(product.eval())
``````

This might seem silly for such a small expression, but one of the key ideas in Tensorflow 1.x is deferred execution: it’s very cheap to build a large and complex expression, and when you want to evaluate it, the back-end (to which you connect with a `Session`) is able to schedule its execution more efficiently (e.g. executing independent parts in parallel and using GPUs).

[A]: To print the value of a tensor without returning it to your Python program, you can use the `tf.print()` operator, as Andrzej suggests in another answer. According to the official documentation:

To make sure the operator runs, users need to pass the produced op to `tf.compat.v1.Session`‘s run method, or to use the op as a control dependency for executed ops by specifying with `tf.compat.v1.control_dependencies([print_op]`), which is printed to standard output.

Also note that:

In Jupyter notebooks and colabs, `tf.print` prints to the notebook cell outputs. It will not write to the notebook kernel’s console logs.

[B]: You might be able to use the `tf.get_static_value()` function to get the constant value of the given tensor if its value is efficiently calculable.

While other answers are correct that you cannot print the value until you evaluate the graph, they do not talk about one easy way of actually printing a value inside the graph, once you evaluate it.

The easiest way to see a value of a tensor whenever the graph is evaluated (using `run` or `eval`) is to use the `Print` operation as in this example:

``````# Initialize session
import tensorflow as tf
sess = tf.InteractiveSession()
# Some tensor we want to print the value of
a = tf.constant([1.0, 3.0])
a = tf.Print(a, [a], message="This is a: ")
# Add more elements of the graph using a
``````

Now, whenever we evaluate the whole graph, e.g. using `b.eval()`, we get:

``````I tensorflow/core/kernels/logging_ops.cc:79] This is a: [1 3]
``````

Reiterating what others said, its not possible to check the values without running the graph.

A simple snippet for anyone looking for an easy example to print values is as below. The code can be executed without any modification in ipython notebook

``````import tensorflow as tf
#define a variable to hold normal random values
normal_rv = tf.Variable( tf.truncated_normal([2,3],stddev = 0.1))
#initialize the variable
init_op = tf.initialize_all_variables()
#run the graph
with tf.Session() as sess:
sess.run(init_op) #execute init_op
#print the random values that we sample
print (sess.run(normal_rv))
``````

Output:

``````[[-0.16702934  0.07173464 -0.04512421]
[-0.02265321  0.06509651 -0.01419079]]
``````

No, you can not see the content of the tensor without running the graph (doing `session.run()`). The only things you can see are:

• the dimensionality of the tensor (but I assume it is not hard to calculate it for the list of the operations that TF has)
• type of the operation that will be used to generate the tensor (`transpose_1:0`, `random_uniform:0`)
• type of elements in the tensor (`float32`)

I have not found this in documentation, but I believe that the values of the variables (and some of the constants are not calculated at the time of assignment).

Take a look at this example:

``````import tensorflow as tf
from datetime import datetime
dim = 7000
``````

The first example where I just initiate a constant Tensor of random numbers run approximately the same time irrespectibly of dim (`0:00:00.003261`)

``````startTime = datetime.now()
m1 = tf.truncated_normal([dim, dim], mean=0.0, stddev=0.02, dtype=tf.float32, seed=1)
print datetime.now() - startTime
``````

In the second case, where the constant is actually gets evaluated and the values are assigned, the time clearly depends on dim (`0:00:01.244642`)

``````startTime = datetime.now()
m1 = tf.truncated_normal([dim, dim], mean=0.0, stddev=0.02, dtype=tf.float32, seed=1)
sess = tf.Session()
sess.run(m1)
print datetime.now() - startTime
``````

And you can make it more clear by calculating something (`d = tf.matrix_determinant(m1)`, keeping in mind that the time will run in `O(dim^2.8)`)

P.S. I found were it is explained in documentation:

A Tensor object is a symbolic handle to the result of an operation,
but does not actually hold the values of the operation’s output.

I think you need to get some fundamentals right. With the examples above you have created tensors (multi dimensional array). But for tensor flow to really work you have to initiate a “session” and run your “operation” in the session. Notice the word “session” and “operation”.
You need to know 4 things to work with tensorflow:

1. tensors
2. Operations
3. Sessions
4. Graphs

Now from what you wrote out you have given the tensor, and the operation but you have no session running nor a graph. Tensor (edges of the graph) flow through graphs and are manipulated by operations (nodes of the graph). There is default graph but you can initiate yours in a session.

When you say print , you only access the shape of the variable or constant you defined.

So you can see what you are missing :

`````` with tf.Session() as sess:
print(sess.run(product))
print (product.eval())
``````

Hope it helps!

In `Tensorflow 1.x`

``````import tensorflow as tf
tf.enable_eager_execution()
matrix1 = tf.constant([[3., 3.]])
matrix2 = tf.constant([[2.],[2.]])
product = tf.matmul(matrix1, matrix2)
#print the product
print(product)         # tf.Tensor([[12.]], shape=(1, 1), dtype=float32)
print(product.numpy()) # [[12.]]
``````

With Tensorflow 2.x, eager mode is enabled by default. so the following code works with TF2.0.

``````import tensorflow as tf
matrix1 = tf.constant([[3., 3.]])
matrix2 = tf.constant([[2.],[2.]])
product = tf.matmul(matrix1, matrix2)
#print the product
print(product)         # tf.Tensor([[12.]], shape=(1, 1), dtype=float32)
print(product.numpy()) # [[12.]]
``````

`tf.keras.backend.eval` is useful for evaluating small expressions.

``````tf.keras.backend.eval(op)
``````

TF 1.x and TF 2.0 compatible.

Minimal Verifiable Example

``````from tensorflow.keras.backend import eval
m1 = tf.constant([[3., 3.]])
m2 = tf.constant([[2.],[2.]])
eval(tf.matmul(m1, m2))
# array([[12.]], dtype=float32)
``````

This is useful because you do not have to explicitly create a `Session` or `InteractiveSession`.

Based on the answers above, with your particular code snippet you can print the product like this:

``````import tensorflow as tf
#Initialize the session
sess = tf.InteractiveSession()
matrix1 = tf.constant([[3., 3.]])
matrix2 = tf.constant([[2.],[2.]])
product = tf.matmul(matrix1, matrix2)
#print the product
print(product.eval())
#close the session to release resources
sess.close()
``````