How to understand static shape and dynamic shape in TensorFlow?

Question

In TensorFlow FAQ, it says:

In TensorFlow, a tensor has both a static (inferred) shape and a dynamic (true) shape. The static shape can be read using the tf.Tensor.get_shape() method: this shape is inferred from the operations that were used to create the tensor, and may be partially complete. If the static shape is not fully defined, the dynamic shape of a Tensor t can be determined by evaluating tf.shape(t).

But I still cannot fully understand the relationship between static shape and dynamic shape. Are there any examples showing their differences? Thanks.

Answer 1

Sometimes the shape of a tensor depends on a value that is computed at runtime. Let's take the following example, where x is defined as a tf.placeholder() vector with four elements:

x = tf.placeholder(tf.int32, shape=[4])
print x.get_shape()
# ==> '(4,)'

The value of x.get_shape() is the static shape of x, and the (4,) means that it is a vector of length 4. Now let's apply the tf.unique() op to x

y, _ = tf.unique(x)
print y.get_shape()
# ==> '(?,)'

The (?,) means that y is a vector of unknown length. Why is it unknown? tf.unique(x) returns the unique values from x, and the values of x are unknown because it is a tf.placeholder(), so it doesn't have a value until you feed it. Let's see what happens if you feed two different values:

sess = tf.Session()
print sess.run(y, feed_dict={x: [0, 1, 2, 3]}).shape
# ==> '(4,)'
print sess.run(y, feed_dict={x: [0, 0, 0, 0]}).shape
# ==> '(1,)'

Hopefully this makes it clear that a tensor can have a different static and dynamic shape. The dynamic shape is always fully defined—it has no ? dimensions—but the static shape can be less specific. This is what allows TensorFlow to support operations like tf.unique() and tf.dynamic_partition(), which can have variable-sized outputs, and are used in advanced applications.

Finally, the tf.shape() op can be used to get the dynamic shape of a tensor and use it in a TensorFlow computation:

z = tf.shape(y)
print sess.run(z, feed_dict={x: [0, 1, 2, 3]})
# ==> [4]
print sess.run(z, feed_dict={x: [0, 0, 0, 0]})
# ==> [1]

Here's a schematic image showing both:

Answer 2

Tensorflow 2.0 Compatible Answer: Mentioning the Code which mrry has specified in his Answer, in Tensorflow Version 2.x (> 2.0), for the benefit of the Community.

# Installing the Tensorflow Version 2.1
!pip install tensorflow==2.1

# If we don't Disable the Eager Execution, usage of Placeholder results in RunTimeError

tf.compat.v1.disable_eager_execution()

x = tf.compat.v1.placeholder(tf.int32, shape=[4])
print(x.get_shape())

# ==> 4

y, _ = tf.unique(x)
print(y.get_shape())

# ==> (None,)

sess = tf.compat.v1.Session()
print(sess.run(y, feed_dict={x: [0, 1, 2, 3]}).shape)
# ==> '(4,)'
print(sess.run(y, feed_dict={x: [0, 0, 0, 0]}).shape)
# ==> '(1,)'

z = tf.shape(y)
print(sess.run(z, feed_dict={x: [0, 1, 2, 3]}))
# ==> [4]
print(sess.run(z, feed_dict={x: [0, 0, 0, 0]}))
# ==> [1]

Answer 3

It is defined well in the above answer, up voted that. There are some more observations i experienced, so i want to share.

tf.Tensor.get_shape(), can be used to infer output using the operation that created it, means we can infer it without using sess.run() (running the operation), as hinted by the name, static shape. For example,

c=tf.random_uniform([1,3,1,1])

is a tf.Tensor, and we want to know its shape at any step in the code, before running the graph, so we can use

c.get_shape()

The reason of tf.Tensor.get_shape unable to be dynamic (sess.run()) is because of the output type TensorShape instead of tf.tensor, outputting the TensorShape restricts the usage of sess.run().

sess.run(c.get_shape())

if we do we get an error that TensorShape has an invalid type it must be a Tensor/operation or a string.

On the other hand, the dynamic shape needs the operation to be run via sess.run() to get the shape

sess.run(tf.shape(c))

Output: array([1, 3, 1, 1])

or

sess.run(c).shape

(1, 3, 1, 1) # tuple

Hope it helps to clarify tensorflow concepts.