tf.Variable

tf.Variable(
    initial_value=None, trainable=None, validate_shape=True, caching_device=None,
    name=None, variable_def=None, dtype=None, import_scope=None, constraint=None,
    synchronization=tf.VariableSynchronization.AUTO,
    aggregation=tf.compat.v1.VariableAggregation.NONE, shape=None
)

Used in the notebooks

Used in the guide	Used in the tutorials
Migrating model checkpoints Introduction to gradients and automatic differentiation Introduction to Variables Advanced automatic differentiation Better performance with tf.function	Custom training loop with Keras and MultiWorkerMirroredStrategy Neural style transfer Learnable Distributions Zoo Wiki Talk Comments Toxicity Prediction Federated Learning for Image Classification

A variable maintains shared, persistent state manipulated by a program.

The Variable() constructor requires an initial value for the variable, which can be a Tensor of any type and shape. This initial value defines the type and shape of the variable. After construction, the type and shape of the variable are fixed. The value can be changed using one of the assign methods.

v = tf.Variable(1.)
v.assign(2.)
<tf.Variable ... shape=() dtype=float32, numpy=2.0>
v.assign_add(0.5)
<tf.Variable ... shape=() dtype=float32, numpy=2.5>

The shape argument to Variable's constructor allows you to construct a variable with a less defined shape than its initial_value:

v = tf.Variable(1., shape=tf.TensorShape(None))
v.assign([[1.]])
<tf.Variable ... shape=<unknown> dtype=float32, numpy=array([[1.]], ...)>

Just like any Tensor, variables created with Variable() can be used as inputs to operations. Additionally, all the operators overloaded for the Tensor class are carried over to variables.

w = tf.Variable([[1.], [2.]])
x = tf.constant([[3., 4.]])
tf.matmul(w, x)
<tf.Tensor:... shape=(2, 2), ... numpy=
  array([[3., 4.],
         [6., 8.]], dtype=float32)>
tf.sigmoid(w + x)
<tf.Tensor:... shape=(2, 2), ...>

When building a machine learning model it is often convenient to distinguish between variables holding trainable model parameters and other variables such as a step variable used to count training steps. To make this easier, the variable constructor supports a trainable=<bool> parameter. tf.GradientTape watches trainable variables by default:

with tf.GradientTape(persistent=True) as tape:
  trainable = tf.Variable(1.)
  non_trainable = tf.Variable(2., trainable=False)
  x1 = trainable * 2.
  x2 = non_trainable * 3.
tape.gradient(x1, trainable)
<tf.Tensor:... shape=(), dtype=float32, numpy=2.0>