# tf.contrib.seq2seq.AttentionWrapper

## Class AttentionWrapper

Inherits From: RNNCell

See the guide: Seq2seq Library (contrib) > Attention

Wraps another RNNCell with attention.

## Properties

### activity_regularizer

Optional regularizer function for the output of this layer.

### input

Retrieves the input tensor(s) of a layer.

Only applicable if the layer has exactly one input, i.e. if it is connected to one incoming layer.

#### Returns:

Input tensor or list of input tensors.

#### Raises:

• AttributeError: if the layer is connected to more than one incoming layers.

#### Raises:

• RuntimeError: If called in Eager mode.
• AttributeError: If no inbound nodes are found.

### input_shape

Retrieves the input shape(s) of a layer.

Only applicable if the layer has exactly one input, i.e. if it is connected to one incoming layer, or if all inputs have the same shape.

#### Returns:

Input shape, as an integer shape tuple (or list of shape tuples, one tuple per input tensor).

#### Raises:

• AttributeError: if the layer has no defined input_shape.
• RuntimeError: if called in Eager mode.

### losses

Losses which are associated with this Layer.

Note that when executing eagerly, getting this property evaluates regularizers. When using graph execution, variable regularization ops have already been created and are simply returned here.

#### Returns:

A list of tensors.

### output

Retrieves the output tensor(s) of a layer.

Only applicable if the layer has exactly one output, i.e. if it is connected to one incoming layer.

#### Returns:

Output tensor or list of output tensors.

#### Raises:

• AttributeError: if the layer is connected to more than one incoming layers.
• RuntimeError: if called in Eager mode.

### output_shape

Retrieves the output shape(s) of a layer.

Only applicable if the layer has one output, or if all outputs have the same shape.

#### Returns:

Output shape, as an integer shape tuple (or list of shape tuples, one tuple per output tensor).

#### Raises:

• AttributeError: if the layer has no defined output shape.
• RuntimeError: if called in Eager mode.

### state_size

The state_size property of AttentionWrapper.

#### Returns:

An AttentionWrapperState tuple containing shapes used by this object.

### variables

Returns the list of all layer variables/weights.

#### Returns:

A list of variables.

### weights

Returns the list of all layer variables/weights.

#### Returns:

A list of variables.

## Methods

### __init__

__init__(
cell,
attention_mechanism,
attention_layer_size=None,
alignment_history=False,
cell_input_fn=None,
output_attention=True,
initial_cell_state=None,
name=None
)


Construct the AttentionWrapper.

NOTE If you are using the BeamSearchDecoder with a cell wrapped in AttentionWrapper, then you must ensure that:

• The encoder output has been tiled to beam_width via tf.contrib.seq2seq.tile_batch (NOT tf.tile).
• The batch_size argument passed to the zero_state method of this wrapper is equal to true_batch_size * beam_width.
• The initial state created with zero_state above contains a cell_state value containing properly tiled final state from the encoder.

An example:

tiled_encoder_outputs = tf.contrib.seq2seq.tile_batch(
encoder_outputs, multiplier=beam_width)
tiled_encoder_final_state = tf.conrib.seq2seq.tile_batch(
encoder_final_state, multiplier=beam_width)
tiled_sequence_length = tf.contrib.seq2seq.tile_batch(
sequence_length, multiplier=beam_width)
attention_mechanism = MyFavoriteAttentionMechanism(
num_units=attention_depth,
memory=tiled_inputs,
memory_sequence_length=tiled_sequence_length)
attention_cell = AttentionWrapper(cell, attention_mechanism, ...)
decoder_initial_state = attention_cell.zero_state(
dtype, batch_size=true_batch_size * beam_width)
decoder_initial_state = decoder_initial_state.clone(
cell_state=tiled_encoder_final_state)


#### Args:

• cell: An instance of RNNCell.
• attention_mechanism: A list of AttentionMechanism instances or a single instance.
• attention_layer_size: A list of Python integers or a single Python integer, the depth of the attention (output) layer(s). If None (default), use the context as attention at each time step. Otherwise, feed the context and cell output into the attention layer to generate attention at each time step. If attention_mechanism is a list, attention_layer_size must be a list of the same length.
• alignment_history: Python boolean, whether to store alignment history from all time steps in the final output state (currently stored as a time major TensorArray on which you must call stack()).
• cell_input_fn: (optional) A callable. The default is: lambda inputs, attention: array_ops.concat([inputs, attention], -1).
• output_attention: Python bool. If True (default), the output at each time step is the attention value. This is the behavior of Luong-style attention mechanisms. If False, the output at each time step is the output of cell. This is the behavior of Bhadanau-style attention mechanisms. In both cases, the attention tensor is propagated to the next time step via the state and is used there. This flag only controls whether the attention mechanism is propagated up to the next cell in an RNN stack or to the top RNN output.
• initial_cell_state: The initial state value to use for the cell when the user calls zero_state(). Note that if this value is provided now, and the user uses a batch_size argument of zero_state which does not match the batch size of initial_cell_state, proper behavior is not guaranteed.
• name: Name to use when creating ops.

#### Raises:

• TypeError: attention_layer_size is not None and (attention_mechanism is a list but attention_layer_size is not; or vice versa).
• ValueError: if attention_layer_size is not None, attention_mechanism is a list, and its length does not match that of attention_layer_size.

### __call__

__call__(
inputs,
state,
scope=None
)


Run this RNN cell on inputs, starting from the given state.

#### Args:

• inputs: 2-D tensor with shape [batch_size, input_size].
• state: if self.state_size is an integer, this should be a 2-D Tensor with shape [batch_size, self.state_size]. Otherwise, if self.state_size is a tuple of integers, this should be a tuple with shapes [batch_size, s] for s in self.state_size.
• scope: VariableScope for the created subgraph; defaults to class name.

#### Returns:

A pair containing:

• Output: A 2-D tensor with shape [batch_size, self.output_size].
• New state: Either a single 2-D tensor, or a tuple of tensors matching the arity and shapes of state.

### __deepcopy__

__deepcopy__(memo)


### add_loss

add_loss(
losses,
inputs=None
)


Add loss tensor(s), potentially dependent on layer inputs.

Some losses (for instance, activity regularization losses) may be dependent on the inputs passed when calling a layer. Hence, when reusing the same layer on different inputs a and b, some entries in layer.losses may be dependent on a and some on b. This method automatically keeps track of dependencies.

The get_losses_for method allows to retrieve the losses relevant to a specific set of inputs.

Note that add_loss is not supported when executing eagerly. Instead, variable regularizers may be added through add_variable. Activity regularization is not supported directly (but such losses may be returned from Layer.call()).

#### Arguments:

• losses: Loss tensor, or list/tuple of tensors.
• inputs: If anything other than None is passed, it signals the losses are conditional on some of the layer's inputs, and thus they should only be run where these inputs are available. This is the case for activity regularization losses, for instance. If None is passed, the losses are assumed to be unconditional, and will apply across all dataflows of the layer (e.g. weight regularization losses).

#### Raises:

• RuntimeError: If called in Eager mode.

### add_update

add_update(
inputs=None
)


Add update op(s), potentially dependent on layer inputs.

Weight updates (for instance, the updates of the moving mean and variance in a BatchNormalization layer) may be dependent on the inputs passed when calling a layer. Hence, when reusing the same layer on different inputs a and b, some entries in layer.updates may be dependent on a and some on b. This method automatically keeps track of dependencies.

The get_updates_for method allows to retrieve the updates relevant to a specific set of inputs.

This call is ignored in Eager mode.

#### Arguments:

• updates: Update op, or list/tuple of update ops.
• inputs: If anything other than None is passed, it signals the updates are conditional on some of the layer's inputs, and thus they should only be run where these inputs are available. This is the case for BatchNormalization updates, for instance. If None, the updates will be taken into account unconditionally, and you are responsible for making sure that any dependency they might have is available at runtime. A step counter might fall into this category.

### add_variable

add_variable(
name,
shape,
dtype=None,
initializer=None,
regularizer=None,
trainable=True,
constraint=None,
partitioner=None
)


Adds a new variable to the layer, or gets an existing one; returns it.

#### Arguments:

• name: variable name.
• shape: variable shape.
• dtype: The type of the variable. Defaults to self.dtype or float32.
• initializer: initializer instance (callable).
• regularizer: regularizer instance (callable).
• trainable: whether the variable should be part of the layer's "trainable_variables" (e.g. variables, biases) or "non_trainable_variables" (e.g. BatchNorm mean, stddev). Note, if the current variable scope is marked as non-trainable then this parameter is ignored and any added variables are also marked as non-trainable.
• constraint: constraint instance (callable).
• partitioner: (optional) partitioner instance (callable). If provided, when the requested variable is created it will be split into multiple partitions according to partitioner. In this case, an instance of PartitionedVariable is returned. Available partitioners include tf.fixed_size_partitioner and tf.variable_axis_size_partitioner. For more details, see the documentation of tf.get_variable and the "Variable Partitioners and Sharding" section of the API guide.

#### Returns:

The created variable. Usually either a Variable or ResourceVariable instance. If partitioner is not None, a PartitionedVariable instance is returned.

#### Raises:

• RuntimeError: If called with partioned variable regularization and eager execution is enabled.

### apply

apply(
inputs,
*args,
**kwargs
)


Apply the layer on a input.

This simply wraps self.__call__.

#### Arguments:

• inputs: Input tensor(s).
• *args: additional positional arguments to be passed to self.call.
• **kwargs: additional keyword arguments to be passed to self.call.

#### Returns:

Output tensor(s).

### build

build(_)


### call

call(
inputs,
state
)


Perform a step of attention-wrapped RNN.

• Step 1: Mix the inputs and previous step's attention output via cell_input_fn.
• Step 2: Call the wrapped cell with this input and its previous state.
• Step 3: Score the cell's output with attention_mechanism.
• Step 4: Calculate the alignments by passing the score through the normalizer.
• Step 5: Calculate the context vector as the inner product between the alignments and the attention_mechanism's values (memory).
• Step 6: Calculate the attention output by concatenating the cell output and context through the attention layer (a linear layer with attention_layer_size outputs).

#### Args:

• inputs: (Possibly nested tuple of) Tensor, the input at this time step.
• state: An instance of AttentionWrapperState containing tensors from the previous time step.

#### Returns:

A tuple (attention_or_cell_output, next_state), where:

• attention_or_cell_output depending on output_attention.
• next_state is an instance of AttentionWrapperState containing the state calculated at this time step.

#### Raises:

• TypeError: If state is not an instance of AttentionWrapperState.

### compute_output_shape

compute_output_shape(input_shape)


Computes the output shape of the layer given the input shape.

#### Args:

• input_shape: A (possibly nested tuple of) TensorShape. It need not be fully defined (e.g. the batch size may be unknown).

#### Returns:

A (possibly nested tuple of) TensorShape.

#### Raises:

• TypeError: if input_shape is not a (possibly nested tuple of) TensorShape.
• ValueError: if input_shape is incomplete or is incompatible with the the layer.

### count_params

count_params()


Count the total number of scalars composing the weights.

#### Returns:

An integer count.

#### Raises:

• ValueError: if the layer isn't yet built (in which case its weights aren't yet defined).

### get_input_at

get_input_at(node_index)


Retrieves the input tensor(s) of a layer at a given node.

#### Arguments:

• node_index: Integer, index of the node from which to retrieve the attribute. E.g. node_index=0 will correspond to the first time the layer was called.

#### Returns:

A tensor (or list of tensors if the layer has multiple inputs).

#### Raises:

• RuntimeError: If called in Eager mode.

### get_input_shape_at

get_input_shape_at(node_index)


Retrieves the input shape(s) of a layer at a given node.

#### Arguments:

• node_index: Integer, index of the node from which to retrieve the attribute. E.g. node_index=0 will correspond to the first time the layer was called.

#### Returns:

A shape tuple (or list of shape tuples if the layer has multiple inputs).

#### Raises:

• RuntimeError: If called in Eager mode.

### get_losses_for

get_losses_for(inputs)


Retrieves losses relevant to a specific set of inputs.

#### Arguments:

• inputs: Input tensor or list/tuple of input tensors.

#### Returns:

List of loss tensors of the layer that depend on inputs.

#### Raises:

• RuntimeError: If called in Eager mode.

### get_output_at

get_output_at(node_index)


Retrieves the output tensor(s) of a layer at a given node.

#### Arguments:

• node_index: Integer, index of the node from which to retrieve the attribute. E.g. node_index=0 will correspond to the first time the layer was called.

#### Returns:

A tensor (or list of tensors if the layer has multiple outputs).

#### Raises:

• RuntimeError: If called in Eager mode.

### get_output_shape_at

get_output_shape_at(node_index)


Retrieves the output shape(s) of a layer at a given node.

#### Arguments:

• node_index: Integer, index of the node from which to retrieve the attribute. E.g. node_index=0 will correspond to the first time the layer was called.

#### Returns:

A shape tuple (or list of shape tuples if the layer has multiple outputs).

#### Raises:

• RuntimeError: If called in Eager mode.

### get_updates_for

get_updates_for(inputs)


Retrieves updates relevant to a specific set of inputs.

#### Arguments:

• inputs: Input tensor or list/tuple of input tensors.

#### Returns:

List of update ops of the layer that depend on inputs.

#### Raises:

• RuntimeError: If called in Eager mode.

### zero_state

zero_state(
batch_size,
dtype
)


Return an initial (zero) state tuple for this AttentionWrapper.

NOTE Please see the initializer documentation for details of how to call zero_state if using an AttentionWrapper with a BeamSearchDecoder.

#### Args:

• batch_size: 0D integer tensor: the batch size.
• dtype: The internal state data type.

#### Returns:

An AttentionWrapperState tuple containing zeroed out tensors and, possibly, empty TensorArray objects.

#### Raises:

• ValueError: (or, possibly at runtime, InvalidArgument), if batch_size does not match the output size of the encoder passed to the wrapper object at initialization time.