# Creating custom layers¶

## A simple layer¶

To implement a custom layer in Lasagne, you will have to write a Python class that subclasses Layer and implement at least one method: get_output_for(). This method computes the output of the layer given its input. Note that both the output and the input are Theano expressions, so they are symbolic.

The following is an example implementation of a layer that multiplies its input by 2:

class DoubleLayer(lasagne.layers.Layer):
def get_output_for(self, input, **kwargs):
return 2 * input


This is all that’s required to implement a functioning custom layer class in Lasagne.

## A layer that changes the shape¶

If the layer does not change the shape of the data (for example because it applies an elementwise operation), then implementing only this one method is sufficient. Lasagne will assume that the output of the layer has the same shape as its input.

However, if the operation performed by the layer changes the shape of the data, you also need to implement get_output_shape_for(). This method computes the shape of the layer output given the shape of its input. Note that this shape computation should result in a tuple of integers, so it is not symbolic.

This method exists because Lasagne needs a way to propagate shape information when a network is defined, so it can determine what sizes the parameter tensors should be, for example. This mechanism allows each layer to obtain the size of its input from the previous layer, which means you don’t have to specify the input size manually. This also prevents errors stemming from inconsistencies between the layers’ expected and actual shapes.

We can implement a layer that computes the sum across the trailing axis of its input as follows:

class SumLayer(lasagne.layers.Layer):
def get_output_for(self, input, **kwargs):
return input.sum(axis=-1)

def get_output_shape_for(self, input_shape):
return input_shape[:-1]


It is important that the shape computation is correct, as this shape information may be used to initialize other layers in the network.

## A layer with parameters¶

If the layer has parameters, these should be initialized in the constructor. In Lasagne, parameters are represented by Theano shared variables. A method is provided to create and register parameter variables: lasagne.layers.Layer.add_param().

To show how this can be used, here is a layer that multiplies its input by a matrix W (much like a typical fully connected layer in a neural network would). This matrix is a parameter of the layer. The shape of the matrix will be (num_inputs, num_units), where num_inputs is the number of input features and num_units has to be specified when the layer is created.

class DotLayer(lasagne.layers.Layer):
def __init__(self, incoming, num_units, W=lasagne.init.Normal(0.01), **kwargs):
super(DotLayer, self).__init__(incoming, **kwargs)
num_inputs = self.input_shape[1]
self.num_units = num_units
self.W = self.add_param(W, (num_inputs, num_units), name='W')

def get_output_for(self, input, **kwargs):
return T.dot(input, self.W)

def get_output_shape_for(self, input_shape):
return (input_shape[0], self.num_units)


A few things are worth noting here: when overriding the constructor, we need to call the superclass constructor on the first line. This is important to ensure the layer functions properly. Note that we pass **kwargs - although this is not strictly necessary, it enables some other cool Lasagne features, such as making it possible to give the layer a name:

>>> l_dot = DotLayer(l_in, num_units=50, name='my_dot_layer')


The call to self.add_param() creates the Theano shared variable representing the parameter, and registers it so it can later be retrieved using lasagne.layers.Layer.get_params(). It returns the created variable, which we tuck away in self.W for easy access.

Note that we’ve also made it possible to specify a custom initialization strategy for W by adding a constructor argument for it, e.g.:

>>> l_dot = DotLayer(l_in, num_units=50, W=lasagne.init.Constant(0.0))


This ‘Lasagne idiom’ of tucking away a created parameter variable in an attribute for easy access and adding a constructor argument with the same name to specify the initialization strategy is very common throughout the library.

Finally, note that we used self.input_shape to determine the shape of the parameter matrix. This property is available in all Lasagne layers, once the superclass constructor has been called.

## A layer with multiple behaviors¶

Some layers can have multiple behaviors. For example, a layer implementing dropout should be able to be switched on or off. During training, we want it to apply dropout noise to its input and scale up the remaining values, but during evaluation we don’t want it to do anything.

For this purpose, the get_output_for() method takes optional keyword arguments (kwargs). When get_output() is called to compute an expression for the output of a network, all specified keyword arguments are passed to the get_output_for() methods of all layers in the network.

For layers that add noise for regularization purposes, such as dropout, the convention in Lasagne is to use the keyword argument deterministic to control its behavior.

Lasagne’s lasagne.layers.DropoutLayer looks roughly like this (simplified implementation for illustration purposes):

from theano.sandbox.rng_mrg import MRG_RandomStreams as RandomStreams
_srng = RandomStreams()

class DropoutLayer(Layer):
def __init__(self, incoming, p=0.5, **kwargs):
super(DropoutLayer, self).__init__(incoming, **kwargs)
self.p = p

def get_output_for(self, input, deterministic=False, **kwargs):
if deterministic:  # do nothing in the deterministic case
return input
else:  # add dropout noise otherwise
retain_prob = 1 - self.p
input /= retain_prob
return input * _srng.binomial(input.shape, p=retain_prob,
dtype=theano.config.floatX)