Create custom Keras Layer object using Lambda

Question

I am want to build a custom Keras layer keeping the k top activation values. I am currently doing this (and its working fine) :

def max_topk_pool(x,k):
    import tensorflow as tf
    k_max =  tf.nn.top_k(x,k=k,sorted=True,name=None)
    return  k_max

def KMax(k):
    return Lambda(max_topk_pool,
                  arguments={'k':k},
                  output_shape=lambda x: (None, k))

Do you know if there is a way to build a custom Layer class "KMax" in the way shown by Keras in https://keras.io/layers/writing-your-own-keras-layers/

from keras import backend as K
from keras.layers import Layer

class MyLayer(Layer):

def __init__(self, output_dim, **kwargs):
    self.output_dim = output_dim
    super(MyLayer, self).__init__(**kwargs)

def build(self, input_shape):
    # Create a trainable weight variable for this layer.
    self.kernel = self.add_weight(name='kernel', 
                                  shape=(input_shape[1], self.output_dim),
                                  initializer='uniform',
                                  trainable=True)
    super(MyLayer, self).build(input_shape)  # Be sure to call this at the end

def call(self, x):
    return K.dot(x, self.kernel)

def compute_output_shape(self, input_shape):
    return (input_shape[0], self.output_dim)

I would like something like this :

from keras import backend as K
from keras.layers import Layer

class KMax(Layer):

def __init__(self, output_dim, **kwargs):
    self.K = K
    super(MyLayer, self).__init__(**kwargs)

def build(self, input_shape):
    <... Lambda here ?>

def compute_output_shape(self, input_shape):
    return (input_shape[0], self.K)

Thank you very much !

Andrey Kulagin · Accepted Answer · 2019-01-26 16:55:25Z

Here is what you need (based on https://github.com/keras-team/keras/issues/373):

from keras.engine import Layer, InputSpec
from keras.layers import Flatten
import tensorflow as tf


# https://github.com/keras-team/keras/issues/373
class KMaxPooling(Layer):
    """
    K-max pooling layer that extracts the k-highest activations from a sequence (2nd dimension).
    TensorFlow backend.
    """

    def __init__(self, k=1, **kwargs):
        super().__init__(**kwargs)
        self.input_spec = InputSpec(ndim=3)
        self.k = k

    def compute_output_shape(self, input_shape):
        return input_shape[0], (input_shape[2] * self.k)

    def call(self, inputs):
        # swap last two dimensions since top_k will be applied along the last dimension
        shifted_input = tf.transpose(inputs, [0, 2, 1])

        # extract top_k, returns two tensors [values, indices]
        top_k = tf.nn.top_k(shifted_input, k=self.k, sorted=True, name=None)[0]

        # return flattened output
        return Flatten()(top_k)

    def get_config(self):
        config = {'k': self.k}
        base_config = super().get_config()
        return {**base_config, **config}

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