AttributeError: The layer has never been called and thus has no defined input shape

Question

I'm tring to build an autoencoder in TensorFlow 2.0 by creating three classes: Encoder, Decoder and AutoEncoder. Since I don't want to manually set input shapes I'm trying to infer the output shape of the decoder from the encoder's input_shape.

import os
import shutil

import numpy as np
import tensorflow as tf
from tensorflow.keras import Model
from tensorflow.keras.layers import Dense, Layer


def mse(model, original):
    return tf.reduce_mean(tf.square(tf.subtract(model(original), original)))


def train_autoencoder(loss, model, opt, original):
    with tf.GradientTape() as tape:
        gradients = tape.gradient(
            loss(model, original), model.trainable_variables)
        gradient_variables = zip(gradients, model.trainable_variables)
        opt.apply_gradients(gradient_variables)


def log_results(model, X, max_outputs, epoch, prefix):
    loss_values = mse(model, X)

    sample_img = X[sample(range(X.shape[0]), max_outputs), :]
    original = tf.reshape(sample_img, (max_outputs, 28, 28, 1))
    encoded = tf.reshape(
        model.encode(sample_img), (sample_img.shape[0], 8, 8, 1))
    decoded = tf.reshape(
        model(tf.constant(sample_img)), (sample_img.shape[0], 28, 28, 1))
    tf.summary.scalar("{}_loss".format(prefix), loss_values, step=epoch + 1)
    tf.summary.image(
        "{}_original".format(prefix),
        original,
        max_outputs=max_outputs,
        step=epoch + 1)
    tf.summary.image(
        "{}_encoded".format(prefix),
        encoded,
        max_outputs=max_outputs,
        step=epoch + 1)
    tf.summary.image(
        "{}_decoded".format(prefix),
        decoded,
        max_outputs=max_outputs,
        step=epoch + 1)

    return loss_values


def preprocess_mnist(batch_size):
    (X_train, y_train), (X_test, y_test) = tf.keras.datasets.mnist.load_data()

    X_train = X_train / np.max(X_train)
    X_train = X_train.reshape(X_train.shape[0],
                              X_train.shape[1] * X_train.shape[2]).astype(
                                  np.float32)
    train_dataset = tf.data.Dataset.from_tensor_slices(X_train).batch(
        batch_size)

    y_train = y_train.astype(np.int32)
    train_labels = tf.data.Dataset.from_tensor_slices(y_train).batch(
        batch_size)

    X_test = X_test / np.max(X_test)
    X_test = X_test.reshape(
        X_test.shape[0], X_test.shape[1] * X_test.shape[2]).astype(np.float32)

    y_test = y_test.astype(np.int32)

    return X_train, X_test, train_dataset, y_train, y_test, train_labels


class Encoder(Layer):
    def __init__(self, units):
        super(Encoder, self).__init__()
        self.units = units

    def build(self, input_shape):
        self.output_layer = Dense(units=self.units, activation=tf.nn.relu)

    @tf.function
    def call(self, X):
        return self.output_layer(X)


class Decoder(Layer):
    def __init__(self, encoder):
        super(Decoder, self).__init__()
        self.encoder = encoder

    def build(self, input_shape):
        self.output_layer = Dense(units=self.encoder.input_shape)

    @tf.function
    def call(self, X):
        return self.output_layer(X)


class AutoEncoder(Model):
    def __init__(self, units):
        super(AutoEncoder, self).__init__()
        self.units = units

    def build(self, input_shape):
        self.encoder = Encoder(units=self.units)
        self.encoder.build(input_shape)
        self.decoder = Decoder(encoder=self.encoder)

    @tf.function
    def call(self, X):
        Z = self.encoder(X)
        return self.decoder(Z)

    @tf.function
    def encode(self, X):
        return self.encoder(X)

    @tf.function
    def decode(self, Z):
        return self.decode(Z)


def test_autoencoder(batch_size,
                     learning_rate,
                     epochs,
                     max_outputs=4,
                     seed=None):

    tf.random.set_seed(seed)

    X_train, X_test, train_dataset, _, _, _ = preprocess_mnist(
        batch_size=batch_size)

    autoencoder = AutoEncoder(units=64)
    opt = tf.optimizers.Adam(learning_rate=learning_rate)

    log_path = 'logs/autoencoder'
    if os.path.exists(log_path):
        shutil.rmtree(log_path)

    writer = tf.summary.create_file_writer(log_path)

    with writer.as_default():
        with tf.summary.record_if(True):
            for epoch in range(epochs):
                for step, batch in enumerate(train_dataset):
                    train_autoencoder(mse, autoencoder, opt, batch)

                # logs (train)
                train_loss = log_results(
                    model=autoencoder,
                    X=X_train,
                    max_outputs=max_outputs,
                    epoch=epoch,
                    prefix='train')

                # logs (test)
                test_loss = log_results(
                    model=autoencoder,
                    X=X_test,
                    max_outputs=max_outputs,
                    epoch=epoch,
                    prefix='test')

                writer.flush()

                template = 'Epoch {}, Train loss: {:.5f}, Test loss: {:.5f}'
                print(
                    template.format(epoch + 1, train_loss.numpy(),
                                    test_loss.numpy()))

    if not os.path.exists('saved_models'):
        os.makedirs('saved_models')
    np.savez_compressed('saved_models/encoder.npz',
                        *autoencoder.encoder.get_weights())


if __name__ == '__main__':
    test_autoencoder(batch_size=128, learning_rate=1e-3, epochs=20, seed=42)

Since the encoder's input shape is used in the build function of the decoder, I'd expect that when I train the autoencoder the encoder is built first, then the decoder, but that doesn't seem to be the case. I've also tried to build the encoder in the build function of the decoder by calling self.encoder.build() at the start of the decoder's build function, but it didn't make any difference. What am I doing wrong?

Error I am receiving:

AttributeError: The layer has never been called and thus has no defined input shape.

Answer 1

You were almost there, just overcomplicated things a bit. You are receiving this error because Decoder layer is dependent on the Encoder layer which wasn't built yet (as the call to build was unsuccessful) and it's input_shape attribute was not set.

Solution would be to pass correct output shapes from AutoEncoder object like this:

class Decoder(Layer):
    def __init__(self, units):
        super(Decoder, self).__init__()
        self.units = units

    def build(self, _):
        self.output_layer = Dense(units=self.units)

    def call(self, X):
        return self.output_layer(X)


class AutoEncoder(Model):
    def __init__(self, units):
        super(AutoEncoder, self).__init__()
        self.units = units

    def build(self, input_shape):
        self.encoder = Encoder(units=self.units)
        self.decoder = Decoder(units=input_shape[-1])

Notice I have removed @tf,function decorator as you are unlikely to get any efficiency boost (keras already creates the static graph under the hood for you).

Furthermore, as one can see, your build is not dependent on input_shape information so all the creation can be safely moved to the constructor like this:

class Encoder(Layer):
    def __init__(self, units):
        super(Encoder, self).__init__()
        self.output_layer = Dense(units=units, activation=tf.nn.relu)

    def call(self, X):
        return self.output_layer(X)


class Decoder(Layer):
    def __init__(self, units):
        super(Decoder, self).__init__()
        self.output_layer = Dense(units=units)

    def call(self, X):
        return self.output_layer(X)


class AutoEncoder(Model):
    def __init__(self, units):
        super(AutoEncoder, self).__init__()
        self.units = units

    def build(self, input_shape):
        self.encoder = Encoder(units=self.units)
        self.decoder = Decoder(units=input_shape[-1])

    def call(self, X):
        Z = self.encoder(X)
        return self.decoder(Z)

    def encode(self, X):
        return self.encoder(X)

    def decode(self, Z):
        return self.decode(Z)

Above begs a question whether separate Decoder and Encoder layers are really needed. IMO those should be left out, which leaves us only with this short and readable snippet:

class AutoEncoder(Model):
    def __init__(self, units):
        super(AutoEncoder, self).__init__()
        self.units = units

    def build(self, input_shape):
        self.encoder = Dense(units=self.units, activation=tf.nn.relu)
        self.decoder = Dense(units=input_shape[-1])

    def call(self, X):
        Z = self.encoder(X)
        return self.decoder(Z)

    def encode(self, X):
        return self.encoder(X)

    def decode(self, Z):
        return self.decode(Z)

BTW. You have an error in sample but that's a minor you can handle on your own no doubt.