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Writing your own Tuner to support a custom training loop

Authors: Tom O'Malley, Haifeng Jin
Date created: 2019/10/28
Last modified: 2021/06/02
Description: Subclassing the Tuner class in KerasTuner for more customization like custom training loops.

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The Tuner class at keras_tuner.engine.tuner.Tuner can be subclassed to support advanced uses such as:

  • Custom training loops (GANs, reinforement learning, etc.)
  • Adding hyperparameters outside of the model builing function (preprocessing, data augmentation, test time augmentation, etc.)

This tutorial will not cover subclassing to support non-Keras models. To accomplish this, you can subclass the keras_tuner.engine.base_tuner.BaseTuner class (See keras_tuner.tuners.sklearn.Sklearn for an example).

Understanding the search process

Tuner.search can be passed any arguments. These arguments will be passed directly to Tuner.run_trial, along with a Trial object that contains information about the current trial, including hyperparameters and the status of the trial. Typically, Tuner.run_trial is the only method that users need to override when subclassing Tuner.

Overriding run_trial

There are two ways to write run_trial. One is to leverage Tuner's built-in callback hooks, which send the value of the objective to the Oracle and save the latest state of the Model. These hooks are:

  • self.on_epoch_end: Must be called. Reports results to the Oracle and saves the Model. The logs dictionary passed to this method must contain the objective name.
  • self.on_epoch_begin, self.on_batch_begin, self.on_batch_end: Optional. These methods do nothing in Tuner, but are useful to provide as hooks if you expect users of your subclass to create their own subclasses that override these parts of the training process.
class MyTuner(kt.Tuner):

    def run_trial(self, trial, ...):
        model = self.hypermodel.build(trial.hyperparameters)
        for epoch in range(10):
              epoch_loss = ...
              self.on_epoch_end(trial, model, epoch, logs={'loss': epoch_loss})

Alternatively, you can instead directly call the methods used to report results to the Oracle and save the Model. This can allow more flexibility for use cases where there is no natural concept of epoch or where you do not want to report results to the Oracle after each epoch. These methods are:

  • self.oracle.update_trial: Reports current results to the Oracle. The metrics dictionary passed to this method must contain the objective name.
  • self.save_model: Saves the trained model.
class MyTuner(kt.Tuner):

    def run_trial(self, trial, ...):
        model = self.hypermodel.build(trial.hyperparameters)
        score = ...
        self.oracle.update_trial(trial.trial_id, {'score': score})
        self.save_model(trial.trial_id, model)

Adding HyperParameters during preprocessing, evaluation, etc.

New HyperParameters can be defined anywhere in run_trial, in the same way that HyperParameters are defined in a HyperModel. These hyperparameters take on their default value the first time they are encountered, and thereafter are tuned by the Oracle.

class MyTuner(kt.Tuner):

    def run_trial(self, trial, ...):
        hp = trial.hyperparameters
        model = self.hypermodel.build(hp)

        batch_size = hp.Int('batch_size', 32, 128, step=32)
        random_flip = hp.Boolean('random_flip')

End-to-end Example:

import keras_tuner as kt
import tensorflow as tf
import numpy as np

def build_model(hp):
    """Builds a convolutional model."""
    inputs = tf.keras.Input(shape=(28, 28, 1))
    x = inputs
    for i in range(hp.Int("conv_layers", 1, 3, default=3)):
        x = tf.keras.layers.Conv2D(
            filters=hp.Int("filters_" + str(i), 4, 32, step=4, default=8),
            kernel_size=hp.Int("kernel_size_" + str(i), 3, 5),

        if hp.Choice("pooling" + str(i), ["max", "avg"]) == "max":
            x = tf.keras.layers.MaxPooling2D()(x)
            x = tf.keras.layers.AveragePooling2D()(x)

        x = tf.keras.layers.BatchNormalization()(x)
        x = tf.keras.layers.ReLU()(x)

    if hp.Choice("global_pooling", ["max", "avg"]) == "max":
        x = tf.keras.layers.GlobalMaxPooling2D()(x)
        x = tf.keras.layers.GlobalAveragePooling2D()(x)
    outputs = tf.keras.layers.Dense(10, activation="softmax")(x)

    model = tf.keras.Model(inputs, outputs)

    optimizer = hp.Choice("optimizer", ["adam", "sgd"])
        optimizer, loss="sparse_categorical_crossentropy", metrics=["accuracy"]
    return model

class MyTuner(kt.Tuner):
    def run_trial(self, trial, train_ds):
        hp = trial.hyperparameters

        # Hyperparameters can be added anywhere inside `run_trial`.
        # When the first trial is run, they will take on their default values.
        # Afterwards, they will be tuned by the `Oracle`.
        train_ds = train_ds.batch(hp.Int("batch_size", 32, 128, step=32, default=64))

        model = self.hypermodel.build(trial.hyperparameters)
        lr = hp.Float("learning_rate", 1e-4, 1e-2, sampling="log", default=1e-3)
        optimizer = tf.keras.optimizers.Adam(lr)
        epoch_loss_metric = tf.keras.metrics.Mean()
        loss_fn = tf.keras.losses.SparseCategoricalCrossentropy()

        # @tf.function
        def run_train_step(data):
            images = tf.dtypes.cast(data[0], "float32") / 255.0
            labels = data[1]
            with tf.GradientTape() as tape:
                logits = model(images)
                loss = loss_fn(labels, logits)
                # Add any regularization losses.
                if model.losses:
                    loss += tf.math.add_n(model.losses)
            gradients = tape.gradient(loss, model.trainable_variables)
            optimizer.apply_gradients(zip(gradients, model.trainable_variables))
            return loss

        # `self.on_epoch_end` reports results to the `Oracle` and saves the
        # current state of the Model. The other hooks called here only log values
        # for display but can also be overridden. For use cases where there is no
        # natural concept of epoch, you do not have to call any of these hooks. In
        # this case you should instead call `self.oracle.update_trial` and
        # `self.oracle.save_model` manually.
        for epoch in range(2):
            print("Epoch: {}".format(epoch))

            self.on_epoch_begin(trial, model, epoch, logs={})
            for batch, data in enumerate(train_ds):
                self.on_batch_begin(trial, model, batch, logs={})
                batch_loss = float(run_train_step(data))
                self.on_batch_end(trial, model, batch, logs={"loss": batch_loss})

                if batch % 100 == 0:
                    loss = epoch_loss_metric.result().numpy()
                    print("Batch: {}, Average Loss: {}".format(batch, loss))

            epoch_loss = epoch_loss_metric.result().numpy()
            self.on_epoch_end(trial, model, epoch, logs={"loss": epoch_loss})

tuner = MyTuner(
        objective=kt.Objective("loss", "min"), max_trials=2

(x_train, y_train), (x_test, y_test) = tf.keras.datasets.mnist.load_data()

# Reshape the images to have the channel dimension.
x_train = x_train.reshape(x_train.shape + (1,))[:1000]
y_train = y_train.astype(np.int64)[:1000]

mnist_train = tf.data.Dataset.from_tensor_slices((x_train, y_train))


best_hps = tuner.get_best_hyperparameters()[0]

best_model = tuner.get_best_models()[0]
Trial 2 Complete [00h 00m 00s]
loss: 2.2953741550445557
Best loss So Far: 2.294417381286621
Total elapsed time: 00h 00m 01s
INFO:tensorflow:Oracle triggered exit
{'conv_layers': 2, 'filters_0': 12, 'kernel_size_0': 4, 'pooling0': 'max', 'filters_1': 16, 'kernel_size_1': 4, 'pooling1': 'avg', 'filters_2': 20, 'kernel_size_2': 4, 'pooling2': 'avg', 'global_pooling': 'avg', 'optimizer': 'adam', 'batch_size': 64, 'learning_rate': 0.001}