Training a neural network on MNIST with Keras¶

This simple example demonstrates how to plug TensorFlow Datasets (TFDS) into a Keras model.

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import tensorflow as tf
import tensorflow_datasets as tfds

Step 1: Create your input pipeline¶

Start by building an efficient input pipeline using advices from:

The Performance tips guide
The Better performance with the tf.data API guide

Load a dataset¶

Load the MNIST dataset with the following arguments:

shuffle_files=True: The MNIST data is only stored in a single file, but for larger datasets with multiple files on disk, it's good practice to shuffle them when training.
as_supervised=True: Returns a tuple (img, label) instead of a dictionary {'image': img, 'label': label}.

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(ds_train, ds_test), ds_info = tfds.load(
    'mnist',
    split=['train', 'test'],
    shuffle_files=True,
    as_supervised=True,
    with_info=True,
)

Build a training pipeline¶

Apply the following transformations:

tf.data.Dataset.map: TFDS provide images of type tf.uint8, while the model expects tf.float32. Therefore, you need to normalize images.
tf.data.Dataset.cache As you fit the dataset in memory, cache it before shuffling for a better performance.

Note: Random transformations should be applied after caching.

tf.data.Dataset.shuffle: For true randomness, set the shuffle buffer to the full dataset size.

Note: For large datasets that can't fit in memory, use buffer_size=1000 if your system allows it.

tf.data.Dataset.batch: Batch elements of the dataset after shuffling to get unique batches at each epoch.
tf.data.Dataset.prefetch: It is good practice to end the pipeline by prefetching for performance.

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def normalize_img(image, label):
  """Normalizes images: `uint8` -> `float32`."""
  return tf.cast(image, tf.float32) / 255., label

ds_train = ds_train.map(
    normalize_img, num_parallel_calls=tf.data.AUTOTUNE)
ds_train = ds_train.cache()
ds_train = ds_train.shuffle(ds_info.splits['train'].num_examples)
ds_train = ds_train.batch(128)
ds_train = ds_train.prefetch(tf.data.AUTOTUNE)

Build an evaluation pipeline¶

Your testing pipeline is similar to the training pipeline with small differences:

You don't need to call tf.data.Dataset.shuffle.
Caching is done after batching because batches can be the same between epochs.

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ds_test = ds_test.map(
    normalize_img, num_parallel_calls=tf.data.AUTOTUNE)
ds_test = ds_test.batch(128)
ds_test = ds_test.cache()
ds_test = ds_test.prefetch(tf.data.AUTOTUNE)

Step 2: Create and train the model¶

Plug the TFDS input pipeline into a simple Keras model, compile the model, and train it.

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model = tf.keras.models.Sequential([
  tf.keras.layers.Flatten(input_shape=(28, 28)),
  tf.keras.layers.Dense(128, activation='relu'),
  tf.keras.layers.Dense(10)
])
model.compile(
    optimizer=tf.keras.optimizers.Adam(0.001),
    loss=tf.keras.losses.SparseCategoricalCrossentropy(from_logits=True),
    metrics=[tf.keras.metrics.SparseCategoricalAccuracy()],
)

model.fit(
    ds_train,
    epochs=6,
    validation_data=ds_test,
)