Text generation with an RNN

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This tutorial demonstrates how to generate text using a character-based RNN. You will work with a dataset of Shakespeare's writing from Andrej Karpathy's The Unreasonable Effectiveness of Recurrent Neural Networks. Given a sequence of characters from this data ("Shakespear"), train a model to predict the next character in the sequence ("e"). Longer sequences of text can be generated by calling the model repeatedly.

This tutorial includes runnable code implemented using tf.keras and eager execution. The following is sample output when the model in this tutorial trained for 30 epochs, and started with the string "Q":

I had thought thou hadst a Roman; for the oracle,
Thus by All bids the man against the word,
Which are so weak of care, by old care done;
Your children were in your holy love,
And the precipitation through the bleeding throne.

Marry, and will, my lord, to weep in such a one were prettiest;
Yet now I was adopted heir
Of the world's lamentable day,
To watch the next way with his father with his face?

The cause why then we are all resolved more sons.

O, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, it is no sin it should be dead,
And love and pale as any will to that word.

But how long have I heard the soul for this world,
And show his hands of life be proved to stand.

I say he look'd on, if I must be content
To stay him from the fatal of our country's bliss.
His lordship pluck'd from this sentence then for prey,
And then let us twain, being the moon,
were she such a case as fills m

While some of the sentences are grammatical, most do not make sense. The model has not learned the meaning of words, but consider:

  • The model is character-based. When training started, the model did not know how to spell an English word, or that words were even a unit of text.

  • The structure of the output resembles a play—blocks of text generally begin with a speaker name, in all capital letters similar to the dataset.

  • As demonstrated below, the model is trained on small batches of text (100 characters each), and is still able to generate a longer sequence of text with coherent structure.


Import TensorFlow and other libraries

import tensorflow as tf

import numpy as np
import os
import time

Download the Shakespeare dataset

Change the following line to run this code on your own data.

path_to_file = tf.keras.utils.get_file('shakespeare.txt', 'https://storage.googleapis.com/download.tensorflow.org/data/shakespeare.txt')
Downloading data from https://storage.googleapis.com/download.tensorflow.org/data/shakespeare.txt
1122304/1115394 [==============================] - 0s 0us/step

Read the data

First, look in the text:

# Read, then decode for py2 compat.
text = open(path_to_file, 'rb').read().decode(encoding='utf-8')
# length of text is the number of characters in it
print('Length of text: {} characters'.format(len(text)))
Length of text: 1115394 characters

# Take a look at the first 250 characters in text
First Citizen:
Before we proceed any further, hear me speak.

Speak, speak.

First Citizen:
You are all resolved rather to die than to famish?

Resolved. resolved.

First Citizen:
First, you know Caius Marcius is chief enemy to the people.

# The unique characters in the file
vocab = sorted(set(text))
print('{} unique characters'.format(len(vocab)))
65 unique characters

Process the text

Vectorize the text

Before training, you need to map strings to a numerical representation. Create two lookup tables: one mapping characters to numbers, and another for numbers to characters.

# Creating a mapping from unique characters to indices
char2idx = {u:i for i, u in enumerate(vocab)}
idx2char = np.array(vocab)

text_as_int = np.array([char2idx[c] for c in text])

Now you have an integer representation for each character. Notice that you mapped the character as indexes from 0 to len(unique).

for char,_ in zip(char2idx, range(20)):
    print('  {:4s}: {:3d},'.format(repr(char), char2idx[char]))
print('  ...\n}')
  '\n':   0,
  ' ' :   1,
  '!' :   2,
  '$' :   3,
  '&' :   4,
  "'" :   5,
  ',' :   6,
  '-' :   7,
  '.' :   8,
  '3' :   9,
  ':' :  10,
  ';' :  11,
  '?' :  12,
  'A' :  13,
  'B' :  14,
  'C' :  15,
  'D' :  16,
  'E' :  17,
  'F' :  18,
  'G' :  19,

# Show how the first 13 characters from the text are mapped to integers
print('{} ---- characters mapped to int ---- > {}'.format(repr(text[:13]), text_as_int[:13]))
'First Citizen' ---- characters mapped to int ---- > [18 47 56 57 58  1 15 47 58 47 64 43 52]

The prediction task

Given a character, or a sequence of characters, what is the most probable next character? This is the task you're training the model to perform. The input to the model will be a sequence of characters, and you train the model to predict the output—the following character at each time step.

Since RNNs maintain an internal state that depends on the previously seen elements, given all the characters computed until this moment, what is the next character?

Create training examples and targets

Next divide the text into example sequences. Each input sequence will contain seq_length characters from the text.

For each input sequence, the corresponding targets contain the same length of text, except shifted one character to the right.

So break the text into chunks of seq_length+1. For example, say seq_length is 4 and our text is "Hello". The input sequence would be "Hell", and the target sequence "ello".

To do this first use the tf.data.Dataset.from_tensor_slices function to convert the text vector into a stream of character indices.

# The maximum length sentence you want for a single input in characters
seq_length = 100
examples_per_epoch = len(text)//(seq_length+1)

# Create training examples / targets
char_dataset = tf.data.Dataset.from_tensor_slices(text_as_int)

for i in char_dataset.take(5):

The batch method lets us easily convert these individual characters to sequences of the desired size.

sequences = char_dataset.batch(seq_length+1, drop_remainder=True)

for item in sequences.take(5):
'First Citizen:\nBefore we proceed any further, hear me speak.\n\nAll:\nSpeak, speak.\n\nFirst Citizen:\nYou '
'are all resolved rather to die than to famish?\n\nAll:\nResolved. resolved.\n\nFirst Citizen:\nFirst, you k'
"now Caius Marcius is chief enemy to the people.\n\nAll:\nWe know't, we know't.\n\nFirst Citizen:\nLet us ki"
"ll him, and we'll have corn at our own price.\nIs't a verdict?\n\nAll:\nNo more talking on't; let it be d"
'one: away, away!\n\nSecond Citizen:\nOne word, good citizens.\n\nFirst Citizen:\nWe are accounted poor citi'

For each sequence, duplicate and shift it to form the input and target text by using the map method to apply a simple function to each batch:

def split_input_target(chunk):
    input_text = chunk[:-1]
    target_text = chunk[1:]
    return input_text, target_text

dataset = sequences.map(split_input_target)

Print the first example input and target values:

for input_example, target_example in  dataset.take(1):
    print('Input data: ', repr(''.join(idx2char[input_example.numpy()])))
    print('Target data:', repr(''.join(idx2char[target_example.numpy()])))
Input data:  'First Citizen:\nBefore we proceed any further, hear me speak.\n\nAll:\nSpeak, speak.\n\nFirst Citizen:\nYou'
Target data: 'irst Citizen:\nBefore we proceed any further, hear me speak.\n\nAll:\nSpeak, speak.\n\nFirst Citizen:\nYou '

Each index of these vectors is processed as a one time step. For the input at time step 0, the model receives the index for "F" and tries to predict the index for "i" as the next character. At the next timestep, it does the same thing but the RNN considers the previous step context in addition to the current input character.

for i, (input_idx, target_idx) in enumerate(zip(input_example[:5], target_example[:5])):
    print("Step {:4d}".format(i))
    print("  input: {} ({:s})".format(input_idx, repr(idx2char[input_idx])))
    print("  expected output: {} ({:s})".format(target_idx, repr(idx2char[target_idx])))
Step    0
  input: 18 ('F')
  expected output: 47 ('i')
Step    1
  input: 47 ('i')
  expected output: 56 ('r')
Step    2
  input: 56 ('r')
  expected output: 57 ('s')
Step    3
  input: 57 ('s')
  expected output: 58 ('t')
Step    4
  input: 58 ('t')
  expected output: 1 (' ')

Create training batches

You used tf.data to split the text into manageable sequences. But before feeding this data into the model, you need to shuffle the data and pack it into batches.

# Batch size

# Buffer size to shuffle the dataset
# (TF data is designed to work with possibly infinite sequences,
# so it doesn't attempt to shuffle the entire sequence in memory. Instead,
# it maintains a buffer in which it shuffles elements).

dataset = dataset.shuffle(BUFFER_SIZE).batch(BATCH_SIZE, drop_remainder=True)

<BatchDataset shapes: ((64, 100), (64, 100)), types: (tf.int64, tf.int64)>

Build The Model

Use tf.keras.Sequential to define the model. For this simple example three layers are used to define our model:

# Length of the vocabulary in chars
vocab_size = len(vocab)

# The embedding dimension
embedding_dim = 256

# Number of RNN units
rnn_units = 1024
def build_model(vocab_size, embedding_dim, rnn_units, batch_size):
    model = tf.keras.Sequential([
        tf.keras.layers.Embedding(vocab_size, embedding_dim,
                                  batch_input_shape=[batch_size, None]),
    return model
model = build_model(

For each character the model looks up the embedding, runs the GRU one timestep with the embedding as input, and applies the dense layer to generate logits predicting the log-likelihood of the next character:

A drawing of the data passing through the model

Please note that Keras sequential model is used here since all the layers in the model only have single input and produce single output. In case you want to retrieve and reuse the states from stateful RNN layer, you might want to build your model with Keras functional API or model subclassing. Please check Keras RNN guide for more details.

Try the model

Now run the model to see that it behaves as expected.

First check the shape of the output:

for input_example_batch, target_example_batch in dataset.take(1):
    example_batch_predictions = model(input_example_batch)
    print(example_batch_predictions.shape, "# (batch_size, sequence_length, vocab_size)")
(64, 100, 65) # (batch_size, sequence_length, vocab_size)

In the above example the sequence length of the input is 100 but the model can be run on inputs of any length:

Model: "sequential"
Layer (type)                 Output Shape              Param #   
embedding (Embedding)        (64, None, 256)           16640     
gru (GRU)                    (64, None, 1024)          3938304   
dense (Dense)                (64, None, 65)            66625     
Total params: 4,021,569
Trainable params: 4,021,569
Non-trainable params: 0

To get actual predictions from the model you need to sample from the output distribution, to get actual character indices. This distribution is defined by the logits over the character vocabulary.

Try it for the first example in the batch:

sampled_indices = tf.random.categorical(example_batch_predictions[0], num_samples=1)
sampled_indices = tf.squeeze(sampled_indices,axis=-1).numpy()

This gives us, at each timestep, a prediction of the next character index:

array([41, 60,  3, 31, 47, 21, 61,  6, 56, 42, 39, 40, 52, 60, 37, 37, 27,
       11,  6, 56, 64, 62, 43, 42,  6, 34,  1, 30, 16, 45, 46, 11, 17,  8,
       26,  8,  1, 46, 37, 21, 37, 53, 34, 49,  5, 58, 11,  9, 42, 62, 14,
       56, 56, 30, 31, 32, 63, 53, 10, 23, 35,  5, 19, 19, 46,  3, 23, 63,
       61, 11, 57,  0, 35, 48, 32,  4, 37,  7, 48, 23, 39, 30, 20, 26,  1,
       52, 57, 23, 46, 56, 11, 22,  7, 47, 16, 27, 38, 51, 55, 28])

Decode these to see the text predicted by this untrained model:

print("Input: \n", repr("".join(idx2char[input_example_batch[0]])))
print("Next Char Predictions: \n", repr("".join(idx2char[sampled_indices ])))
 "dness! Make not impossible\nThat which but seems unlike: 'tis not impossible\nBut one, the wicked'st c"

Next Char Predictions: 
 "cv$SiIw,rdabnvYYO;,rzxed,V RDgh;E.N. hYIYoVk't;3dxBrrRSTyo:KW'GGh$Kyw;s\nWjT&Y-jKaRHN nsKhr;J-iDOZmqP"

Train the model

At this point the problem can be treated as a standard classification problem. Given the previous RNN state, and the input this time step, predict the class of the next character.

Attach an optimizer, and a loss function

The standard tf.keras.losses.sparse_categorical_crossentropy loss function works in this case because it is applied across the last dimension of the predictions.

Because your model returns logits, you need to set the from_logits flag.

def loss(labels, logits):
    return tf.keras.losses.sparse_categorical_crossentropy(labels, logits, from_logits=True)

example_batch_loss = loss(target_example_batch, example_batch_predictions)
print("Prediction shape: ", example_batch_predictions.shape, " # (batch_size, sequence_length, vocab_size)")
print("scalar_loss:      ", example_batch_loss.numpy().mean())
Prediction shape:  (64, 100, 65)  # (batch_size, sequence_length, vocab_size)
scalar_loss:       4.174373

Configure the training procedure using the tf.keras.Model.compile method. Use tf.keras.optimizers.Adam with default arguments and the loss function.

model.compile(optimizer='adam', loss=loss)

Configure checkpoints

Use a tf.keras.callbacks.ModelCheckpoint to ensure that checkpoints are saved during training:

# Directory where the checkpoints will be saved
checkpoint_dir = './training_checkpoints'
# Name of the checkpoint files
checkpoint_prefix = os.path.join(checkpoint_dir, "ckpt_{epoch}")

checkpoint_callback = tf.keras.callbacks.ModelCheckpoint(

Execute the training

To keep training time reasonable, use 10 epochs to train the model. In Colab, set the runtime to GPU for faster training.

history = model.fit(dataset, epochs=EPOCHS, callbacks=[checkpoint_callback])
Epoch 1/10
172/172 [==============================] - 5s 27ms/step - loss: 2.6807
Epoch 2/10
172/172 [==============================] - 5s 27ms/step - loss: 1.9748
Epoch 3/10
172/172 [==============================] - 5s 26ms/step - loss: 1.7063
Epoch 4/10
172/172 [==============================] - 5s 26ms/step - loss: 1.5543
Epoch 5/10
172/172 [==============================] - 5s 27ms/step - loss: 1.4633
Epoch 6/10
172/172 [==============================] - 5s 26ms/step - loss: 1.4028
Epoch 7/10
172/172 [==============================] - 5s 26ms/step - loss: 1.3568
Epoch 8/10
172/172 [==============================] - 5s 26ms/step - loss: 1.3187
Epoch 9/10
172/172 [==============================] - 5s 26ms/step - loss: 1.2845
Epoch 10/10
172/172 [==============================] - 5s 26ms/step - loss: 1.2528

Generate text

Restore the latest checkpoint

To keep this prediction step simple, use a batch size of 1.

Because of the way the RNN state is passed from timestep to timestep, the model only accepts a fixed batch size once built.

To run the model with a different batch_size, you need to rebuild the model and restore the weights from the checkpoint.

model = build_model(vocab_size, embedding_dim, rnn_units, batch_size=1)


model.build(tf.TensorShape([1, None]))
Model: "sequential_1"
Layer (type)                 Output Shape              Param #   
embedding_1 (Embedding)      (1, None, 256)            16640     
gru_1 (GRU)                  (1, None, 1024)           3938304   
dense_1 (Dense)              (1, None, 65)             66625     
Total params: 4,021,569
Trainable params: 4,021,569
Non-trainable params: 0

The prediction loop

The following code block generates the text:

  • Begin by choosing a start string, initializing the RNN state and setting the number of characters to generate.

  • Get the prediction distribution of the next character using the start string and the RNN state.

  • Then, use a categorical distribution to calculate the index of the predicted character. Use this predicted character as our next input to the model.

  • The RNN state returned by the model is fed back into the model so that it now has more context, instead of only one character. After predicting the next character, the modified RNN states are again fed back into the model, which is how it learns as it gets more context from the previously predicted characters.

To generate text the model's output is fed back to the input

Looking at the generated text, you'll see the model knows when to capitalize, make paragraphs and imitates a Shakespeare-like writing vocabulary. With the small number of training epochs, it has not yet learned to form coherent sentences.

def generate_text(model, start_string):
    # Evaluation step (generating text using the learned model)

    # Number of characters to generate
    num_generate = 1000

    # Converting our start string to numbers (vectorizing)
    input_eval = [char2idx[s] for s in start_string]
    input_eval = tf.expand_dims(input_eval, 0)

    # Empty string to store our results
    text_generated = []

    # Low temperature results in more predictable text.
    # Higher temperature results in more surprising text.
    # Experiment to find the best setting.
    temperature = 1.0

    # Here batch size == 1
    for i in range(num_generate):
        predictions = model(input_eval)
        # remove the batch dimension
        predictions = tf.squeeze(predictions, 0)

        # using a categorical distribution to predict the character returned by the model
        predictions = predictions / temperature
        predicted_id = tf.random.categorical(predictions, num_samples=1)[-1,0].numpy()

        # Pass the predicted character as the next input to the model
        # along with the previous hidden state
        input_eval = tf.expand_dims([predicted_id], 0)


    return (start_string + ''.join(text_generated))
print(generate_text(model, start_string=u"ROMEO: "))
ROMEO: ghast I cut go,
Know the normander and the wrong:
To our Morsuis misdress are behiod;
And after as if no other husion.

Your father and of worms?

Your hot can dost.

Then, atient the bade, truckle aid,
Dearve your tongue should be cred to our face,
Bear trouble my father valiant,' in the company.

O God!'Sir afeard?

Come, good med,---or whom by the duke?

Yes, that are bore indocation!

None not, my lord's sons.

Of some King?'
And, if thou was, a partanot young to thee.

O, tell; then I'll see them again? There's not so reder
no mother, and my three here to us. You might shall not speak, these this
same this within; what armpy I might
but though some way.

Our daughter of the fool, that great come.
So, not the sun summer so all the sends,
Your ludgers made before the souls of years, and thereby there. Lady, father, were well the sold, pass, remeate.

Second King Richard's daughter,
Which chee

The easiest thing you can do to improve the results is to train it for longer (try EPOCHS = 30).

You can also experiment with a different start string, try adding another RNN layer to improve the model's accuracy, or adjust the temperature parameter to generate more or less random predictions.

Advanced: Customized Training

The above training procedure is simple, but does not give you much control.

So now that you've seen how to run the model manually let's unpack the training loop, and implement it ourselves. This gives a starting point if, for example, you want to implement curriculum learning to help stabilize the model's open-loop output.

Use tf.GradientTape to track the gradients. You can learn more about this approach by reading the eager execution guide.

The procedure works as follows:

  • First, reset the RNN state. You do this by calling the tf.keras.Model.reset_states method.

  • Next, iterate over the dataset (batch by batch) and calculate the predictions associated with each.

  • Open a tf.GradientTape, and calculate the predictions and loss in that context.

  • Calculate the gradients of the loss with respect to the model variables using the tf.GradientTape.grads method.

  • Finally, take a step downwards by using the optimizer's tf.train.Optimizer.apply_gradients method.

model = build_model(
WARNING:tensorflow:Unresolved object in checkpoint: (root).optimizer
WARNING:tensorflow:Unresolved object in checkpoint: (root).optimizer.iter
WARNING:tensorflow:Unresolved object in checkpoint: (root).optimizer.beta_1
WARNING:tensorflow:Unresolved object in checkpoint: (root).optimizer.beta_2
WARNING:tensorflow:Unresolved object in checkpoint: (root).optimizer.decay
WARNING:tensorflow:Unresolved object in checkpoint: (root).optimizer.learning_rate
WARNING:tensorflow:Unresolved object in checkpoint: (root).optimizer's state 'm' for (root).layer_with_weights-0.embeddings
WARNING:tensorflow:Unresolved object in checkpoint: (root).optimizer's state 'm' for (root).layer_with_weights-2.kernel
WARNING:tensorflow:Unresolved object in checkpoint: (root).optimizer's state 'm' for (root).layer_with_weights-2.bias
WARNING:tensorflow:Unresolved object in checkpoint: (root).optimizer's state 'm' for (root).layer_with_weights-1.cell.kernel
WARNING:tensorflow:Unresolved object in checkpoint: (root).optimizer's state 'm' for (root).layer_with_weights-1.cell.recurrent_kernel
WARNING:tensorflow:Unresolved object in checkpoint: (root).optimizer's state 'm' for (root).layer_with_weights-1.cell.bias
WARNING:tensorflow:Unresolved object in checkpoint: (root).optimizer's state 'v' for (root).layer_with_weights-0.embeddings
WARNING:tensorflow:Unresolved object in checkpoint: (root).optimizer's state 'v' for (root).layer_with_weights-2.kernel
WARNING:tensorflow:Unresolved object in checkpoint: (root).optimizer's state 'v' for (root).layer_with_weights-2.bias
WARNING:tensorflow:Unresolved object in checkpoint: (root).optimizer's state 'v' for (root).layer_with_weights-1.cell.kernel
WARNING:tensorflow:Unresolved object in checkpoint: (root).optimizer's state 'v' for (root).layer_with_weights-1.cell.recurrent_kernel
WARNING:tensorflow:Unresolved object in checkpoint: (root).optimizer's state 'v' for (root).layer_with_weights-1.cell.bias
WARNING:tensorflow:A checkpoint was restored (e.g. tf.train.Checkpoint.restore or tf.keras.Model.load_weights) but not all checkpointed values were used. See above for specific issues. Use expect_partial() on the load status object, e.g. tf.train.Checkpoint.restore(...).expect_partial(), to silence these warnings, or use assert_consumed() to make the check explicit. See https://www.tensorflow.org/guide/checkpoint#loading_mechanics for details.

optimizer = tf.keras.optimizers.Adam()
def train_step(inp, target):
    with tf.GradientTape() as tape:
        predictions = model(inp)
        loss = tf.reduce_mean(
                target, predictions, from_logits=True))
    grads = tape.gradient(loss, model.trainable_variables)
    optimizer.apply_gradients(zip(grads, model.trainable_variables))

    return loss
# Training step

for epoch in range(EPOCHS):
    start = time.time()

    # resetting the hidden state at the start of every epoch

    for (batch_n, (inp, target)) in enumerate(dataset):
        loss = train_step(inp, target)

        if batch_n % 100 == 0:
            template = 'Epoch {} Batch {} Loss {}'
            print(template.format(epoch + 1, batch_n, loss))

    # saving (checkpoint) the model every 5 epochs
    if (epoch + 1) % 5 == 0:

    print('Epoch {} Loss {:.4f}'.format(epoch + 1, loss))
    print('Time taken for 1 epoch {} sec\n'.format(time.time() - start))

Epoch 1 Batch 0 Loss 4.174976348876953
Epoch 1 Batch 100 Loss 2.351067304611206
Epoch 1 Loss 2.1421
Time taken for 1 epoch 6.3171796798706055 sec

Epoch 2 Batch 0 Loss 2.166642665863037
Epoch 2 Batch 100 Loss 1.9492360353469849
Epoch 2 Loss 1.7901
Time taken for 1 epoch 5.3413612842559814 sec

Epoch 3 Batch 0 Loss 1.804692029953003
Epoch 3 Batch 100 Loss 1.6545528173446655
Epoch 3 Loss 1.6328
Time taken for 1 epoch 5.337632179260254 sec

Epoch 4 Batch 0 Loss 1.6188888549804688
Epoch 4 Batch 100 Loss 1.5314372777938843
Epoch 4 Loss 1.5319
Time taken for 1 epoch 5.2844321727752686 sec

Epoch 5 Batch 0 Loss 1.470827579498291
Epoch 5 Batch 100 Loss 1.4400928020477295
Epoch 5 Loss 1.4442
Time taken for 1 epoch 5.46646785736084 sec

Epoch 6 Batch 0 Loss 1.4113285541534424
Epoch 6 Batch 100 Loss 1.387071132659912
Epoch 6 Loss 1.3713
Time taken for 1 epoch 5.243147373199463 sec

Epoch 7 Batch 0 Loss 1.3486154079437256
Epoch 7 Batch 100 Loss 1.353363037109375
Epoch 7 Loss 1.3270
Time taken for 1 epoch 5.295132160186768 sec

Epoch 8 Batch 0 Loss 1.2960264682769775
Epoch 8 Batch 100 Loss 1.3038402795791626
Epoch 8 Loss 1.3556
Time taken for 1 epoch 5.228798151016235 sec

Epoch 9 Batch 0 Loss 1.2495232820510864
Epoch 9 Batch 100 Loss 1.30863618850708
Epoch 9 Loss 1.2699
Time taken for 1 epoch 5.33559775352478 sec

Epoch 10 Batch 0 Loss 1.2161246538162231
Epoch 10 Batch 100 Loss 1.2242770195007324
Epoch 10 Loss 1.2360
Time taken for 1 epoch 5.377742528915405 sec