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Migrating feature_columns to TF2's Keras Preprocessing Layers

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Training a model will usually come with some amount of feature preprocessing, particularly when dealing with structured data. When training a tf.estimator.Estimator in TF1, this feature preprocessing is usually done with the tf.feature_column API. In TF2, this preprocessing can be done directly with Keras layers, called preprocessing layers.

In this migration guide, you will perform some common feature transformations using both feature columns and preprocessing layers, followed by training a complete model with both APIs.

First, start with a couple of necessary imports,

import tensorflow as tf
import tensorflow.compat.v1 as tf1
import math

and add a utility for calling a feature column for demonstration:

def call_feature_columns(feature_columns, inputs):
  # This is a convenient way to call a `feature_column` outside of an estimator
  # to display its output.
  feature_layer = tf1.keras.layers.DenseFeatures(feature_columns)
  return feature_layer(inputs)

Input handling

To use feature columns with an estimator, model inputs are always expected to be a dictionary of tensors:

input_dict = {
  'foo': tf.constant([1]),
  'bar': tf.constant([0]),
  'baz': tf.constant([-1])
}

Each feature column needs to be created with a key to index into the source data. The output of all feature columns is concatenated and used by the estimator model.

columns = [
  tf1.feature_column.numeric_column('foo'),
  tf1.feature_column.numeric_column('bar'),
  tf1.feature_column.numeric_column('baz'),
]
call_feature_columns(columns, input_dict)
<tf.Tensor: shape=(1, 3), dtype=float32, numpy=array([[ 0., -1.,  1.]], dtype=float32)>

In Keras, model input is much more flexible. A tf.keras.Model can handle a single tensor input, a list of tensor features, or a dictionary of tensor features. You can handle dictionary input by passing a dictionary of tf.keras.Input on model creation. Inputs will not be concatenated automatically, which allows them to be used in much more flexible ways. They can be concatenated with tf.keras.layers.Concatenate.

inputs = {
  'foo': tf.keras.Input(shape=()),
  'bar': tf.keras.Input(shape=()),
  'baz': tf.keras.Input(shape=()),
}
# Inputs are typically transformed by preprocessing layers before concatenation.
outputs = tf.keras.layers.Concatenate()(inputs.values())
model = tf.keras.Model(inputs=inputs, outputs=outputs)
model(input_dict)
<tf.Tensor: shape=(3,), dtype=float32, numpy=array([ 1.,  0., -1.], dtype=float32)>

One-hot encoding integer IDs

A common feature transformation is one-hot encoding integer inputs of a known range. Here is an example using feature columns:

categorical_col = tf1.feature_column.categorical_column_with_identity(
    'type', num_buckets=3)
indicator_col = tf1.feature_column.indicator_column(categorical_col)
call_feature_columns(indicator_col, {'type': [0, 1, 2]})
<tf.Tensor: shape=(3, 3), dtype=float32, numpy=
array([[1., 0., 0.],
       [0., 1., 0.],
       [0., 0., 1.]], dtype=float32)>

Using Keras preprocessing layers, these columns can be replaced by a single tf.keras.layers.CategoryEncoding layer with output_mode set to 'one_hot':

one_hot_layer = tf.keras.layers.CategoryEncoding(
    num_tokens=3, output_mode='one_hot')
one_hot_layer([0, 1, 2])
<tf.Tensor: shape=(3, 3), dtype=float32, numpy=
array([[1., 0., 0.],
       [0., 1., 0.],
       [0., 0., 1.]], dtype=float32)>

Normalizing numeric features

When handling continuous, floating-point features with feature columns, you need to use a tf.feature_column.numeric_column. In the case where the input is already normalized, converting this to Keras is trivial. You can simply use a tf.keras.Input directly into your model, as shown above.

A numeric_column can also be used to normalize input:

def normalize(x):
  mean, variance = (2.0, 1.0)
  return (x - mean) / math.sqrt(variance)
numeric_col = tf1.feature_column.numeric_column('col', normalizer_fn=normalize)
call_feature_columns(numeric_col, {'col': tf.constant([[0.], [1.], [2.]])})
<tf.Tensor: shape=(3, 1), dtype=float32, numpy=
array([[-2.],
       [-1.],
       [ 0.]], dtype=float32)>

In contrast, with Keras, this normalization can be done with tf.keras.layers.Normalization.

normalization_layer = tf.keras.layers.Normalization(mean=2.0, variance=1.0)
normalization_layer(tf.constant([[0.], [1.], [2.]]))
<tf.Tensor: shape=(3, 1), dtype=float32, numpy=
array([[-2.],
       [-1.],
       [ 0.]], dtype=float32)>

Bucketizing and one-hot encoding numeric features

Another common transformation of continuous, floating point inputs is to bucketize then to integers of a fixed range.

In feature columns, this can be achieved with a tf.feature_column.bucketized_column:

numeric_col = tf1.feature_column.numeric_column('col')
bucketized_col = tf1.feature_column.bucketized_column(numeric_col, [1, 4, 5])
call_feature_columns(bucketized_col, {'col': tf.constant([1., 2., 3., 4., 5.])})
<tf.Tensor: shape=(5, 4), dtype=float32, numpy=
array([[0., 1., 0., 0.],
       [0., 1., 0., 0.],
       [0., 1., 0., 0.],
       [0., 0., 1., 0.],
       [0., 0., 0., 1.]], dtype=float32)>

In Keras, this can be replaced by tf.keras.layers.Discretization:

discretization_layer = tf.keras.layers.Discretization(bin_boundaries=[1, 4, 5])
one_hot_layer = tf.keras.layers.CategoryEncoding(
    num_tokens=4, output_mode='one_hot')
one_hot_layer(discretization_layer([1., 2., 3., 4., 5.]))
<tf.Tensor: shape=(5, 4), dtype=float32, numpy=
array([[0., 1., 0., 0.],
       [0., 1., 0., 0.],
       [0., 1., 0., 0.],
       [0., 0., 1., 0.],
       [0., 0., 0., 1.]], dtype=float32)>

One-hot encoding string data with a vocabulary

Handling string features often requires a vocabulary lookup to translate strings into indices. Here is an example using feature columns to lookup strings and then one-hot encode the indices:

vocab_col = tf1.feature_column.categorical_column_with_vocabulary_list(
    'sizes',
    vocabulary_list=['small', 'medium', 'large'],
    num_oov_buckets=0)
indicator_col = tf1.feature_column.indicator_column(vocab_col)
call_feature_columns(indicator_col, {'sizes': ['small', 'medium', 'large']})
<tf.Tensor: shape=(3, 3), dtype=float32, numpy=
array([[1., 0., 0.],
       [0., 1., 0.],
       [0., 0., 1.]], dtype=float32)>

Using Keras preprocessing layers, use the tf.keras.layers.StringLookup layer with output_mode set to 'one_hot':

string_lookup_layer = tf.keras.layers.StringLookup(
    vocabulary=['small', 'medium', 'large'],
    num_oov_indices=0,
    output_mode='one_hot')
string_lookup_layer(['small', 'medium', 'large'])
<tf.Tensor: shape=(3, 3), dtype=float32, numpy=
array([[1., 0., 0.],
       [0., 1., 0.],
       [0., 0., 1.]], dtype=float32)>

Embedding string data with a vocabulary

For larger vocabularies, an embedding is often needed for good performance. Here is an example embedding a string feature using feature columns:

vocab_col = tf1.feature_column.categorical_column_with_vocabulary_list(
    'col',
    vocabulary_list=['small', 'medium', 'large'],
    num_oov_buckets=0)
embedding_col = tf1.feature_column.embedding_column(vocab_col, 4)
call_feature_columns(embedding_col, {'col': ['small', 'medium', 'large']})
<tf.Tensor: shape=(3, 4), dtype=float32, numpy=
array([[-0.01798586, -0.2808677 ,  0.27639154,  0.06081508],
       [ 0.05771849,  0.02464074,  0.20080602,  0.50164527],
       [-0.9208247 , -0.40816694, -0.49132794,  0.9203153 ]],
      dtype=float32)>

Using Keras preprocessing layers, this can be achieved by combining a tf.keras.layers.StringLookup layer and an tf.keras.layers.Embedding layer. The default output for the StringLookup will be integer indices which can be fed directly into an embedding.

string_lookup_layer = tf.keras.layers.StringLookup(
    vocabulary=['small', 'medium', 'large'], num_oov_indices=0)
embedding = tf.keras.layers.Embedding(3, 4)
embedding(string_lookup_layer(['small', 'medium', 'large']))
<tf.Tensor: shape=(3, 4), dtype=float32, numpy=
array([[ 0.04838837, -0.04014301,  0.02001903, -0.01150769],
       [-0.04580117, -0.04319514,  0.03725603, -0.00572466],
       [-0.0401094 ,  0.00997342,  0.00111955,  0.00132702]],
      dtype=float32)>

Summing weighted categorical data

In some cases, you need to deal with categorical data where each occurance of a category comes with an associated weight. In feature columns, this is handled with tf.feature_column.weighted_categorical_column. When paired with an indicator_column, this has the effect of summing weights per category.

ids = tf.constant([[5, 11, 5, 17, 17]])
weights = tf.constant([[0.5, 1.5, 0.7, 1.8, 0.2]])

categorical_col = tf1.feature_column.categorical_column_with_identity(
    'ids', num_buckets=20)
weighted_categorical_col = tf1.feature_column.weighted_categorical_column(
    categorical_col, 'weights')
indicator_col = tf1.feature_column.indicator_column(weighted_categorical_col)
call_feature_columns(indicator_col, {'ids': ids, 'weights': weights})
WARNING:tensorflow:From /tmpfs/src/tf_docs_env/lib/python3.7/site-packages/tensorflow/python/feature_column/feature_column_v2.py:4203: sparse_merge (from tensorflow.python.ops.sparse_ops) is deprecated and will be removed in a future version.
Instructions for updating:
No similar op available at this time.
<tf.Tensor: shape=(1, 20), dtype=float32, numpy=
array([[0. , 0. , 0. , 0. , 0. , 1.2, 0. , 0. , 0. , 0. , 0. , 1.5, 0. ,

        0. , 0. , 0. , 0. , 2. , 0. , 0. ]], dtype=float32)>

In Keras, this can be done by passing a count_weights input to tf.keras.layers.CategoryEncoding with output_mode='count'.

ids = tf.constant([[5, 11, 5, 17, 17]])
weights = tf.constant([[0.5, 1.5, 0.7, 1.8, 0.2]])

# Using sparse output is more efficient when `num_tokens` is large.
count_layer = tf.keras.layers.CategoryEncoding(
    num_tokens=20, output_mode='count', sparse=True)
tf.sparse.to_dense(count_layer(ids, count_weights=weights))
<tf.Tensor: shape=(1, 20), dtype=float32, numpy=
array([[0. , 0. , 0. , 0. , 0. , 1.2, 0. , 0. , 0. , 0. , 0. , 1.5, 0. ,

        0. , 0. , 0. , 0. , 2. , 0. , 0. ]], dtype=float32)>

Embedding weighted categorical data

You might alternately want to embed weighted categorical inputs. In feature columns, the embedding_column contains a combiner argument. If any sample contains multiple entries for a category, they will be combined according to the argument setting (by default 'mean').

ids = tf.constant([[5, 11, 5, 17, 17]])
weights = tf.constant([[0.5, 1.5, 0.7, 1.8, 0.2]])

categorical_col = tf1.feature_column.categorical_column_with_identity(
    'ids', num_buckets=20)
weighted_categorical_col = tf1.feature_column.weighted_categorical_column(
    categorical_col, 'weights')
embedding_col = tf1.feature_column.embedding_column(
    weighted_categorical_col, 4, combiner='mean')
call_feature_columns(embedding_col, {'ids': ids, 'weights': weights})
<tf.Tensor: shape=(1, 4), dtype=float32, numpy=
array([[ 0.02666993,  0.289671  ,  0.18065728, -0.21045178]],
      dtype=float32)>

In Keras, there is no combiner option to tf.keras.layers.Embedding, but you can acheive the same effect with tf.keras.layers.Dense. The embedding_column above is simply linearly combining embedding vectors according to category weight. Though not obvious at first, it is exactly equivalent to representing your categorical inputs as a sparse weight vector of size (num_tokens), and mutiplying them by a Dense kernel of shape (embedding_size, num_tokens).

ids = tf.constant([[5, 11, 5, 17, 17]])
weights = tf.constant([[0.5, 1.5, 0.7, 1.8, 0.2]])

# For `combiner='mean'`, normalize your weights to sum to 1. Removing this line
# would be eqivalent to an `embedding_column` with `combiner='sum'`.
weights = weights / tf.reduce_sum(weights, axis=-1, keepdims=True)

count_layer = tf.keras.layers.CategoryEncoding(
    num_tokens=20, output_mode='count', sparse=True)
embedding_layer = tf.keras.layers.Dense(4, use_bias=False)
embedding_layer(count_layer(ids, count_weights=weights))
<tf.Tensor: shape=(1, 4), dtype=float32, numpy=
array([[-0.03897291, -0.27131438,  0.09332469,  0.04333957]],
      dtype=float32)>

Complete training example

To show a complete training workflow, first prepare some data with three features of different types:

features = {
    'type': [0, 1, 1],
    'size': ['small', 'small', 'medium'],
    'weight': [2.7, 1.8, 1.6],
}
labels = [1, 1, 0]
predict_features = {'type': [0], 'size': ['foo'], 'weight': [-0.7]}

Define some common constants for both TF1 and TF2 workflows:

vocab = ['small', 'medium', 'large']
one_hot_dims = 3
embedding_dims = 4
weight_mean = 2.0
weight_variance = 1.0

With feature columns

Feature columns must be passed as a list to the estimator on creation, and will be called implicitly during training.

categorical_col = tf1.feature_column.categorical_column_with_identity(
    'type', num_buckets=one_hot_dims)
# Convert index to one-hot; e.g. [2] -> [0,0,1].
indicator_col = tf1.feature_column.indicator_column(categorical_col)

# Convert strings to indices; e.g. ['small'] -> [1].
vocab_col = tf1.feature_column.categorical_column_with_vocabulary_list(
    'size', vocabulary_list=vocab, num_oov_buckets=1)
# Embed the indices.
embedding_col = tf1.feature_column.embedding_column(vocab_col, embedding_dims)

normalizer_fn = lambda x: (x - weight_mean) / math.sqrt(weight_variance)
# Normalize the numeric inputs; e.g. [2.0] -> [0.0].
numeric_col = tf1.feature_column.numeric_column(
    'weight', normalizer_fn=normalizer_fn)

estimator = tf1.estimator.DNNClassifier(
    feature_columns=[indicator_col, embedding_col, numeric_col],
    hidden_units=[1])

def _input_fn():
  return tf1.data.Dataset.from_tensor_slices((features, labels)).batch(1)

estimator.train(_input_fn)
INFO:tensorflow:Using default config.
WARNING:tensorflow:Using temporary folder as model directory: /tmp/tmp8lwbuor2
INFO:tensorflow:Using config: {'_model_dir': '/tmp/tmp8lwbuor2', '_tf_random_seed': None, '_save_summary_steps': 100, '_save_checkpoints_steps': None, '_save_checkpoints_secs': 600, '_session_config': allow_soft_placement: true
graph_options {
  rewrite_options {
    meta_optimizer_iterations: ONE
  }
}
, '_keep_checkpoint_max': 5, '_keep_checkpoint_every_n_hours': 10000, '_log_step_count_steps': 100, '_train_distribute': None, '_device_fn': None, '_protocol': None, '_eval_distribute': None, '_experimental_distribute': None, '_experimental_max_worker_delay_secs': None, '_session_creation_timeout_secs': 7200, '_checkpoint_save_graph_def': True, '_service': None, '_cluster_spec': ClusterSpec({}), '_task_type': 'worker', '_task_id': 0, '_global_id_in_cluster': 0, '_master': '', '_evaluation_master': '', '_is_chief': True, '_num_ps_replicas': 0, '_num_worker_replicas': 1}
WARNING:tensorflow:From /tmpfs/src/tf_docs_env/lib/python3.7/site-packages/tensorflow/python/training/training_util.py:236: Variable.initialized_value (from tensorflow.python.ops.variables) is deprecated and will be removed in a future version.
Instructions for updating:
Use Variable.read_value. Variables in 2.X are initialized automatically both in eager and graph (inside tf.defun) contexts.
INFO:tensorflow:Calling model_fn.
WARNING:tensorflow:From /tmpfs/src/tf_docs_env/lib/python3.7/site-packages/tensorflow/python/training/adagrad.py:77: calling Constant.__init__ (from tensorflow.python.ops.init_ops) with dtype is deprecated and will be removed in a future version.
Instructions for updating:
Call initializer instance with the dtype argument instead of passing it to the constructor
INFO:tensorflow:Done calling model_fn.
INFO:tensorflow:Create CheckpointSaverHook.
INFO:tensorflow:Graph was finalized.
INFO:tensorflow:Running local_init_op.
INFO:tensorflow:Done running local_init_op.
INFO:tensorflow:Calling checkpoint listeners before saving checkpoint 0...
INFO:tensorflow:Saving checkpoints for 0 into /tmp/tmp8lwbuor2/model.ckpt.
INFO:tensorflow:Calling checkpoint listeners after saving checkpoint 0...
INFO:tensorflow:loss = 0.54634213, step = 0
INFO:tensorflow:Calling checkpoint listeners before saving checkpoint 3...
INFO:tensorflow:Saving checkpoints for 3 into /tmp/tmp8lwbuor2/model.ckpt.
INFO:tensorflow:Calling checkpoint listeners after saving checkpoint 3...
INFO:tensorflow:Loss for final step: 0.7308526.
<tensorflow_estimator.python.estimator.canned.dnn.DNNClassifier at 0x7f90685d53d0>

The feature columns will also be used to transform input data when running inference on the model.

def _predict_fn():
  return tf1.data.Dataset.from_tensor_slices(predict_features).batch(1)

next(estimator.predict(_predict_fn))
INFO:tensorflow:Calling model_fn.
INFO:tensorflow:Done calling model_fn.
INFO:tensorflow:Graph was finalized.
INFO:tensorflow:Restoring parameters from /tmp/tmp8lwbuor2/model.ckpt-3
INFO:tensorflow:Running local_init_op.
INFO:tensorflow:Done running local_init_op.
{'logits': array([0.5172372], dtype=float32),
 'logistic': array([0.6265015], dtype=float32),
 'probabilities': array([0.37349847, 0.6265015 ], dtype=float32),
 'class_ids': array([1]),
 'classes': array([b'1'], dtype=object),
 'all_class_ids': array([0, 1], dtype=int32),
 'all_classes': array([b'0', b'1'], dtype=object)}

With Keras preprocessing layers

Keras preprocessing layers are more flexible in where they can be called. A layer can be applied directly to tensors, used inside a tf.data input pipeline, or built directly into a trainable Keras model.

In this example, you will apply preprocessing layers inside a tf.data input pipeline. To do this, you can define a separate tf.keras.Model to preprocess your input features. This model is not trainable, but is a convenient way to group preprocessing layers.

inputs = {
  'type': tf.keras.Input(shape=(), dtype='int64'),
  'size': tf.keras.Input(shape=(), dtype='string'),
  'weight': tf.keras.Input(shape=(), dtype='float32'),
}
# Convert index to one-hot; e.g. [2] -> [0,0,1].
type_output = tf.keras.layers.CategoryEncoding(
      one_hot_dims, output_mode='one_hot')(inputs['type'])
# Convert size strings to indices; e.g. ['small'] -> [1].
size_output = tf.keras.layers.StringLookup(vocabulary=vocab)(inputs['size'])
# Normalize the numeric inputs; e.g. [2.0] -> [0.0].
weight_output = tf.keras.layers.Normalization(
      axis=None, mean=weight_mean, variance=weight_variance)(inputs['weight'])
outputs = {
  'type': type_output,
  'size': size_output,
  'weight': weight_output,
}
preprocessing_model = tf.keras.Model(inputs, outputs)

You can now apply this model inside a call to tf.data.Dataset.map. Please note that the function passed to map will automatically be converted into a tf.function, and usual caveats for writing tf.function code apply (no side effects).

# Apply the preprocessing in tf.data.Dataset.map.
dataset = tf.data.Dataset.from_tensor_slices((features, labels)).batch(1)
dataset = dataset.map(lambda x, y: (preprocessing_model(x), y),
                      num_parallel_calls=tf.data.AUTOTUNE)
# Display a preprocessed input sample.
next(dataset.take(1).as_numpy_iterator())
({'type': array([[1., 0., 0.]], dtype=float32),
  'size': array([1]),
  'weight': array([0.70000005], dtype=float32)},
 array([1], dtype=int32))

Next, you can define a separate Model containing the trainable layers. Note how the inputs to this model now reflect the preprocessed feature types and shapes.

inputs = {
  'type': tf.keras.Input(shape=(one_hot_dims,), dtype='float32'),
  'size': tf.keras.Input(shape=(), dtype='int64'),
  'weight': tf.keras.Input(shape=(), dtype='float32'),
}
# Since the embedding is trainable, it needs to be part of the training model.
embedding = tf.keras.layers.Embedding(len(vocab), embedding_dims)
outputs = tf.keras.layers.Concatenate()([
  inputs['type'],
  embedding(inputs['size']),
  tf.expand_dims(inputs['weight'], -1),
])
outputs = tf.keras.layers.Dense(1)(outputs)
training_model = tf.keras.Model(inputs, outputs)

You can now train the training_model with tf.keras.Model.fit.

# Train on the preprocessed data.
training_model.compile(
    loss=tf.keras.losses.BinaryCrossentropy(from_logits=True))
training_model.fit(dataset)
3/3 [==============================] - 0s 3ms/step - loss: 0.7248
<keras.callbacks.History at 0x7f9041a294d0>

Finally, at inference time, it can be useful to combine these separate stages into a single model that handles raw feature inputs.

inputs = preprocessing_model.input
outpus = training_model(preprocessing_model(inputs))
inference_model = tf.keras.Model(inputs, outpus)

predict_dataset = tf.data.Dataset.from_tensor_slices(predict_features).batch(1)
inference_model.predict(predict_dataset)
array([[0.936637]], dtype=float32)

This composed model can be saved as a SavedModel for later use.

inference_model.save('model')
restored_model = tf.keras.models.load_model('model')
restored_model.predict(predict_dataset)
WARNING:tensorflow:Compiled the loaded model, but the compiled metrics have yet to be built. `model.compile_metrics` will be empty until you train or evaluate the model.
2021-10-27 01:23:25.649967: W tensorflow/python/util/util.cc:348] Sets are not currently considered sequences, but this may change in the future, so consider avoiding using them.
INFO:tensorflow:Assets written to: model/assets
WARNING:tensorflow:No training configuration found in save file, so the model was *not* compiled. Compile it manually.
array([[0.936637]], dtype=float32)

Feature column equivalence table

For reference, here is an approximate correspondence between feature columns and preprocessing layers:

Feature Column Keras Layer
feature_column.bucketized_column layers.Discretization
feature_column.categorical_column_with_hash_bucket layers.Hashing
feature_column.categorical_column_with_identity layers.CategoryEncoding
feature_column.categorical_column_with_vocabulary_file layers.StringLookup or layers.IntegerLookup
feature_column.categorical_column_with_vocabulary_list layers.StringLookup or layers.IntegerLookup
feature_column.crossed_column Not implemented.
feature_column.embedding_column layers.Embedding
feature_column.indicator_column output_mode='one_hot' or output_mode='multi_hot'*
feature_column.numeric_column layers.Normalization
feature_column.sequence_categorical_column_with_hash_bucket layers.Hashing
feature_column.sequence_categorical_column_with_identity layers.CategoryEncoding
feature_column.sequence_categorical_column_with_vocabulary_file layers.StringLookup, layers.IntegerLookup, or layer.TextVectorization
feature_column.sequence_categorical_column_with_vocabulary_list layers.StringLookup, layers.IntegerLookup, or layer.TextVectorization
feature_column.sequence_numeric_column layers.Normalization
feature_column.weighted_categorical_column layers.CategoryEncoding

* output_mode can be passed to layers.CategoryEncoding, layers.StringLookup, layers.IntegerLookup, and layers.TextVectorization.

layers.TextVectorization can handle freeform text input directly (e.g. entire sentences or paragraphs). This is not one-to-one replacement for categorical sequence handling in TF1, but may offer a convinient replacement for ad-hoc text preprocessing.

Next Steps