Copyright 2019 The TensorFlow Authors.



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TFRecord and tf.Example

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To read data efficiently it can be helpful to serialize your data and store it in a set of files (100-200MB each) that can each be read linearly. This is especially true if the data is being streamed over a network. This can also be useful for caching any data-preprocessing.

The TFRecord format is a simple format for storing a sequence of binary records.

Protocol buffers are a cross-platform, cross-language library for efficient serialization of structured data.

Protocol messages are defined by .proto files, these are often the easiest way to understand a message type.

The tf.Example message (or protobuf) is a flexible message type that represents a {"string": value} mapping. It is designed for use with TensorFlow and is used throughout the higher-level APIs such as TFX.

This notebook will demonstrate how to create, parse, and use the tf.Example message, and then serialize, write, and read tf.Example messages to and from .tfrecord files.

Note: While useful, these structures are optional. There is no need to convert existing code to use TFRecords, unless you are using tf.data and reading data is still the bottleneck to training. See Data Input Pipeline Performance for dataset performance tips.

Setup



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import tensorflow as tf

import numpy as np
import IPython.display as display

`tf.Example`

Data types for `tf.Example`

Fundamentally, a tf.Example is a {"string": tf.train.Feature} mapping.

The tf.train.Feature message type can accept one of the following three types (See the .proto file for reference). Most other generic types can be coerced into one of these:

tf.train.BytesList (the following types can be coerced)
- string
- byte
tf.train.FloatList (the following types can be coerced)
- float (float32)
- double (float64)
tf.train.Int64List (the following types can be coerced)
- bool
- enum
- int32
- uint32
- int64
- uint64

In order to convert a standard TensorFlow type to a tf.Example-compatible tf.train.Feature, you can use the shortcut functions below. Note that each function takes a scalar input value and returns a tf.train.Feature containing one of the three list types above:



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# The following functions can be used to convert a value to a type compatible
# with tf.Example.

def _bytes_feature(value):
  """Returns a bytes_list from a string / byte."""
  if isinstance(value, type(tf.constant(0))):
    value = value.numpy() # BytesList won't unpack a string from an EagerTensor.
  return tf.train.Feature(bytes_list=tf.train.BytesList(value=[value]))

def _float_feature(value):
  """Returns a float_list from a float / double."""
  return tf.train.Feature(float_list=tf.train.FloatList(value=[value]))

def _int64_feature(value):
  """Returns an int64_list from a bool / enum / int / uint."""
  return tf.train.Feature(int64_list=tf.train.Int64List(value=[value]))

Note: To stay simple, this example only uses scalar inputs. The simplest way to handle non-scalar features is to use tf.io.serialize_tensor to convert tensors to binary-strings. Strings are scalars in tensorflow. Use tf.io.parse_tensor to convert the binary-string back to a tensor.

Below are some examples of how these functions work. Note the varying input types and the standardized output types. If the input type for a function does not match one of the coercible types stated above, the function will raise an exception (e.g. _int64_feature(1.0) will error out, since 1.0 is a float, so should be used with the _float_feature function instead):



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print(_bytes_feature(b'test_string'))
print(_bytes_feature(u'test_bytes'.encode('utf-8')))

print(_float_feature(np.exp(1)))

print(_int64_feature(True))
print(_int64_feature(1))

All proto messages can be serialized to a binary-string using the .SerializeToString method:



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feature = _float_feature(np.exp(1))

feature.SerializeToString()

Creating a `tf.Example` message

Suppose you want to create a tf.Example message from existing data. In practice, the dataset may come from anywhere, but the procedure of creating the tf.Example message from a single observation will be the same:

Within each observation, each value needs to be converted to a tf.train.Feature containing one of the 3 compatible types, using one of the functions above.
You create a map (dictionary) from the feature name string to the encoded feature value produced in #1.
The map produced in step 2 is converted to a Features message.

In this notebook, you will create a dataset using NumPy.

This dataset will have 4 features:

a boolean feature, False or True with equal probability
an integer feature uniformly randomly chosen from [0, 5]
a string feature generated from a string table by using the integer feature as an index
a float feature from a standard normal distribution

Consider a sample consisting of 10,000 independently and identically distributed observations from each of the above distributions:



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# The number of observations in the dataset.
n_observations = int(1e4)

# Boolean feature, encoded as False or True.
feature0 = np.random.choice([False, True], n_observations)

# Integer feature, random from 0 to 4.
feature1 = np.random.randint(0, 5, n_observations)

# String feature
strings = np.array([b'cat', b'dog', b'chicken', b'horse', b'goat'])
feature2 = strings[feature1]

# Float feature, from a standard normal distribution
feature3 = np.random.randn(n_observations)

Each of these features can be coerced into a tf.Example-compatible type using one of _bytes_feature, _float_feature, _int64_feature. You can then create a tf.Example message from these encoded features:



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def serialize_example(feature0, feature1, feature2, feature3):
  """
  Creates a tf.Example message ready to be written to a file.
  """
  # Create a dictionary mapping the feature name to the tf.Example-compatible
  # data type.
  feature = {
      'feature0': _int64_feature(feature0),
      'feature1': _int64_feature(feature1),
      'feature2': _bytes_feature(feature2),
      'feature3': _float_feature(feature3),
  }

  # Create a Features message using tf.train.Example.

  example_proto = tf.train.Example(features=tf.train.Features(feature=feature))
  return example_proto.SerializeToString()

For example, suppose you have a single observation from the dataset, [False, 4, bytes('goat'), 0.9876]. You can create and print the tf.Example message for this observation using create_message(). Each single observation will be written as a Features message as per the above. Note that the tf.Example message is just a wrapper around the Features message:



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# This is an example observation from the dataset.

example_observation = []

serialized_example = serialize_example(False, 4, b'goat', 0.9876)
serialized_example

To decode the message use the tf.train.Example.FromString method.



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example_proto = tf.train.Example.FromString(serialized_example)
example_proto

TFRecords format details

A TFRecord file contains a sequence of records. The file can only be read sequentially.

Each record contains a byte-string, for the data-payload, plus the data-length, and CRC32C (32-bit CRC using the Castagnoli polynomial) hashes for integrity checking.

Each record is stored in the following formats:

uint64 length
uint32 masked_crc32_of_length
byte   data[length]
uint32 masked_crc32_of_data

The records are concatenated together to produce the file. CRCs are described here, and the mask of a CRC is:

masked_crc = ((crc >> 15) | (crc << 17)) + 0xa282ead8ul

Note: There is no requirement to use tf.Example in TFRecord files. tf.Example is just a method of serializing dictionaries to byte-strings. Lines of text, encoded image data, or serialized tensors (using tf.io.serialize_tensor, and tf.io.parse_tensor when loading). See the tf.io module for more options.

TFRecord files using `tf.data`

The tf.data module also provides tools for reading and writing data in TensorFlow.

Writing a TFRecord file

The easiest way to get the data into a dataset is to use the from_tensor_slices method.

Applied to an array, it returns a dataset of scalars:



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tf.data.Dataset.from_tensor_slices(feature1)

Applied to a tuple of arrays, it returns a dataset of tuples:



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features_dataset = tf.data.Dataset.from_tensor_slices((feature0, feature1, feature2, feature3))
features_dataset



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# Use `take(1)` to only pull one example from the dataset.
for f0,f1,f2,f3 in features_dataset.take(1):
  print(f0)
  print(f1)
  print(f2)
  print(f3)

Use the tf.data.Dataset.map method to apply a function to each element of a Dataset.

The mapped function must operate in TensorFlow graph mode—it must operate on and return tf.Tensors. A non-tensor function, like serialize_example, can be wrapped with tf.py_function to make it compatible.

Using tf.py_function requires to specify the shape and type information that is otherwise unavailable:



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def tf_serialize_example(f0,f1,f2,f3):
  tf_string = tf.py_function(
    serialize_example,
    (f0,f1,f2,f3),  # pass these args to the above function.
    tf.string)      # the return type is `tf.string`.
  return tf.reshape(tf_string, ()) # The result is a scalar



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tf_serialize_example(f0,f1,f2,f3)

Apply this function to each element in the dataset:



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serialized_features_dataset = features_dataset.map(tf_serialize_example)
serialized_features_dataset



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def generator():
  for features in features_dataset:
    yield serialize_example(*features)



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serialized_features_dataset = tf.data.Dataset.from_generator(
    generator, output_types=tf.string, output_shapes=())



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serialized_features_dataset

And write them to a TFRecord file:



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filename = 'test.tfrecord'
writer = tf.data.experimental.TFRecordWriter(filename)
writer.write(serialized_features_dataset)

Reading a TFRecord file

You can also read the TFRecord file using the tf.data.TFRecordDataset class.

More information on consuming TFRecord files using tf.data can be found here.

Using TFRecordDatasets can be useful for standardizing input data and optimizing performance.



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filenames = [filename]
raw_dataset = tf.data.TFRecordDataset(filenames)
raw_dataset

At this point the dataset contains serialized tf.train.Example messages. When iterated over it returns these as scalar string tensors.

Use the .take method to only show the first 10 records.

Note: iterating over a tf.data.Dataset only works with eager execution enabled.



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for raw_record in raw_dataset.take(10):
  print(repr(raw_record))

These tensors can be parsed using the function below. Note that the feature_description is necessary here because datasets use graph-execution, and need this description to build their shape and type signature:



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# Create a description of the features.
feature_description = {
    'feature0': tf.io.FixedLenFeature([], tf.int64, default_value=0),
    'feature1': tf.io.FixedLenFeature([], tf.int64, default_value=0),
    'feature2': tf.io.FixedLenFeature([], tf.string, default_value=''),
    'feature3': tf.io.FixedLenFeature([], tf.float32, default_value=0.0),
}

def _parse_function(example_proto):
  # Parse the input `tf.Example` proto using the dictionary above.
  return tf.io.parse_single_example(example_proto, feature_description)

Alternatively, use tf.parse example to parse the whole batch at once. Apply this function to each item in the dataset using the tf.data.Dataset.map method:



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parsed_dataset = raw_dataset.map(_parse_function)
parsed_dataset

Use eager execution to display the observations in the dataset. There are 10,000 observations in this dataset, but you will only display the first 10. The data is displayed as a dictionary of features. Each item is a tf.Tensor, and the numpy element of this tensor displays the value of the feature:



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for parsed_record in parsed_dataset.take(10):
  print(repr(parsed_record))

Here, the tf.parse_example function unpacks the tf.Example fields into standard tensors.

TFRecord files in Python

The tf.io module also contains pure-Python functions for reading and writing TFRecord files.

Writing a TFRecord file

Next, write the 10,000 observations to the file test.tfrecord. Each observation is converted to a tf.Example message, then written to file. You can then verify that the file test.tfrecord has been created:



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# Write the `tf.Example` observations to the file.
with tf.io.TFRecordWriter(filename) as writer:
  for i in range(n_observations):
    example = serialize_example(feature0[i], feature1[i], feature2[i], feature3[i])
    writer.write(example)



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!du -sh {filename}

Reading a TFRecord file

These serialized tensors can be easily parsed using tf.train.Example.ParseFromString:



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filenames = [filename]
raw_dataset = tf.data.TFRecordDataset(filenames)
raw_dataset



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for raw_record in raw_dataset.take(1):
  example = tf.train.Example()
  example.ParseFromString(raw_record.numpy())
  print(example)

Walkthrough: Reading and writing image data

This is an end-to-end example of how to read and write image data using TFRecords. Using an image as input data, you will write the data as a TFRecord file, then read the file back and display the image.

This can be useful if, for example, you want to use several models on the same input dataset. Instead of storing the image data raw, it can be preprocessed into the TFRecords format, and that can be used in all further processing and modelling.

First, let's download this image of a cat in the snow and this photo of the Williamsburg Bridge, NYC under construction.

Fetch the images



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cat_in_snow  = tf.keras.utils.get_file('320px-Felis_catus-cat_on_snow.jpg', 'https://storage.googleapis.com/download.tensorflow.org/example_images/320px-Felis_catus-cat_on_snow.jpg')
williamsburg_bridge = tf.keras.utils.get_file('194px-New_East_River_Bridge_from_Brooklyn_det.4a09796u.jpg','https://storage.googleapis.com/download.tensorflow.org/example_images/194px-New_East_River_Bridge_from_Brooklyn_det.4a09796u.jpg')



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display.display(display.Image(filename=cat_in_snow))
display.display(display.HTML('Image cc-by: <a "href=https://commons.wikimedia.org/wiki/File:Felis_catus-cat_on_snow.jpg">Von.grzanka</a>'))



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display.display(display.Image(filename=williamsburg_bridge))
display.display(display.HTML('<a "href=https://commons.wikimedia.org/wiki/File:New_East_River_Bridge_from_Brooklyn_det.4a09796u.jpg">From Wikimedia</a>'))

Write the TFRecord file

As before, encode the features as types compatible with tf.Example. This stores the raw image string feature, as well as the height, width, depth, and arbitrary label feature. The latter is used when you write the file to distinguish between the cat image and the bridge image. Use 0 for the cat image, and 1 for the bridge image:



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image_labels = {
    cat_in_snow : 0,
    williamsburg_bridge : 1,
}



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# This is an example, just using the cat image.
image_string = open(cat_in_snow, 'rb').read()

label = image_labels[cat_in_snow]

# Create a dictionary with features that may be relevant.
def image_example(image_string, label):
  image_shape = tf.image.decode_jpeg(image_string).shape

  feature = {
      'height': _int64_feature(image_shape[0]),
      'width': _int64_feature(image_shape[1]),
      'depth': _int64_feature(image_shape[2]),
      'label': _int64_feature(label),
      'image_raw': _bytes_feature(image_string),
  }

  return tf.train.Example(features=tf.train.Features(feature=feature))

for line in str(image_example(image_string, label)).split('\n')[:15]:
  print(line)
print('...')

Notice that all of the features are now stored in the tf.Example message. Next, functionalize the code above and write the example messages to a file named images.tfrecords:



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# Write the raw image files to `images.tfrecords`.
# First, process the two images into `tf.Example` messages.
# Then, write to a `.tfrecords` file.
record_file = 'images.tfrecords'
with tf.io.TFRecordWriter(record_file) as writer:
  for filename, label in image_labels.items():
    image_string = open(filename, 'rb').read()
    tf_example = image_example(image_string, label)
    writer.write(tf_example.SerializeToString())



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!du -sh {record_file}

Read the TFRecord file

You now have the file—images.tfrecords—and can now iterate over the records in it to read back what you wrote. Given that in this example you will only reproduce the image, the only feature you will need is the raw image string. Extract it using the getters described above, namely example.features.feature['image_raw'].bytes_list.value[0]. You can also use the labels to determine which record is the cat and which one is the bridge:



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raw_image_dataset = tf.data.TFRecordDataset('images.tfrecords')

# Create a dictionary describing the features.
image_feature_description = {
    'height': tf.io.FixedLenFeature([], tf.int64),
    'width': tf.io.FixedLenFeature([], tf.int64),
    'depth': tf.io.FixedLenFeature([], tf.int64),
    'label': tf.io.FixedLenFeature([], tf.int64),
    'image_raw': tf.io.FixedLenFeature([], tf.string),
}

def _parse_image_function(example_proto):
  # Parse the input tf.Example proto using the dictionary above.
  return tf.io.parse_single_example(example_proto, image_feature_description)

parsed_image_dataset = raw_image_dataset.map(_parse_image_function)
parsed_image_dataset

Recover the images from the TFRecord file:



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for image_features in parsed_image_dataset:
  image_raw = image_features['image_raw'].numpy()
  display.display(display.Image(data=image_raw))

Copyright 2019 The TensorFlow Authors.

TFRecord and tf.Example

Setup

tf.Example

Data types for tf.Example

Creating a tf.Example message

TFRecords format details

TFRecord files using tf.data

Writing a TFRecord file

Reading a TFRecord file

TFRecord files in Python

Writing a TFRecord file

Reading a TFRecord file

Walkthrough: Reading and writing image data

Fetch the images

Write the TFRecord file

Read the TFRecord file

`tf.Example`

Data types for `tf.Example`

Creating a `tf.Example` message

TFRecord files using `tf.data`