def __init__(self, schema, serialized=True):
    """Build an ExampleProtoCoder.

    Args:
      schema: A `Schema` proto.
      serialized: Whether to encode serialized Example protos (as opposed to
        in-memory Example protos).

    Raises:
      ValueError: If `schema` is invalid.
    """
    self._schema = schema
    self._serialized = serialized

    # Using pre-allocated tf.train.Example and FeatureHandler objects for
    # performance reasons.
    #
    # Since the output of "encode" is deep as opposed to shallow
    # transformations, and since the schema always fully defines the Example's
    # FeatureMap (ie all fields are always cleared/assigned or copied), the
    # optimization and implementation are correct and thread-compatible.
    self._encode_example_cache = tf.train.Example()
    self._feature_handlers = []
    for name, feature_spec in schema_utils.schema_as_feature_spec(
        schema).feature_spec.items():
      if isinstance(feature_spec, tf.io.FixedLenFeature):
        self._feature_handlers.append(
            _FixedLenFeatureHandler(name, feature_spec))
      elif isinstance(feature_spec, tf.io.VarLenFeature):
        self._feature_handlers.append(
            _VarLenFeatureHandler(name, feature_spec.dtype))
      elif isinstance(feature_spec, tf.io.SparseFeature):
        index_keys = (
            feature_spec.index_key if isinstance(feature_spec.index_key, list)
            else [feature_spec.index_key])
        for index_key in index_keys:
          self._feature_handlers.append(
              _VarLenFeatureHandler(index_key, tf.int64))
        self._feature_handlers.append(
            _VarLenFeatureHandler(feature_spec.value_key, feature_spec.dtype))
      elif isinstance(feature_spec, tf.io.RaggedFeature):
        uniform_partition = False
        for partition in feature_spec.partitions:
          if isinstance(partition, tf.io.RaggedFeature.RowLengths):
            if uniform_partition:
              raise ValueError(
                  'Encountered ragged dimension after uniform for feature '
                  '"{}": only inner dimensions can be uniform. Feature spec '
                  'is {}'.format(name, feature_spec))
            self._feature_handlers.append(
                _VarLenFeatureHandler(partition.key, tf.int64))
          elif isinstance(partition, tf.io.RaggedFeature.UniformRowLength):
            # We don't encode uniform partitions since they can be recovered
            # from the shape information.
            uniform_partition = True
          else:
            raise ValueError(
                'Only `RowLengths` and `UniformRowLength` partitions of ragged '
                'features are supported, got {}'.format(type(partition)))
        self._feature_handlers.append(
            _VarLenFeatureHandler(feature_spec.value_key, feature_spec.dtype))
      else:
        raise ValueError('feature_spec should be one of tf.io.FixedLenFeature, '
                         'tf.io.VarLenFeature, tf.io.SparseFeature or '
                         'tf.io.RaggedFeature: "{}" was {}'.format(
                             name, type(feature_spec)))

    for feature_handler in self._feature_handlers:
      feature_handler.initialize_encode_cache(self._encode_example_cache)
Build an ExampleProtoCoder.

    Args:
      schema: A `Schema` proto.
      serialized: Whether to encode serialized Example protos (as opposed to
        in-memory Example protos).

    Raises:
      ValueError: If `schema` is invalid.
    
tensorflow_transform/coders/example_proto_coder.py