content: # Copyright 2017 Google Inc.
  #
  # Licensed under the Apache License, Version 2.0 (the "License");
  # you may not use this file except in compliance with the License.
  # You may obtain a copy of the License at
  #
  #      http://www.apache.org/licenses/LICENSE-2.0
  #
  # Unless required by applicable law or agreed to in writing, software
  # distributed under the License is distributed on an "AS IS" BASIS,
  # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  # See the License for the specific language governing permissions and
  # limitations under the License.
  """
  Base class for sequence decoders.
  """
  
  from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  from __future__ import unicode_literals
  
  import abc
  from collections import namedtuple
  
  import six
  import tensorflow as tf
  from tensorflow.python.util import nest  # pylint: disable=E0611
  
  from seq2seq.graph_module import GraphModule
  from seq2seq.configurable import Configurable
  from seq2seq.contrib.seq2seq.decoder import Decoder, dynamic_decode
  from seq2seq.encoders.rnn_encoder import _default_rnn_cell_params
  from seq2seq.encoders.rnn_encoder import _toggle_dropout
  from seq2seq.training import utils as training_utils
  
  
  class DecoderOutput(
      namedtuple("DecoderOutput", ["logits", "predicted_ids", "cell_output"])):
    """Output of an RNN decoder.
  
    Note that we output both the logits and predictions because during
    dynamic decoding the predictions may not correspond to max(logits).
    For example, we may be sampling from the logits instead.
    """
    pass
  
  
  @six.add_metaclass(abc.ABCMeta)
  class RNNDecoder(Decoder, GraphModule, Configurable):
    """Base class for RNN decoders.
  
    Args:
      cell: An instance of ` tf.contrib.rnn.RNNCell`
      helper: An instance of `tf.contrib.seq2seq.Helper` to assist decoding
      initial_state: A tensor or tuple of tensors used as the initial cell
        state.
      name: A name for this module
    """
  
    def __init__(self, params, mode, name):
      GraphModule.__init__(self, name)
      Configurable.__init__(self, params, mode)
      self.params["rnn_cell"] = _toggle_dropout(self.params["rnn_cell"], mode)
      self.cell = training_utils.get_rnn_cell(**self.params["rnn_cell"])
      # Not initialized yet
      self.initial_state = None
      self.helper = None
  
    @abc.abstractmethod
    def initialize(self, name=None):
      raise NotImplementedError
  
    @abc.abstractmethod
    def step(self, name=None):
      raise NotImplementedError
  
    @property
    def batch_size(self):
      return tf.shape(nest.flatten([self.initial_state])[0])[0]
  
    def _setup(self, initial_state, helper):
      """Sets the initial state and helper for the decoder.
      """
      self.initial_state = initial_state
      self.helper = helper
  
    def finalize(self, outputs, final_state):
      """Applies final transformation to the decoder output once decoding is
      finished.
      """
      #pylint: disable=R0201
      return (outputs, final_state)
  
    @staticmethod
    def default_params():
      return {
          "max_decode_length": 100,
          "rnn_cell": _default_rnn_cell_params(),
          "init_scale": 0.04,
      }
  
    def _build(self, initial_state, helper):
      if not self.initial_state:
        self._setup(initial_state, helper)
  
      scope = tf.get_variable_scope()
      scope.set_initializer(tf.random_uniform_initializer(
          -self.params["init_scale"],
          self.params["init_scale"]))
  
      maximum_iterations = None
      if self.mode == tf.contrib.learn.ModeKeys.INFER:
        maximum_iterations = self.params["max_decode_length"]
  
      outputs, final_state = dynamic_decode(
          decoder=self,
          output_time_major=True,
          impute_finished=False,
          maximum_iterations=maximum_iterations)
      return self.finalize(outputs, final_state)
  