xref: /external/tensorflow/tensorflow/python/training/basic_session_run_hooks.py

# Copyright 2016 The TensorFlow Authors. All Rights Reserved.
#
# 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.
# ==============================================================================
"""Some common SessionRunHook classes."""

from __future__ import absolute_import
from __future__ import division
from __future__ import print_function

import os
import time

import numpy as np
import six

from tensorflow.core.framework.summary_pb2 import Summary
from tensorflow.core.protobuf import config_pb2
from tensorflow.core.util.event_pb2 import SessionLog
from tensorflow.python.client import timeline
from tensorflow.python.framework import dtypes
from tensorflow.python.framework import errors
from tensorflow.python.framework import meta_graph
from tensorflow.python.framework import ops
from tensorflow.python.ops import init_ops
from tensorflow.python.ops import variable_scope
from tensorflow.python.platform import gfile
from tensorflow.python.platform import tf_logging as logging
from tensorflow.python.training import session_run_hook
from tensorflow.python.training import training_util
from tensorflow.python.training.session_run_hook import SessionRunArgs
from tensorflow.python.training.summary_io import SummaryWriterCache
from tensorflow.python.util.tf_export import tf_export


_HOOKS = "hooks"
_STEPS_PER_RUN_VAR = "steps_per_run"


class _HookTimer(object):
  """Base timer for determining when Hooks should trigger.

  Should not be instantiated directly.
  """

  def __init__(self):
    pass

  def reset(self):
    """Resets the timer."""
    pass

  def should_trigger_for_step(self, step):
    """Return true if the timer should trigger for the specified step."""
    raise NotImplementedError

  def update_last_triggered_step(self, step):
    """Update the last triggered time and step number.

    Args:
      step: The current step.

    Returns:
      A pair `(elapsed_time, elapsed_steps)`, where `elapsed_time` is the number
      of seconds between the current trigger and the last one (a float), and
      `elapsed_steps` is the number of steps between the current trigger and
      the last one. Both values will be set to `None` on the first trigger.
    """
    raise NotImplementedError

  def last_triggered_step(self):
    """Returns the last triggered time step or None if never triggered."""
    raise NotImplementedError


@tf_export(v1=["train.SecondOrStepTimer"])
class SecondOrStepTimer(_HookTimer):
  """Timer that triggers at most once every N seconds or once every N steps.
  """

  def __init__(self, every_secs=None, every_steps=None):
    self.reset()
    self._every_secs = every_secs
    self._every_steps = every_steps

    if self._every_secs is None and self._every_steps is None:
      raise ValueError("Either every_secs or every_steps should be provided.")
    if (self._every_secs is not None) and (self._every_steps is not None):
      raise ValueError("Can not provide both every_secs and every_steps.")

    super(SecondOrStepTimer, self).__init__()

  def reset(self):
    self._last_triggered_step = None
    self._last_triggered_time = None

  def should_trigger_for_step(self, step):
    """Return true if the timer should trigger for the specified step.

    Args:
      step: Training step to trigger on.

    Returns:
      True if the difference between the current time and the time of the last
      trigger exceeds `every_secs`, or if the difference between the current
      step and the last triggered step exceeds `every_steps`. False otherwise.
    """
    if self._last_triggered_step is None:
      return True

    if self._last_triggered_step == step:
      return False

    if self._every_secs is not None:
      if time.time() >= self._last_triggered_time + self._every_secs:
        return True

    if self._every_steps is not None:
      if step >= self._last_triggered_step + self._every_steps:
        return True

    return False

  def update_last_triggered_step(self, step):
    current_time = time.time()
    if self._last_triggered_time is None:
      elapsed_secs = None
      elapsed_steps = None
    else:
      elapsed_secs = current_time - self._last_triggered_time
      elapsed_steps = step - self._last_triggered_step

    self._last_triggered_time = current_time
    self._last_triggered_step = step
    return (elapsed_secs, elapsed_steps)

  def last_triggered_step(self):
    return self._last_triggered_step


class NeverTriggerTimer(_HookTimer):
  """Timer that never triggers."""

  def should_trigger_for_step(self, step):
    _ = step
    return False

  def update_last_triggered_step(self, step):
    _ = step
    return (None, None)

  def last_triggered_step(self):
    return None


@tf_export(v1=["train.LoggingTensorHook"])
class LoggingTensorHook(session_run_hook.SessionRunHook):
  """Prints the given tensors every N local steps, every N seconds, or at end.

  The tensors will be printed to the log, with `INFO` severity. If you are not
  seeing the logs, you might want to add the following line after your imports:

  ```python
    tf.logging.set_verbosity(tf.logging.INFO)
  ```

  Note that if `at_end` is True, `tensors` should not include any tensor
  whose evaluation produces a side effect such as consuming additional inputs.
  """

  def __init__(self, tensors, every_n_iter=None, every_n_secs=None,
               at_end=False, formatter=None):
    """Initializes a `LoggingTensorHook`.

    Args:
      tensors: `dict` that maps string-valued tags to tensors/tensor names,
          or `iterable` of tensors/tensor names.
      every_n_iter: `int`, print the values of `tensors` once every N local
          steps taken on the current worker.
      every_n_secs: `int` or `float`, print the values of `tensors` once every N
          seconds. Exactly one of `every_n_iter` and `every_n_secs` should be
          provided.
      at_end: `bool` specifying whether to print the values of `tensors` at the
          end of the run.
      formatter: function, takes dict of `tag`->`Tensor` and returns a string.
          If `None` uses default printing all tensors.

    Raises:
      ValueError: if `every_n_iter` is non-positive.
    """
    only_log_at_end = (
        at_end and (every_n_iter is None) and (every_n_secs is None))
    if (not only_log_at_end and
        (every_n_iter is None) == (every_n_secs is None)):
      raise ValueError(
          "either at_end and/or exactly one of every_n_iter and every_n_secs "
          "must be provided.")
    if every_n_iter is not None and every_n_iter <= 0:
      raise ValueError("invalid every_n_iter=%s." % every_n_iter)
    if not isinstance(tensors, dict):
      self._tag_order = tensors
      tensors = {item: item for item in tensors}
    else:
      self._tag_order = sorted(tensors.keys())
    self._tensors = tensors
    self._formatter = formatter
    self._timer = (
        NeverTriggerTimer() if only_log_at_end else
        SecondOrStepTimer(every_secs=every_n_secs, every_steps=every_n_iter))
    self._log_at_end = at_end

  def begin(self):
    self._timer.reset()
    self._iter_count = 0
    # Convert names to tensors if given
    self._current_tensors = {tag: _as_graph_element(tensor)
                             for (tag, tensor) in self._tensors.items()}

  def before_run(self, run_context):  # pylint: disable=unused-argument
    self._should_trigger = self._timer.should_trigger_for_step(self._iter_count)
    if self._should_trigger:
      return SessionRunArgs(self._current_tensors)
    else:
      return None

  def _log_tensors(self, tensor_values):
    original = np.get_printoptions()
    np.set_printoptions(suppress=True)
    elapsed_secs, _ = self._timer.update_last_triggered_step(self._iter_count)
    if self._formatter:
      logging.info(self._formatter(tensor_values))
    else:
      stats = []
      for tag in self._tag_order:
        stats.append("%s = %s" % (tag, tensor_values[tag]))
      if elapsed_secs is not None:
        logging.info("%s (%.3f sec)", ", ".join(stats), elapsed_secs)
      else:
        logging.info("%s", ", ".join(stats))
    np.set_printoptions(**original)

  def after_run(self, run_context, run_values):
    _ = run_context
    if self._should_trigger:
      self._log_tensors(run_values.results)

    self._iter_count += 1

  def end(self, session):
    if self._log_at_end:
      values = session.run(self._current_tensors)
      self._log_tensors(values)


def get_or_create_steps_per_run_variable():
  """Gets or creates the steps_per_run variable.

  In Estimator, the user provided computation, the model_fn, is wrapped
  inside a tf.while_loop for peak performance. The iterations of the loop are
  specified by this variable, which adjusts its value on the CPU after each
  device program execution and before the next execution.

  The purpose of using a variable, rather than a constant, is to allow
  Estimator adapt the device training iterations according to the final steps
  specified by users. For example, if the user sets the steps_per_run as
  4 and steps as 10 in Estimator.train(), the steps_per_run
  variable will have the following value before each training run.

      - 1-st execution: steps_per_run = 4
      - 2-nd execution: steps_per_run = 4
      - 3-rd execution: steps_per_run = 2

  As model_fn increases the global step once per train_op invocation, the global
  step is 10 after all executions, matching the steps=10 inputs passed in by
  users.

  Returns:
    A TF non-trainable resource variable.

  Raises:
    RuntimeError: If multi steps_per_run variables were found.
  """
  graph = ops.get_default_graph()
  collection_name = "{}_{}".format(_HOOKS, _STEPS_PER_RUN_VAR)
  steps_per_run_vars = graph.get_collection(collection_name)
  if len(steps_per_run_vars) == 1:
    return steps_per_run_vars[0]
  elif len(steps_per_run_vars) > 1:
    raise RuntimeError("Multiple steps_per_run_var in collection.")

  with variable_scope.variable_scope(_HOOKS, reuse=variable_scope.AUTO_REUSE):
    return variable_scope.get_variable(
        _STEPS_PER_RUN_VAR,
        initializer=init_ops.ones_initializer(),
        shape=[],
        dtype=dtypes.int32,
        trainable=False,
        collections=[collection_name, ops.GraphKeys.LOCAL_VARIABLES],
        use_resource=True)


class _MultiStepStopAtStepHook(session_run_hook.SessionRunHook):
  """Hook that requests stop at a specified step."""

  def __init__(self, num_steps=None, last_step=None, steps_per_run=1):
    """Initializes a `MultiStepStopAtStepHook`.

    This hook requests stop after either a number of steps have been
    executed or a last step has been reached. Only one of the two options can be
    specified.

    if `num_steps` is specified, it indicates the number of steps to execute
    after `begin()` is called. If instead `last_step` is specified, it
    indicates the last step we want to execute, as passed to the `after_run()`
    call.

    In Estimator, the user provided computation, the model_fn, is wrapped
    inside a tf.while_loop for peak performance. The steps_per_run variable
    determines the number of iterations of the loop before returning to the CPU.

    Args:
      num_steps: Number of steps to execute.
      last_step: Step after which to stop.
      steps_per_run: Number of steps executed per run call.

    Raises:
      ValueError: If one of the arguments is invalid.
    """
    if num_steps is None and last_step is None:
      raise ValueError("One of num_steps or last_step must be specified.")
    if num_steps is not None and last_step is not None:
      raise ValueError("Only one of num_steps or last_step can be specified.")
    if steps_per_run is None or steps_per_run < 1:
      raise ValueError("steps_per_run should be greater than 0")
    self._num_steps = num_steps
    self._last_step = last_step
    self._steps_per_run_initial_value = steps_per_run

  def begin(self):
    self._global_step_tensor = training_util.get_global_step()
    if self._global_step_tensor is None:
      raise RuntimeError("Global step should be created to use StopAtStepHook.")
    self._steps_per_run_variable = get_or_create_steps_per_run_variable()

  def _update_steps_per_run_variable(self, global_step, session):
    steps = min(self._last_step - global_step,
                self._steps_per_run_initial_value)
    self._steps_per_run_variable.load(steps, session=session)

  def after_create_session(self, session, coord):
    global_step = session.run(self._global_step_tensor)
    if self._last_step is None:
      self._last_step = global_step + self._num_steps
    self._update_steps_per_run_variable(global_step, session)

  def after_run(self, run_context, run_values):
    # Global step cannot be retrieved via SessionRunArgs and before_run due to
    # race condition in hook execution.
    global_step = run_context.session.run(self._global_step_tensor)
    if global_step >= self._last_step:
      run_context.request_stop()
    else:
      self._update_steps_per_run_variable(global_step, run_context.session)


@tf_export(v1=["train.StopAtStepHook"])
class StopAtStepHook(session_run_hook.SessionRunHook):
  """Hook that requests stop at a specified step."""

  def __init__(self, num_steps=None, last_step=None):
    """Initializes a `StopAtStepHook`.

    This hook requests stop after either a number of steps have been
    executed or a last step has been reached. Only one of the two options can be
    specified.

    if `num_steps` is specified, it indicates the number of steps to execute
    after `begin()` is called. If instead `last_step` is specified, it
    indicates the last step we want to execute, as passed to the `after_run()`
    call.

    Args:
      num_steps: Number of steps to execute.
      last_step: Step after which to stop.

    Raises:
      ValueError: If one of the arguments is invalid.
    """
    if num_steps is None and last_step is None:
      raise ValueError("One of num_steps or last_step must be specified.")
    if num_steps is not None and last_step is not None:
      raise ValueError("Only one of num_steps or last_step can be specified.")
    self._num_steps = num_steps
    self._last_step = last_step

  def begin(self):
    self._global_step_tensor = training_util._get_or_create_global_step_read()  # pylint: disable=protected-access
    if self._global_step_tensor is None:
      raise RuntimeError("Global step should be created to use StopAtStepHook.")

  def after_create_session(self, session, coord):
    if self._last_step is None:
      global_step = session.run(self._global_step_tensor)
      self._last_step = global_step + self._num_steps

  def before_run(self, run_context):  # pylint: disable=unused-argument
    return SessionRunArgs(self._global_step_tensor)

  def after_run(self, run_context, run_values):
    global_step = run_values.results + 1
    if global_step >= self._last_step:
      # Check latest global step to ensure that the targeted last step is
      # reached. global_step read tensor is the value of global step
      # before running the operation. We're not sure whether current session.run
      # incremented the global_step or not. Here we're checking it.

      step = run_context.session.run(self._global_step_tensor)
      if step >= self._last_step:
        run_context.request_stop()


@tf_export(v1=["train.CheckpointSaverListener"])
class CheckpointSaverListener(object):
  """Interface for listeners that take action before or after checkpoint save.

  `CheckpointSaverListener` triggers only in steps when `CheckpointSaverHook` is
  triggered, and provides callbacks at the following points:
   - before using the session
   - before each call to `Saver.save()`
   - after each call to `Saver.save()`
   - at the end of session

  To use a listener, implement a class and pass the listener to a
  `CheckpointSaverHook`, as in this example:

  ```python
  class ExampleCheckpointSaverListener(CheckpointSaverListener):
    def begin(self):
      # You can add ops to the graph here.
      print('Starting the session.')
      self.your_tensor = ...

    def before_save(self, session, global_step_value):
      print('About to write a checkpoint')

    def after_save(self, session, global_step_value):
      print('Done writing checkpoint.')
      if decided_to_stop_training():
        return True

    def end(self, session, global_step_value):
      print('Done with the session.')

  ...
  listener = ExampleCheckpointSaverListener()
  saver_hook = tf.train.CheckpointSaverHook(
      checkpoint_dir, listeners=[listener])
  with tf.train.MonitoredTrainingSession(chief_only_hooks=[saver_hook]):
    ...
  ```

  A `CheckpointSaverListener` may simply take some action after every
  checkpoint save. It is also possible for the listener to use its own schedule
  to act less frequently, e.g. based on global_step_value. In this case,
  implementors should implement the `end()` method to handle actions related to
  the last checkpoint save. But the listener should not act twice if
  `after_save()` already handled this last checkpoint save.

  A `CheckpointSaverListener` can request training to be stopped, by returning
  True in `after_save`. Please note that, in replicated distributed training
  setting, only `chief` should use this behavior. Otherwise each worker will do
  their own evaluation, which may be wasteful of resources.
  """

  def begin(self):
    pass

  def before_save(self, session, global_step_value):
    pass

  def after_save(self, session, global_step_value):
    pass

  def end(self, session, global_step_value):
    pass


@tf_export(v1=["train.CheckpointSaverHook"])
class CheckpointSaverHook(session_run_hook.SessionRunHook):
  """Saves checkpoints every N steps or seconds."""

  def __init__(self,
               checkpoint_dir,
               save_secs=None,
               save_steps=None,
               saver=None,
               checkpoint_basename="model.ckpt",
               scaffold=None,
               listeners=None):
    """Initializes a `CheckpointSaverHook`.

    Args:
      checkpoint_dir: `str`, base directory for the checkpoint files.
      save_secs: `int`, save every N secs.
      save_steps: `int`, save every N steps.
      saver: `Saver` object, used for saving.
      checkpoint_basename: `str`, base name for the checkpoint files.
      scaffold: `Scaffold`, use to get saver object.
      listeners: List of `CheckpointSaverListener` subclass instances.
        Used for callbacks that run immediately before or after this hook saves
        the checkpoint.

    Raises:
      ValueError: One of `save_steps` or `save_secs` should be set.
      ValueError: At most one of `saver` or `scaffold` should be set.
    """
    logging.info("Create CheckpointSaverHook.")
    if saver is not None and scaffold is not None:
      raise ValueError("You cannot provide both saver and scaffold.")
    self._saver = saver
    self._checkpoint_dir = checkpoint_dir
    self._save_path = os.path.join(checkpoint_dir, checkpoint_basename)
    self._scaffold = scaffold
    self._timer = SecondOrStepTimer(every_secs=save_secs,
                                    every_steps=save_steps)
    self._listeners = listeners or []
    self._steps_per_run = 1

  def _set_steps_per_run(self, steps_per_run):
    self._steps_per_run = steps_per_run

  def begin(self):
    self._summary_writer = SummaryWriterCache.get(self._checkpoint_dir)
    self._global_step_tensor = training_util._get_or_create_global_step_read()  # pylint: disable=protected-access
    if self._global_step_tensor is None:
      raise RuntimeError(
          "Global step should be created to use CheckpointSaverHook.")
    for l in self._listeners:
      l.begin()

  def after_create_session(self, session, coord):
    global_step = session.run(self._global_step_tensor)
    # We do write graph and saver_def at the first call of before_run.
    # We cannot do this in begin, since we let other hooks to change graph and
    # add variables in begin. Graph is finalized after all begin calls.
    training_util.write_graph(
        ops.get_default_graph().as_graph_def(add_shapes=True),
        self._checkpoint_dir,
        "graph.pbtxt")
    saver_def = self._get_saver().saver_def if self._get_saver() else None
    graph = ops.get_default_graph()
    meta_graph_def = meta_graph.create_meta_graph_def(
        graph_def=graph.as_graph_def(add_shapes=True),
        saver_def=saver_def)
    self._summary_writer.add_graph(graph)
    self._summary_writer.add_meta_graph(meta_graph_def)
    # The checkpoint saved here is the state at step "global_step".
    self._save(session, global_step)
    self._timer.update_last_triggered_step(global_step)

  def before_run(self, run_context):  # pylint: disable=unused-argument
    return SessionRunArgs(self._global_step_tensor)

  def after_run(self, run_context, run_values):
    stale_global_step = run_values.results
    if self._timer.should_trigger_for_step(
        stale_global_step + self._steps_per_run):
      # get the real value after train op.
      global_step = run_context.session.run(self._global_step_tensor)
      if self._timer.should_trigger_for_step(global_step):
        self._timer.update_last_triggered_step(global_step)
        if self._save(run_context.session, global_step):
          run_context.request_stop()

  def end(self, session):
    last_step = session.run(self._global_step_tensor)
    if last_step != self._timer.last_triggered_step():
      self._save(session, last_step)
    for l in self._listeners:
      l.end(session, last_step)

  def _save(self, session, step):
    """Saves the latest checkpoint, returns should_stop."""
    logging.info("Saving checkpoints for %d into %s.", step, self._save_path)

    for l in self._listeners:
      l.before_save(session, step)

    self._get_saver().save(session, self._save_path, global_step=step)
    self._summary_writer.add_session_log(
        SessionLog(
            status=SessionLog.CHECKPOINT, checkpoint_path=self._save_path),
        step)

    should_stop = False
    for l in self._listeners:
      if l.after_save(session, step):
        logging.info(
            "A CheckpointSaverListener requested that training be stopped. "
            "listener: {}".format(l))
        should_stop = True
    return should_stop

  def _get_saver(self):
    if self._saver is not None:
      return self._saver
    elif self._scaffold is not None:
      return self._scaffold.saver

    # Get saver from the SAVERS collection if present.
    collection_key = ops.GraphKeys.SAVERS
    savers = ops.get_collection(collection_key)
    if not savers:
      raise RuntimeError(
          "No items in collection {}. Please add a saver to the collection "
          "or provide a saver or scaffold.".format(collection_key))
    elif len(savers) > 1:
      raise RuntimeError(
          "More than one item in collection {}. "
          "Please indicate which one to use by passing it to the constructor.".
          format(collection_key))

    self._saver = savers[0]
    return savers[0]


@tf_export(v1=["train.StepCounterHook"])
class StepCounterHook(session_run_hook.SessionRunHook):
  """Hook that counts steps per second."""

  def __init__(self,
               every_n_steps=100,
               every_n_secs=None,
               output_dir=None,
               summary_writer=None):

    if (every_n_steps is None) == (every_n_secs is None):
      raise ValueError(
          "exactly one of every_n_steps and every_n_secs should be provided.")
    self._timer = SecondOrStepTimer(every_steps=every_n_steps,
                                    every_secs=every_n_secs)

    self._summary_writer = summary_writer
    self._output_dir = output_dir
    self._last_global_step = None
    self._global_step_check_count = 0
    self._steps_per_run = 1

  def _set_steps_per_run(self, steps_per_run):
    self._steps_per_run = steps_per_run

  def begin(self):
    if self._summary_writer is None and self._output_dir:
      self._summary_writer = SummaryWriterCache.get(self._output_dir)
    self._global_step_tensor = training_util._get_or_create_global_step_read()  # pylint: disable=protected-access
    if self._global_step_tensor is None:
      raise RuntimeError(
          "Global step should be created to use StepCounterHook.")
    self._summary_tag = training_util.get_global_step().op.name + "/sec"

  def before_run(self, run_context):  # pylint: disable=unused-argument
    return SessionRunArgs(self._global_step_tensor)

  def _log_and_record(self, elapsed_steps, elapsed_time, global_step):
    steps_per_sec = elapsed_steps / elapsed_time
    if self._summary_writer is not None:
      summary = Summary(value=[Summary.Value(
          tag=self._summary_tag, simple_value=steps_per_sec)])
      self._summary_writer.add_summary(summary, global_step)
    logging.info("%s: %g", self._summary_tag, steps_per_sec)

  def after_run(self, run_context, run_values):
    _ = run_context

    stale_global_step = run_values.results
    if self._timer.should_trigger_for_step(
        stale_global_step + self._steps_per_run):
      # get the real value after train op.
      global_step = run_context.session.run(self._global_step_tensor)
      if self._timer.should_trigger_for_step(global_step):
        elapsed_time, elapsed_steps = self._timer.update_last_triggered_step(
            global_step)
        if elapsed_time is not None:
          self._log_and_record(elapsed_steps, elapsed_time, global_step)

    # Check whether the global step has been increased. Here, we do not use the
    # timer.last_triggered_step as the timer might record a different global
    # step value such that the comparison could be unreliable. For simplicity,
    # we just compare the stale_global_step with previously recorded version.
    if stale_global_step == self._last_global_step:
      # Here, we use a counter to count how many times we have observed that the
      # global step has not been increased. For some Optimizers, the global step
      # is not increased each time by design. For example, SyncReplicaOptimizer
      # doesn't increase the global step in worker's main train step.
      self._global_step_check_count += 1
      if self._global_step_check_count % 20 == 0:
        self._global_step_check_count = 0
        logging.warning(
            "It seems that global step (tf.train.get_global_step) has not "
            "been increased. Current value (could be stable): %s vs previous "
            "value: %s. You could increase the global step by passing "
            "tf.train.get_global_step() to Optimizer.apply_gradients or "
            "Optimizer.minimize.", stale_global_step, self._last_global_step)
    else:
      # Whenever we observe the increment, reset the counter.
      self._global_step_check_count = 0

    self._last_global_step = stale_global_step


@tf_export(v1=["train.NanLossDuringTrainingError"])
class NanLossDuringTrainingError(RuntimeError):

  def __str__(self):
    return "NaN loss during training."


@tf_export(v1=["train.NanTensorHook"])
class NanTensorHook(session_run_hook.SessionRunHook):
  """Monitors the loss tensor and stops training if loss is NaN.

  Can either fail with exception or just stop training.
  """

  def __init__(self, loss_tensor, fail_on_nan_loss=True):
    """Initializes a `NanTensorHook`.

    Args:
      loss_tensor: `Tensor`, the loss tensor.
      fail_on_nan_loss: `bool`, whether to raise exception when loss is NaN.
    """
    self._loss_tensor = loss_tensor
    self._fail_on_nan_loss = fail_on_nan_loss

  def before_run(self, run_context):  # pylint: disable=unused-argument
    return SessionRunArgs(self._loss_tensor)

  def after_run(self, run_context, run_values):
    if np.isnan(run_values.results):
      failure_message = "Model diverged with loss = NaN."
      if self._fail_on_nan_loss:
        logging.error(failure_message)
        raise NanLossDuringTrainingError
      else:
        logging.warning(failure_message)
        # We don't raise an error but we request stop without an exception.
        run_context.request_stop()


@tf_export(v1=["train.SummarySaverHook"])
class SummarySaverHook(session_run_hook.SessionRunHook):
  """Saves summaries every N steps."""

  def __init__(self,
               save_steps=None,
               save_secs=None,
               output_dir=None,
               summary_writer=None,
               scaffold=None,
               summary_op=None):
    """Initializes a `SummarySaverHook`.

    Args:
      save_steps: `int`, save summaries every N steps. Exactly one of
          `save_secs` and `save_steps` should be set.
      save_secs: `int`, save summaries every N seconds.
      output_dir: `string`, the directory to save the summaries to. Only used
          if no `summary_writer` is supplied.
      summary_writer: `SummaryWriter`. If `None` and an `output_dir` was passed,
          one will be created accordingly.
      scaffold: `Scaffold` to get summary_op if it's not provided.
      summary_op: `Tensor` of type `string` containing the serialized `Summary`
          protocol buffer or a list of `Tensor`. They are most likely an output
          by TF summary methods like `tf.summary.scalar` or
          `tf.summary.merge_all`. It can be passed in as one tensor; if more
          than one, they must be passed in as a list.

    Raises:
      ValueError: Exactly one of scaffold or summary_op should be set.
    """
    if ((scaffold is None and summary_op is None) or
        (scaffold is not None and summary_op is not None)):
      raise ValueError(
          "Exactly one of scaffold or summary_op must be provided.")
    self._summary_op = summary_op
    self._summary_writer = summary_writer
    self._output_dir = output_dir
    self._scaffold = scaffold
    self._timer = SecondOrStepTimer(every_secs=save_secs,
                                    every_steps=save_steps)
    # TODO(mdan): Throw an error if output_dir and summary_writer are None.

  def begin(self):
    if self._summary_writer is None and self._output_dir:
      self._summary_writer = SummaryWriterCache.get(self._output_dir)
    self._next_step = None
    self._global_step_tensor = training_util._get_or_create_global_step_read()  # pylint: disable=protected-access
    if self._global_step_tensor is None:
      raise RuntimeError(
          "Global step should be created to use SummarySaverHook.")

  def before_run(self, run_context):  # pylint: disable=unused-argument
    self._request_summary = (
        self._next_step is None or
        self._timer.should_trigger_for_step(self._next_step))
    requests = {"global_step": self._global_step_tensor}
    if self._request_summary:
      if self._get_summary_op() is not None:
        requests["summary"] = self._get_summary_op()

    return SessionRunArgs(requests)

  def after_run(self, run_context, run_values):
    _ = run_context
    if not self._summary_writer:
      return

    stale_global_step = run_values.results["global_step"]
    global_step = stale_global_step + 1
    if self._next_step is None or self._request_summary:
      global_step = run_context.session.run(self._global_step_tensor)

    if self._next_step is None:
      self._summary_writer.add_session_log(
          SessionLog(status=SessionLog.START), global_step)

    if self._request_summary:
      self._timer.update_last_triggered_step(global_step)
      if "summary" in run_values.results:
        for summary in run_values.results["summary"]:
          self._summary_writer.add_summary(summary, global_step)

    self._next_step = global_step + 1

  def end(self, session=None):
    if self._summary_writer:
      self._summary_writer.flush()

  def _get_summary_op(self):
    """Fetches the summary op either from self._summary_op or self._scaffold.

    Returns:
      Returns a list of summary `Tensor`.
    """
    summary_op = None
    if self._summary_op is not None:
      summary_op = self._summary_op
    elif self._scaffold.summary_op is not None:
      summary_op = self._scaffold.summary_op

    if summary_op is None:
      return None

    if not isinstance(summary_op, list):
      return [summary_op]
    return summary_op


@tf_export(v1=["train.GlobalStepWaiterHook"])
class GlobalStepWaiterHook(session_run_hook.SessionRunHook):
  """Delays execution until global step reaches `wait_until_step`.

  This hook delays execution until global step reaches to `wait_until_step`. It
  is used to gradually start workers in distributed settings. One example usage
  would be setting `wait_until_step=int(K*log(task_id+1))` assuming that
  task_id=0 is the chief.
  """

  def __init__(self, wait_until_step):
    """Initializes a `GlobalStepWaiterHook`.

    Args:
      wait_until_step: an `int` shows until which global step should we wait.
    """
    self._wait_until_step = wait_until_step

  def begin(self):
    self._worker_is_started = False
    self._global_step_tensor = training_util._get_or_create_global_step_read()  # pylint: disable=protected-access
    if self._global_step_tensor is None:
      raise RuntimeError(
          "Global step should be created to use _GlobalStepWaiterHook.")

  def before_run(self, run_context):
    if self._worker_is_started:
      return None

    if self._wait_until_step <= 0:
      self._worker_is_started = True
      return None

    logging.info("Waiting for global step %d before starting training.",
                 self._wait_until_step)
    last_logged_step = 0
    while True:
      current_step = run_context.session.run(self._global_step_tensor)
      if current_step >= self._wait_until_step:
        self._worker_is_started = True
        return None
      if current_step - last_logged_step > 1000:
        logging.info("Waiting for global step %d before starting training. "
                     "Current step is %d.", self._wait_until_step, current_step)
        last_logged_step = current_step
      time.sleep(0.5)


@tf_export(v1=["train.FinalOpsHook"])
class FinalOpsHook(session_run_hook.SessionRunHook):
  """A hook which evaluates `Tensors` at the end of a session."""

  def __init__(self, final_ops, final_ops_feed_dict=None):
    """Initializes `FinalOpHook` with ops to run at the end of the session.

    Args:
      final_ops: A single `Tensor`, a list of `Tensors` or a dictionary of
        names to `Tensors`.
      final_ops_feed_dict: A feed dictionary to use when running
        `final_ops_dict`.
    """
    self._final_ops = final_ops
    self._final_ops_feed_dict = final_ops_feed_dict
    self._final_ops_values = None

  @property
  def final_ops_values(self):
    return self._final_ops_values

  def end(self, session):
    if self._final_ops is not None:
      try:
        self._final_ops_values = session.run(
            self._final_ops, feed_dict=self._final_ops_feed_dict)
      except (errors.OutOfRangeError, StopIteration) as e:
        logging.warning(
            "An OutOfRangeError or StopIteration exception is raised by the "
            "code in FinalOpsHook. This typically means the Ops running by the "
            "FinalOpsHook have a dependency back to some input source, which "
            "should not happen. For example, for metrics in "
            "tf.estimator.Estimator, all metrics functions return two Ops: "
            "`value_op` and  `update_op`. Estimator.evaluate calls the "
            "`update_op` for each batch of the data in input source and, once "
            "it is exhausted, it call the `value_op` to get the metric values. "
            "The `value_op` here should have dependency back to variables "
            "reading only, rather than reading another batch from input. "
            "Otherwise, the `value_op`, executed by `FinalOpsHook`, triggers "
            "another data reading, which ends OutOfRangeError/StopIteration. "
            "Please fix that.")
        raise e


@tf_export(v1=["train.FeedFnHook"])
class FeedFnHook(session_run_hook.SessionRunHook):
  """Runs `feed_fn` and sets the `feed_dict` accordingly."""

  def __init__(self, feed_fn):
    """Initializes a `FeedFnHook`.

    Args:
      feed_fn: function that takes no arguments and returns `dict` of `Tensor`
        to feed.
    """
    self.feed_fn = feed_fn

  def before_run(self, run_context):  # pylint: disable=unused-argument
    return session_run_hook.SessionRunArgs(
        fetches=None, feed_dict=self.feed_fn())


@tf_export(v1=["train.ProfilerHook"])
class ProfilerHook(session_run_hook.SessionRunHook):
  """Captures CPU/GPU profiling information every N steps or seconds.

  This produces files called "timeline-<step>.json", which are in Chrome
  Trace format.

  For more information see:
  https://github.com/catapult-project/catapult/blob/master/tracing/README.md
  """

  def __init__(self,
               save_steps=None,
               save_secs=None,
               output_dir="",
               show_dataflow=True,
               show_memory=False):
    """Initializes a hook that takes periodic profiling snapshots.

    `options.run_metadata` argument of `tf.Session.Run` is used to collect
    metadata about execution. This hook sets the metadata and dumps it in Chrome
    Trace format.


    Args:
      save_steps: `int`, save profile traces every N steps. Exactly one of
          `save_secs` and `save_steps` should be set.
      save_secs: `int` or `float`, save profile traces every N seconds.
      output_dir: `string`, the directory to save the profile traces to.
          Defaults to the current directory.
      show_dataflow: `bool`, if True, add flow events to the trace connecting
          producers and consumers of tensors.
      show_memory: `bool`, if True, add object snapshot events to the trace
          showing the sizes and lifetimes of tensors.
    """
    self._output_file = os.path.join(output_dir, "timeline-{}.json")
    self._file_writer = SummaryWriterCache.get(output_dir)
    self._show_dataflow = show_dataflow
    self._show_memory = show_memory
    self._timer = SecondOrStepTimer(
        every_secs=save_secs, every_steps=save_steps)

  def begin(self):
    self._next_step = None
    self._global_step_tensor = training_util._get_or_create_global_step_read()  # pylint: disable=protected-access
    if self._global_step_tensor is None:
      raise RuntimeError("Global step should be created to use ProfilerHook.")

  def before_run(self, run_context):
    self._request_summary = (
        self._next_step is not None and
        self._timer.should_trigger_for_step(self._next_step))
    requests = {"global_step": self._global_step_tensor}
    opts = (config_pb2.RunOptions(trace_level=config_pb2.RunOptions.FULL_TRACE)
            if self._request_summary else None)

    return SessionRunArgs(requests, options=opts)

  def after_run(self, run_context, run_values):
    stale_global_step = run_values.results["global_step"]
    if self._next_step is None:
      # Update the timer so that it does not activate until N steps or seconds
      # have passed.
      self._timer.update_last_triggered_step(stale_global_step)
    global_step = stale_global_step + 1
    if self._request_summary:
      global_step = run_context.session.run(self._global_step_tensor)
      self._timer.update_last_triggered_step(global_step)
      self._save(global_step,
                 self._output_file.format(global_step),
                 run_values.run_metadata.step_stats)
      self._file_writer.add_run_metadata(run_values.run_metadata,
                                         "step_%d" % global_step)

    self._next_step = global_step + 1

  def _save(self, step, save_path, step_stats):
    logging.info("Saving timeline for %d into '%s'.", step, save_path)
    with gfile.Open(save_path, "w") as f:
      trace = timeline.Timeline(step_stats)
      f.write(
          trace.generate_chrome_trace_format(
              show_dataflow=self._show_dataflow, show_memory=self._show_memory))


def _as_graph_element(obj):
  """Retrieves Graph element."""
  graph = ops.get_default_graph()
  if not isinstance(obj, six.string_types):
    if not hasattr(obj, "graph") or obj.graph != graph:
      raise ValueError("Passed %s should have graph attribute that is equal "
                       "to current graph %s." % (obj, graph))
    return obj
  if ":" in obj:
    element = graph.as_graph_element(obj)
  else:
    element = graph.as_graph_element(obj + ":0")
    # Check that there is no :1 (e.g. it's single output).
    try:
      graph.as_graph_element(obj + ":1")
    except (KeyError, ValueError):
      pass
    else:
      raise ValueError("Name %s is ambiguous, "
                       "as this `Operation` has multiple outputs "
                       "(at least 2)." % obj)
  return element