xref: /external/tensorflow/tensorflow/python/keras/callbacks.py

# Copyright 2015 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.
# ==============================================================================
# pylint: disable=g-import-not-at-top
"""Callbacks: utilities called at certain points during model training.
"""
from __future__ import absolute_import
from __future__ import division
from __future__ import print_function

import collections
import copy
import csv
import io
import json
import os
import re
import tempfile
import time

import numpy as np
import six

from tensorflow.python.data.ops import iterator_ops
from tensorflow.python.distribute import distributed_file_utils
from tensorflow.python.distribute import multi_worker_util
from tensorflow.python.eager import context
from tensorflow.python.framework import ops
from tensorflow.python.keras import backend as K
from tensorflow.python.keras.distribute import multi_worker_training_state as training_state
from tensorflow.python.keras.utils.data_utils import Sequence
from tensorflow.python.keras.utils.generic_utils import Progbar
from tensorflow.python.keras.utils.mode_keys import ModeKeys
from tensorflow.python.lib.io import file_io
from tensorflow.python.ops import array_ops
from tensorflow.python.ops import math_ops
from tensorflow.python.ops import summary_ops_v2
from tensorflow.python.ops import variables
from tensorflow.python.platform import tf_logging as logging
from tensorflow.python.training import checkpoint_management
from tensorflow.python.util.compat import collections_abc
from tensorflow.python.util.tf_export import keras_export
from tensorflow.tools.docs import doc_controls

try:
  import requests
except ImportError:
  requests = None


def configure_callbacks(callbacks,
                        model,
                        do_validation=False,
                        batch_size=None,
                        epochs=None,
                        steps_per_epoch=None,
                        samples=None,
                        verbose=1,
                        count_mode='steps',
                        mode=ModeKeys.TRAIN):
  """Configures callbacks for use in various training loops.

  Arguments:
      callbacks: List of Callbacks.
      model: Model being trained.
      do_validation: Whether or not validation loop will be run.
      batch_size: Number of samples per batch.
      epochs: Number of epoch to train.
      steps_per_epoch: Number of batches to run per training epoch.
      samples: Number of training samples.
      verbose: int, 0 or 1. Keras logging verbosity to pass to ProgbarLogger.
      count_mode: One of 'steps' or 'samples'. Per-batch or per-sample count.
      mode: String. One of ModeKeys.TRAIN, ModeKeys.TEST, or ModeKeys.PREDICT.
        Which loop mode to configure callbacks for.

  Returns:
      Instance of CallbackList used to control all Callbacks.
  """
  # Check if callbacks have already been configured.
  if isinstance(callbacks, CallbackList):
    return callbacks

  if not callbacks:
    callbacks = []

  # Add additional callbacks during training.
  if mode == ModeKeys.TRAIN:
    model.history = History()
    callbacks = [BaseLogger()] + (callbacks or []) + [model.history]
    if verbose:
      callbacks.append(ProgbarLogger(count_mode))
  callback_list = CallbackList(callbacks)

  # Set callback model
  callback_model = model._get_callback_model()  # pylint: disable=protected-access
  callback_list.set_model(callback_model)

  set_callback_parameters(
      callback_list,
      model,
      do_validation=do_validation,
      batch_size=batch_size,
      epochs=epochs,
      steps_per_epoch=steps_per_epoch,
      samples=samples,
      verbose=verbose,
      mode=mode)

  callback_list.model.stop_training = False
  return callback_list


def set_callback_parameters(callback_list,
                            model,
                            do_validation=False,
                            batch_size=None,
                            epochs=None,
                            steps_per_epoch=None,
                            samples=None,
                            verbose=1,
                            mode=ModeKeys.TRAIN):
  """Sets callback parameters.

  Arguments:
      callback_list: CallbackList instance.
      model: Model being trained.
      do_validation: Whether or not validation loop will be run.
      batch_size: Number of samples per batch.
      epochs: Number of epoch to train.
      steps_per_epoch: Number of batches to run per training epoch.
      samples: Number of training samples.
      verbose: int, 0 or 1. Keras logging verbosity to pass to ProgbarLogger.
      mode: String. One of ModeKeys.TRAIN, ModeKeys.TEST, or ModeKeys.PREDICT.
        Which loop mode to configure callbacks for.
  """
  metric_names = model.metrics_names
  for cbk in callback_list:
    if isinstance(cbk, (BaseLogger, ProgbarLogger)):
      cbk.stateful_metrics = metric_names[1:]  # Exclude `loss`

  # Set callback parameters
  callback_metrics = []
  # When we have deferred build scenario with iterator input, we will compile
  # when we standardize first batch of data.
  if mode != ModeKeys.PREDICT:
    callback_metrics = copy.copy(metric_names)
    if do_validation:
      callback_metrics += ['val_' + n for n in metric_names]
  callback_params = {
      'batch_size': batch_size,
      'epochs': epochs,
      'steps': steps_per_epoch,
      'samples': samples,
      'verbose': verbose,
      'do_validation': do_validation,
      'metrics': callback_metrics,
  }
  callback_list.set_params(callback_params)


def _is_generator_like(data):
  """Checks if data is a generator, Sequence, or Iterator."""
  return (hasattr(data, 'next') or hasattr(data, '__next__') or isinstance(
      data, (Sequence, iterator_ops.Iterator, iterator_ops.OwnedIterator)))


def make_logs(model, logs, outputs, mode, prefix=''):
  """Computes logs for sending to `on_batch_end` methods."""
  metric_names = model.metrics_names
  if mode in {ModeKeys.TRAIN, ModeKeys.TEST} and metric_names:
    for label, output in zip(metric_names, outputs):
      logs[prefix + label] = output
  else:
    logs['outputs'] = outputs
  return logs


class CallbackList(object):
  """Container abstracting a list of callbacks.

  Arguments:
      callbacks: List of `Callback` instances.
      queue_length: Queue length for keeping
          running statistics over callback execution time.
  """

  def __init__(self, callbacks=None, queue_length=10):
    callbacks = callbacks or []
    self.callbacks = [c for c in callbacks]
    self.queue_length = queue_length
    self.params = {}
    self.model = None
    self._reset_batch_timing()

  def _reset_batch_timing(self):
    self._delta_t_batch = 0.
    self._delta_ts = collections.defaultdict(
        lambda: collections.deque([], maxlen=self.queue_length))

  def append(self, callback):
    self.callbacks.append(callback)

  def set_params(self, params):
    self.params = params
    for callback in self.callbacks:
      callback.set_params(params)

  def set_model(self, model):
    self.model = model
    for callback in self.callbacks:
      callback.set_model(model)

  def _call_batch_hook(self, mode, hook, batch, logs=None):
    """Helper function for all batch_{begin | end} methods."""
    if not self.callbacks:
      return
    hook_name = 'on_{mode}_batch_{hook}'.format(mode=mode, hook=hook)
    if hook == 'begin':
      self._t_enter_batch = time.time()
    if hook == 'end':
      # Batch is ending, calculate batch time.
      self._delta_t_batch = time.time() - self._t_enter_batch

    logs = logs or {}
    t_before_callbacks = time.time()
    for callback in self.callbacks:
      batch_hook = getattr(callback, hook_name)
      batch_hook(batch, logs)
    self._delta_ts[hook_name].append(time.time() - t_before_callbacks)

    delta_t_median = np.median(self._delta_ts[hook_name])
    if (self._delta_t_batch > 0. and
        delta_t_median > 0.95 * self._delta_t_batch and delta_t_median > 0.1):
      logging.warning(
          'Method (%s) is slow compared '
          'to the batch update (%f). Check your callbacks.', hook_name,
          delta_t_median)

  def _call_begin_hook(self, mode):
    """Helper function for on_{train|test|predict}_begin methods."""
    if mode == ModeKeys.TRAIN:
      self.on_train_begin()
    elif mode == ModeKeys.TEST:
      self.on_test_begin()
    else:
      self.on_predict_begin()

  def _call_end_hook(self, mode):
    """Helper function for on_{train|test|predict}_end methods."""
    if mode == ModeKeys.TRAIN:
      self.on_train_end()
    elif mode == ModeKeys.TEST:
      self.on_test_end()
    else:
      self.on_predict_end()

  def on_batch_begin(self, batch, logs=None):
    self._call_batch_hook(ModeKeys.TRAIN, 'begin', batch, logs=logs)

  def on_batch_end(self, batch, logs=None):
    self._call_batch_hook(ModeKeys.TRAIN, 'end', batch, logs=logs)

  def on_epoch_begin(self, epoch, logs=None):
    """Calls the `on_epoch_begin` methods of its callbacks.

    This function should only be called during TRAIN mode.

    Arguments:
        epoch: integer, index of epoch.
        logs: dict. Currently no data is passed to this argument for this method
          but that may change in the future.
    """
    logs = logs or {}
    for callback in self.callbacks:
      callback.on_epoch_begin(epoch, logs)
    self._reset_batch_timing()

  def on_epoch_end(self, epoch, logs=None):
    """Calls the `on_epoch_end` methods of its callbacks.

    This function should only be called during TRAIN mode.

    Arguments:
        epoch: integer, index of epoch.
        logs: dict, metric results for this training epoch, and for the
          validation epoch if validation is performed. Validation result keys
          are prefixed with `val_`.
    """
    logs = logs or {}
    for callback in self.callbacks:
      callback.on_epoch_end(epoch, logs)

  def on_train_batch_begin(self, batch, logs=None):
    """Calls the `on_train_batch_begin` methods of its callbacks.

    Arguments:
        batch: integer, index of batch within the current epoch.
        logs: dict. Has keys `batch` and `size` representing the current batch
          number and the size of the batch.
    """
    self._call_batch_hook(ModeKeys.TRAIN, 'begin', batch, logs=logs)

  def on_train_batch_end(self, batch, logs=None):
    """Calls the `on_train_batch_end` methods of its callbacks.

    Arguments:
        batch: integer, index of batch within the current epoch.
        logs: dict. Metric results for this batch.
    """
    self._call_batch_hook(ModeKeys.TRAIN, 'end', batch, logs=logs)

  def on_test_batch_begin(self, batch, logs=None):
    """Calls the `on_test_batch_begin` methods of its callbacks.

    Arguments:
        batch: integer, index of batch within the current epoch.
        logs: dict. Has keys `batch` and `size` representing the current batch
          number and the size of the batch.
    """
    self._call_batch_hook(ModeKeys.TEST, 'begin', batch, logs=logs)

  def on_test_batch_end(self, batch, logs=None):
    """Calls the `on_test_batch_end` methods of its callbacks.

    Arguments:
        batch: integer, index of batch within the current epoch.
        logs: dict. Metric results for this batch.
    """
    self._call_batch_hook(ModeKeys.TEST, 'end', batch, logs=logs)

  def on_predict_batch_begin(self, batch, logs=None):
    """Calls the `on_predict_batch_begin` methods of its callbacks.

    Arguments:
        batch: integer, index of batch within the current epoch.
        logs: dict. Has keys `batch` and `size` representing the current batch
          number and the size of the batch.
    """
    self._call_batch_hook(ModeKeys.PREDICT, 'begin', batch, logs=logs)

  def on_predict_batch_end(self, batch, logs=None):
    """Calls the `on_predict_batch_end` methods of its callbacks.

    Arguments:
        batch: integer, index of batch within the current epoch.
        logs: dict. Metric results for this batch.
    """
    self._call_batch_hook(ModeKeys.PREDICT, 'end', batch, logs=logs)

  def on_train_begin(self, logs=None):
    """Calls the `on_train_begin` methods of its callbacks.

    Arguments:
        logs: dict. Currently no data is passed to this argument for this method
          but that may change in the future.
    """
    for callback in self.callbacks:
      callback.on_train_begin(logs)

  def on_train_end(self, logs=None):
    """Calls the `on_train_end` methods of its callbacks.

    Arguments:
        logs: dict. Currently no data is passed to this argument for this method
          but that may change in the future.
    """
    for callback in self.callbacks:
      callback.on_train_end(logs)

  def on_test_begin(self, logs=None):
    """Calls the `on_test_begin` methods of its callbacks.

    Arguments:
        logs: dict. Currently no data is passed to this argument for this method
          but that may change in the future.
    """
    for callback in self.callbacks:
      callback.on_test_begin(logs)

  def on_test_end(self, logs=None):
    """Calls the `on_test_end` methods of its callbacks.

    Arguments:
        logs: dict. Currently no data is passed to this argument for this method
          but that may change in the future.
    """
    for callback in self.callbacks:
      callback.on_test_end(logs)

  def on_predict_begin(self, logs=None):
    """Calls the 'on_predict_begin` methods of its callbacks.

    Arguments:
        logs: dict. Currently no data is passed to this argument for this method
          but that may change in the future.
    """
    for callback in self.callbacks:
      callback.on_predict_begin(logs)

  def on_predict_end(self, logs=None):
    """Calls the `on_predict_end` methods of its callbacks.

    Arguments:
        logs: dict. Currently no data is passed to this argument for this method
          but that may change in the future.
    """
    for callback in self.callbacks:
      callback.on_predict_end(logs)

  def __iter__(self):
    return iter(self.callbacks)


@keras_export('keras.callbacks.Callback')
class Callback(object):
  """Abstract base class used to build new callbacks.

  Attributes:
      params: dict. Training parameters
          (eg. verbosity, batch size, number of epochs...).
      model: instance of `keras.models.Model`.
          Reference of the model being trained.
      validation_data: Deprecated. Do not use.

  The `logs` dictionary that callback methods
  take as argument will contain keys for quantities relevant to
  the current batch or epoch.

  Currently, the `.fit()` method of the `Model` class
  will include the following quantities in the `logs` that
  it passes to its callbacks:

      on_epoch_end: logs include `acc` and `loss`, and
          optionally include `val_loss`
          (if validation is enabled in `fit`), and `val_acc`
          (if validation and accuracy monitoring are enabled).
      on_batch_begin: logs include `size`,
          the number of samples in the current batch.
      on_batch_end: logs include `loss`, and optionally `acc`
          (if accuracy monitoring is enabled).
  """

  def __init__(self):
    self.validation_data = None
    self.model = None
    # Whether this Callback should only run on the chief worker in a
    # Multi-Worker setting.
    # TODO(omalleyt): Make this attr public once solution is stable.
    self._chief_worker_only = None

  def set_params(self, params):
    self.params = params

  def set_model(self, model):
    self.model = model

  @doc_controls.for_subclass_implementers
  def on_batch_begin(self, batch, logs=None):
    """A backwards compatibility alias for `on_train_batch_begin`."""

  @doc_controls.for_subclass_implementers
  def on_batch_end(self, batch, logs=None):
    """A backwards compatibility alias for `on_train_batch_end`."""

  @doc_controls.for_subclass_implementers
  def on_epoch_begin(self, epoch, logs=None):
    """Called at the start of an epoch.

    Subclasses should override for any actions to run. This function should only
    be called during TRAIN mode.

    Arguments:
        epoch: integer, index of epoch.
        logs: dict. Currently no data is passed to this argument for this method
          but that may change in the future.
    """

  @doc_controls.for_subclass_implementers
  def on_epoch_end(self, epoch, logs=None):
    """Called at the end of an epoch.

    Subclasses should override for any actions to run. This function should only
    be called during TRAIN mode.

    Arguments:
        epoch: integer, index of epoch.
        logs: dict, metric results for this training epoch, and for the
          validation epoch if validation is performed. Validation result keys
          are prefixed with `val_`.
    """

  @doc_controls.for_subclass_implementers
  def on_train_batch_begin(self, batch, logs=None):
    """Called at the beginning of a training batch in `fit` methods.

    Subclasses should override for any actions to run.

    Arguments:
        batch: integer, index of batch within the current epoch.
        logs: dict. Has keys `batch` and `size` representing the current batch
          number and the size of the batch.
    """
    # For backwards compatibility.
    self.on_batch_begin(batch, logs=logs)

  @doc_controls.for_subclass_implementers
  def on_train_batch_end(self, batch, logs=None):
    """Called at the end of a training batch in `fit` methods.

    Subclasses should override for any actions to run.

    Arguments:
        batch: integer, index of batch within the current epoch.
        logs: dict. Metric results for this batch.
    """
    # For backwards compatibility.
    self.on_batch_end(batch, logs=logs)

  @doc_controls.for_subclass_implementers
  def on_test_batch_begin(self, batch, logs=None):
    """Called at the beginning of a batch in `evaluate` methods.

    Also called at the beginning of a validation batch in the `fit`
    methods, if validation data is provided.

    Subclasses should override for any actions to run.

    Arguments:
        batch: integer, index of batch within the current epoch.
        logs: dict. Has keys `batch` and `size` representing the current batch
          number and the size of the batch.
    """

  @doc_controls.for_subclass_implementers
  def on_test_batch_end(self, batch, logs=None):
    """Called at the end of a batch in `evaluate` methods.

    Also called at the end of a validation batch in the `fit`
    methods, if validation data is provided.

    Subclasses should override for any actions to run.

    Arguments:
        batch: integer, index of batch within the current epoch.
        logs: dict. Metric results for this batch.
    """

  @doc_controls.for_subclass_implementers
  def on_predict_batch_begin(self, batch, logs=None):
    """Called at the beginning of a batch in `predict` methods.

    Subclasses should override for any actions to run.

    Arguments:
        batch: integer, index of batch within the current epoch.
        logs: dict. Has keys `batch` and `size` representing the current batch
          number and the size of the batch.
    """

  @doc_controls.for_subclass_implementers
  def on_predict_batch_end(self, batch, logs=None):
    """Called at the end of a batch in `predict` methods.

    Subclasses should override for any actions to run.

    Arguments:
        batch: integer, index of batch within the current epoch.
        logs: dict. Metric results for this batch.
    """

  @doc_controls.for_subclass_implementers
  def on_train_begin(self, logs=None):
    """Called at the beginning of training.

    Subclasses should override for any actions to run.

    Arguments:
        logs: dict. Currently no data is passed to this argument for this method
          but that may change in the future.
    """

  @doc_controls.for_subclass_implementers
  def on_train_end(self, logs=None):
    """Called at the end of training.

    Subclasses should override for any actions to run.

    Arguments:
        logs: dict. Currently no data is passed to this argument for this method
          but that may change in the future.
    """

  @doc_controls.for_subclass_implementers
  def on_test_begin(self, logs=None):
    """Called at the beginning of evaluation or validation.

    Subclasses should override for any actions to run.

    Arguments:
        logs: dict. Currently no data is passed to this argument for this method
          but that may change in the future.
    """

  @doc_controls.for_subclass_implementers
  def on_test_end(self, logs=None):
    """Called at the end of evaluation or validation.

    Subclasses should override for any actions to run.

    Arguments:
        logs: dict. Currently no data is passed to this argument for this method
          but that may change in the future.
    """

  @doc_controls.for_subclass_implementers
  def on_predict_begin(self, logs=None):
    """Called at the beginning of prediction.

    Subclasses should override for any actions to run.

    Arguments:
        logs: dict. Currently no data is passed to this argument for this method
          but that may change in the future.
    """

  @doc_controls.for_subclass_implementers
  def on_predict_end(self, logs=None):
    """Called at the end of prediction.

    Subclasses should override for any actions to run.

    Arguments:
        logs: dict. Currently no data is passed to this argument for this method
          but that may change in the future.
    """


@keras_export('keras.callbacks.BaseLogger')
class BaseLogger(Callback):
  """Callback that accumulates epoch averages of metrics.

  This callback is automatically applied to every Keras model.

  Arguments:
      stateful_metrics: Iterable of string names of metrics that
          should *not* be averaged over an epoch.
          Metrics in this list will be logged as-is in `on_epoch_end`.
          All others will be averaged in `on_epoch_end`.
  """

  def __init__(self, stateful_metrics=None):
    super(BaseLogger, self).__init__()
    self.stateful_metrics = set(stateful_metrics or [])

  def on_epoch_begin(self, epoch, logs=None):
    self.seen = 0
    self.totals = {}

  def on_batch_end(self, batch, logs=None):
    logs = logs or {}
    batch_size = logs.get('size', 0)
    # In case of distribution strategy we can potentially run multiple steps
    # at the same time, we should account for that in the `seen` calculation.
    num_steps = logs.get('num_steps', 1)
    self.seen += batch_size * num_steps

    for k, v in logs.items():
      if k in self.stateful_metrics:
        self.totals[k] = v
      else:
        if k in self.totals:
          self.totals[k] += v * batch_size
        else:
          self.totals[k] = v * batch_size

  def on_epoch_end(self, epoch, logs=None):
    if logs is not None:
      for k in self.params['metrics']:
        if k in self.totals:
          # Make value available to next callbacks.
          if k in self.stateful_metrics:
            logs[k] = self.totals[k]
          else:
            logs[k] = self.totals[k] / self.seen


@keras_export('keras.callbacks.TerminateOnNaN')
class TerminateOnNaN(Callback):
  """Callback that terminates training when a NaN loss is encountered.
  """

  def on_batch_end(self, batch, logs=None):
    logs = logs or {}
    loss = logs.get('loss')
    if loss is not None:
      if np.isnan(loss) or np.isinf(loss):
        print('Batch %d: Invalid loss, terminating training' % (batch))
        self.model.stop_training = True


@keras_export('keras.callbacks.ProgbarLogger')
class ProgbarLogger(Callback):
  """Callback that prints metrics to stdout.

  Arguments:
      count_mode: One of "steps" or "samples".
          Whether the progress bar should
          count samples seen or steps (batches) seen.
      stateful_metrics: Iterable of string names of metrics that
          should *not* be averaged over an epoch.
          Metrics in this list will be logged as-is.
          All others will be averaged over time (e.g. loss, etc).

  Raises:
      ValueError: In case of invalid `count_mode`.
  """

  def __init__(self, count_mode='samples', stateful_metrics=None):
    super(ProgbarLogger, self).__init__()
    if count_mode == 'samples':
      self.use_steps = False
    elif count_mode == 'steps':
      self.use_steps = True
    else:
      raise ValueError('Unknown `count_mode`: ' + str(count_mode))
    self.stateful_metrics = set(stateful_metrics or [])
    self.log_values = None

  def on_train_begin(self, logs=None):
    self.verbose = self.params['verbose']
    self.epochs = self.params['epochs']

  def on_epoch_begin(self, epoch, logs=None):
    self.seen = 0
    if self.use_steps:
      self.target = self.params['steps']
    else:
      self.target = self.params['samples']

    if self.verbose:
      if self.epochs > 1:
        print('Epoch %d/%d' % (epoch + 1, self.epochs))
    self.progbar = Progbar(
        target=self.target,
        verbose=self.verbose,
        stateful_metrics=self.stateful_metrics,
        unit_name='step' if self.use_steps else 'sample')

  def on_batch_begin(self, batch, logs=None):
    self.log_values = []

  def on_batch_end(self, batch, logs=None):
    logs = logs or {}
    batch_size = logs.get('size', 0)
    # In case of distribution strategy we can potentially run multiple steps
    # at the same time, we should account for that in the `seen` calculation.
    num_steps = logs.get('num_steps', 1)
    if self.use_steps:
      self.seen += num_steps
    else:
      self.seen += batch_size * num_steps

    for k in self.params['metrics']:
      if k in logs:
        self.log_values.append((k, logs[k]))

    # Skip progbar update for the last batch;
    # will be handled by on_epoch_end.
    if self.verbose and (self.target is None or self.seen < self.target):
      self.progbar.update(self.seen, self.log_values)

  def on_epoch_end(self, epoch, logs=None):
    logs = logs or {}
    for k in self.params['metrics']:
      if k in logs:
        self.log_values.append((k, logs[k]))
    if self.verbose:
      self.progbar.update(self.seen, self.log_values)


@keras_export('keras.callbacks.History')
class History(Callback):
  """Callback that records events into a `History` object.

  This callback is automatically applied to
  every Keras model. The `History` object
  gets returned by the `fit` method of models.
  """

  def on_train_begin(self, logs=None):
    self.epoch = []
    self.history = {}

  def on_epoch_end(self, epoch, logs=None):
    logs = logs or {}
    self.epoch.append(epoch)
    for k, v in logs.items():
      self.history.setdefault(k, []).append(v)


@keras_export('keras.callbacks.ModelCheckpoint')
class ModelCheckpoint(Callback):
  """Callback to save the Keras model or model weights at some frequency.

  `ModelCheckpoint` callback is used in conjunction with training using
  `model.fit()` to save a model or weights (in a checkpoint file) at some
  interval, so the model or weights can be loaded later to continue the training
  from the state saved.

  A few options this callback provides include:

  - Whether to only keep the model that has achieved the "best performance" so
    far, or whether to save the model at the end of every epoch regardless of
    performance.
  - Definition of 'best'; which quantity to monitor and whether it should be
    maximized or minimized.
  - The frequency it should save at. Currently, the callback supports saving at
    the end of every epoch, or after a fixed number of training samples.
  - Whether only weights are saved, or the whole model is saved.

  Example:

  ```python
  EPOCHS = 10
  checkpoint_filepath = '/tmp/checkpoint'
  model_checkpoint_callback = tf.keras.callbacks.ModelCheckpoint(
      filepath=checkpoint_filepath,
      save_weights_only=True,
      monitor='val_acc',
      mode='max',
      save_best_only=True)

  # Model weights are saved at the end of every epoch, if it's the best seen
  # so far.
  model.fit(epochs=EPOCHS, callbacks=[model_checkpoint_callback])

  # The model weights (that are considered the best) are loaded into the model.
  model.load_weights(checkpoint_filepath)
  ```

  Arguments:
      filepath: string, path to save the model file. `filepath` can contain
        named formatting options, which will be filled the value of `epoch` and
        keys in `logs` (passed in `on_epoch_end`). For example: if `filepath` is
        `weights.{epoch:02d}-{val_loss:.2f}.hdf5`, then the model checkpoints
        will be saved with the epoch number and the validation loss in the
        filename.
      monitor: quantity to monitor.
      verbose: verbosity mode, 0 or 1.
      save_best_only: if `save_best_only=True`, the latest best model according
        to the quantity monitored will not be overwritten.
        If `filepath` doesn't contain formatting options like `{epoch}` then
        `filepath` will be overwritten by each new better model.
      mode: one of {auto, min, max}. If `save_best_only=True`, the decision to
        overwrite the current save file is made based on either the maximization
        or the minimization of the monitored quantity. For `val_acc`, this
        should be `max`, for `val_loss` this should be `min`, etc. In `auto`
        mode, the direction is automatically inferred from the name of the
        monitored quantity.
      save_weights_only: if True, then only the model's weights will be saved
        (`model.save_weights(filepath)`), else the full model is saved
        (`model.save(filepath)`).
      save_freq: `'epoch'` or integer. When using `'epoch'`, the callback saves
        the model after each epoch. When using integer, the callback saves the
        model at end of a batch at which this many samples have been seen since
        last saving. Note that if the saving isn't aligned to epochs, the
        monitored metric may potentially be less reliable (it could reflect as
        little as 1 batch, since the metrics get reset every epoch). Defaults to
        `'epoch'`
      **kwargs: Additional arguments for backwards compatibility. Possible key
        is `period`.
  """

  def __init__(self,
               filepath,
               monitor='val_loss',
               verbose=0,
               save_best_only=False,
               save_weights_only=False,
               mode='auto',
               save_freq='epoch',
               **kwargs):
    super(ModelCheckpoint, self).__init__()
    self.monitor = monitor
    self.verbose = verbose
    self.filepath = filepath
    self.save_best_only = save_best_only
    self.save_weights_only = save_weights_only
    self.save_freq = save_freq
    self.epochs_since_last_save = 0
    self._samples_seen_since_last_saving = 0

    # Deprecated field `load_weights_on_restart` is for loading the checkpoint
    # file from `filepath` at the start of `model.fit()`
    # TODO(rchao): Remove the arg during next breaking release.
    if 'load_weights_on_restart' in kwargs:
      self.load_weights_on_restart = kwargs['load_weights_on_restart']
      logging.warning('`load_weights_on_restart` argument is deprecated. '
                      'Please use `model.load_weights()` for loading weights '
                      'before the start of `model.fit()`.')
    else:
      self.load_weights_on_restart = False

    # Deprecated field `period` is for the number of epochs between which
    # the model is saved.
    if 'period' in kwargs:
      self.period = kwargs['period']
      logging.warning('`period` argument is deprecated. Please use `save_freq` '
                      'to specify the frequency in number of samples seen.')
    else:
      self.period = 1

    if mode not in ['auto', 'min', 'max']:
      logging.warning('ModelCheckpoint mode %s is unknown, '
                      'fallback to auto mode.', mode)
      mode = 'auto'

    if mode == 'min':
      self.monitor_op = np.less
      self.best = np.Inf
    elif mode == 'max':
      self.monitor_op = np.greater
      self.best = -np.Inf
    else:
      if 'acc' in self.monitor or self.monitor.startswith('fmeasure'):
        self.monitor_op = np.greater
        self.best = -np.Inf
      else:
        self.monitor_op = np.less
        self.best = np.Inf

    if self.save_freq != 'epoch' and not isinstance(self.save_freq, int):
      raise ValueError('Unrecognized save_freq: {}'.format(self.save_freq))

    # Only the chief worker writes model checkpoints, but all workers
    # restore checkpoint at on_train_begin().
    self._chief_worker_only = False

  def set_model(self, model):
    self.model = model
    # Use name matching rather than `isinstance` to avoid circular dependencies.
    if (not self.save_weights_only and
        not model._is_graph_network and  # pylint: disable=protected-access
        model.__class__.__name__ != 'Sequential'):
      self.save_weights_only = True

  def on_train_begin(self, logs=None):
    # pylint: disable=protected-access
    if self.model._in_multi_worker_mode():
      # MultiWorkerTrainingState is used to manage the training state needed
      # for preemption-recovery of a worker in multi-worker training.
      self.model._training_state = (
          training_state.MultiWorkerTrainingState(self.model, self.filepath))
      self._training_state = self.model._training_state
      if self._training_state.restore():
        # If the training state needs to be and is successfully restored,
        # it is recovering from a previous failure (or preemption). In such
        # case, do not load the weights from user specified file path.
        return

    # If this is not multi worker training, restoring is not needed, or
    # restoring failed, check if it should load weights on restart.
    if self.load_weights_on_restart:
      if (not self.model._in_multi_worker_mode() or
          multi_worker_util.should_load_checkpoint()):
        filepath_to_load = (
            self._get_most_recently_modified_file_matching_pattern(
                self.filepath))
        if (filepath_to_load is not None and
            training_state.checkpoint_exists(filepath_to_load)):
          try:
            # `filepath` may contain placeholders such as `{epoch:02d}`, and
            # thus it attempts to load the most recently modified file with file
            # name matching the pattern.
            self.model.load_weights(filepath_to_load)
          except (IOError, ValueError) as e:
            raise ValueError('Error loading file from {}. Reason: {}'.format(
                filepath_to_load, e))

  def on_train_end(self, logs=None):
    # pylint: disable=protected-access
    if self.model._in_multi_worker_mode():
      if self.model.stop_training or getattr(
          self.model, '_successful_loop_finish', False):
        # In multi-worker training, on successful exit of training, delete the
        # training state backup file that was saved for the purpose of worker
        # recovery.
        self._training_state.delete_backup()
        # Restore the training state so the model is ready for next (possible)
        # multi worker training.
        del self._training_state
        del self.model._training_state

  def on_batch_end(self, batch, logs=None):
    logs = logs or {}
    if isinstance(self.save_freq, int):
      self._samples_seen_since_last_saving += logs.get('size', 1)
      if self._samples_seen_since_last_saving >= self.save_freq:
        self._save_model(epoch=self._current_epoch, logs=logs)
        self._samples_seen_since_last_saving = 0

  def on_epoch_begin(self, epoch, logs=None):
    self._current_epoch = epoch

  def on_epoch_end(self, epoch, logs=None):
    self.epochs_since_last_save += 1
    # pylint: disable=protected-access
    if self.save_freq == 'epoch':
      if self.model._in_multi_worker_mode():
        # Exclude training state variables in user-requested checkpoint file.
        with self._training_state.untrack_vars():
          self._save_model(epoch=epoch, logs=logs)
      else:
        self._save_model(epoch=epoch, logs=logs)
    if self.model._in_multi_worker_mode():
      # For multi-worker training, back up the weights and current training
      # state for possible future recovery.
      # TODO(rchao): Call `back_up` at finer period such as N steps.
      self._training_state.back_up(epoch)

  def _save_model(self, epoch, logs):
    """Saves the model.

    Arguments:
        epoch: the epoch this iteration is in.
        logs: the `logs` dict passed in to `on_batch_end` or `on_epoch_end`.
    """
    logs = logs or {}

    if isinstance(self.save_freq,
                  int) or self.epochs_since_last_save >= self.period:
      self.epochs_since_last_save = 0
      filepath = self._get_file_path(epoch, logs)

      try:
        if self.save_best_only:
          current = logs.get(self.monitor)
          if current is None:
            logging.warning('Can save best model only with %s available, '
                            'skipping.', self.monitor)
          else:
            if self.monitor_op(current, self.best):
              if self.verbose > 0:
                print('\nEpoch %05d: %s improved from %0.5f to %0.5f,'
                      ' saving model to %s' % (epoch + 1, self.monitor,
                                               self.best, current, filepath))
              self.best = current
              if self.save_weights_only:
                self.model.save_weights(filepath, overwrite=True)
              else:
                self.model.save(filepath, overwrite=True)
            else:
              if self.verbose > 0:
                print('\nEpoch %05d: %s did not improve from %0.5f' %
                      (epoch + 1, self.monitor, self.best))
        else:
          if self.verbose > 0:
            print('\nEpoch %05d: saving model to %s' % (epoch + 1, filepath))
          if self.save_weights_only:
            self.model.save_weights(filepath, overwrite=True)
          else:
            self.model.save(filepath, overwrite=True)

        self._maybe_remove_file()
      except IOError as e:
        # `e.errno` appears to be `None` so checking the content of `e.args[0]`.
        if 'is a directory' in six.ensure_str(e.args[0]):
          raise IOError('Please specify a non-directory filepath for '
                        'ModelCheckpoint. Filepath used is an existing '
                        'directory: {}'.format(filepath))

  def _get_file_path(self, epoch, logs):
    """Returns the file path for checkpoint."""
    # pylint: disable=protected-access
    if not self.model._in_multi_worker_mode(
    ) or multi_worker_util.should_save_checkpoint():
      try:
        # `filepath` may contain placeholders such as `{epoch:02d}` and
        # `{mape:.2f}`. A mismatch between logged metrics and the path's
        # placeholders can cause formatting to fail.
        return self.filepath.format(epoch=epoch + 1, **logs)
      except KeyError as e:
        raise KeyError('Failed to format this callback filepath: "{}". '
                       'Reason: {}'.format(self.filepath, e))
    else:
      # If this is multi-worker training, and this worker should not
      # save checkpoint, we use a temp filepath to store a dummy checkpoint, so
      # it writes to a file that will be removed at the end of `_save_model()`
      # call. This is because the SyncOnReadVariable needs to be synced across
      # all the workers in order to be read, and all workers need to initiate
      # that.
      self._temp_file_dir = tempfile.mkdtemp()
      extension = os.path.splitext(self.filepath)[1]
      return os.path.join(self._temp_file_dir, 'temp' + extension)

  def _maybe_remove_file(self):
    # Remove the checkpoint directory in multi-worker training where this worker
    # should not checkpoint. It is a dummy directory previously saved for sync
    # distributed training.

    if (self.model._in_multi_worker_mode() and  # pylint: disable=protected-access
        not multi_worker_util.should_save_checkpoint()):
      file_io.delete_recursively(self._temp_file_dir)
      del self._temp_file_dir

  def _get_most_recently_modified_file_matching_pattern(self, pattern):
    """Returns the most recently modified filepath matching pattern.

    Pattern may contain python formatting placeholder. If
    `tf.train.latest_checkpoint()` does not return None, use that; otherwise,
    check for most recently modified one that matches the pattern.

    In the rare case where there are more than one pattern-matching file having
    the same modified time that is most recent among all, return the filepath
    that is largest (by `>` operator, lexicographically using the numeric
    equivalents). This provides a tie-breaker when multiple files are most
    recent. Note that a larger `filepath` can sometimes indicate a later time of
    modification (for instance, when epoch/batch is used as formatting option),
    but not necessarily (when accuracy or loss is used). The tie-breaker is
    put in the logic as best effort to return the most recent, and to avoid
    undeterministic result.

    Modified time of a file is obtained with `os.path.getmtime()`.

    This utility function is best demonstrated via an example:

    ```python
    file_pattern = 'f.batch{batch:02d}epoch{epoch:02d}.h5'
    test_dir = self.get_temp_dir()
    path_pattern = os.path.join(test_dir, file_pattern)
    file_paths = [
        os.path.join(test_dir, file_name) for file_name in
        ['f.batch03epoch02.h5', 'f.batch02epoch02.h5', 'f.batch01epoch01.h5']
    ]
    for file_path in file_paths:
      # Write something to each of the files
    self.assertEqual(
        _get_most_recently_modified_file_matching_pattern(path_pattern),
        file_paths[-1])
    ```

    Arguments:
        pattern: The file pattern that may optionally contain python placeholder
            such as `{epoch:02d}`.

    Returns:
        The most recently modified file's full filepath matching `pattern`. If
        `pattern` does not contain any placeholder, this returns the filepath
        that
        exactly matches `pattern`. Returns `None` if no match is found.
    """
    dir_name = os.path.dirname(pattern)
    base_name = os.path.basename(pattern)
    base_name_regex = '^' + re.sub(r'{.*}', r'.*', base_name) + '$'

    # If tf.train.latest_checkpoint tells us there exists a latest checkpoint,
    # use that as it is more robust than `os.path.getmtime()`.
    latest_tf_checkpoint = checkpoint_management.latest_checkpoint(dir_name)
    if latest_tf_checkpoint is not None and re.match(
        base_name_regex, os.path.basename(latest_tf_checkpoint)):
      return latest_tf_checkpoint

    latest_mod_time = 0
    file_path_with_latest_mod_time = None
    n_file_with_latest_mod_time = 0
    file_path_with_largest_file_name = None

    if file_io.file_exists(dir_name):
      for file_name in os.listdir(dir_name):
        # Only consider if `file_name` matches the pattern.
        if re.match(base_name_regex, file_name):
          file_path = os.path.join(dir_name, file_name)
          mod_time = os.path.getmtime(file_path)
          if (file_path_with_largest_file_name is None or
              file_path > file_path_with_largest_file_name):
            file_path_with_largest_file_name = file_path
          if mod_time > latest_mod_time:
            latest_mod_time = mod_time
            file_path_with_latest_mod_time = file_path
            # In the case a file with later modified time is found, reset
            # the counter for the number of files with latest modified time.
            n_file_with_latest_mod_time = 1
          elif mod_time == latest_mod_time:
            # In the case a file has modified time tied with the most recent,
            # increment the counter for the number of files with latest modified
            # time by 1.
            n_file_with_latest_mod_time += 1

    if n_file_with_latest_mod_time == 1:
      # Return the sole file that has most recent modified time.
      return file_path_with_latest_mod_time
    else:
      # If there are more than one file having latest modified time, return
      # the file path with the largest file name.
      return file_path_with_largest_file_name


@keras_export('keras.callbacks.EarlyStopping')
class EarlyStopping(Callback):
  """Stop training when a monitored metric has stopped improving.

  Assuming the goal of a training is to minimize the loss. With this, the
  metric to be monitored would be 'loss', and mode would be 'min'. A
  `model.fit()` training loop will check at end of every epoch whether
  the loss is no longer decreasing, considering the `min_delta` and
  `patience` if applicable. Once it's found no longer decreasing,
  `model.stop_training` is marked True and the training terminates.

  The quantity to be monitored needs to be available in `logs` dict.
  To make it so, pass the loss or metrics at `model.compile()`.

  Example:

  >>> callback = tf.keras.callbacks.EarlyStopping(monitor='loss', patience=3)
  >>> # This callback will stop the training when there is no improvement in
  >>> # the validation loss for three consecutive epochs.
  >>> model = tf.keras.models.Sequential([tf.keras.layers.Dense(10)])
  >>> model.compile(tf.keras.optimizers.SGD(), loss='mse')
  >>> history = model.fit(np.arange(100).reshape(5, 20), np.zeros(5),
  ...                     epochs=10, callbacks=[callback])
      Train on 5 samples
      Epoch 1/10
  5/5 [==============================] - ... loss: 6533.1904
      Epoch 2/10
  5/5 [==============================] - ... loss: 110183360.0000
      Epoch 3/10
  5/5 [==============================] - ... loss: 1862575718400.0000
      Epoch 4/10
  5/5 [==============================] - ... loss: 31485597793124352.0000
  """

  def __init__(self,
               monitor='val_loss',
               min_delta=0,
               patience=0,
               verbose=0,
               mode='auto',
               baseline=None,
               restore_best_weights=False):
    """Initialize an EarlyStopping callback.

    Arguments:
        monitor: Quantity to be monitored.
        min_delta: Minimum change in the monitored quantity
            to qualify as an improvement, i.e. an absolute
            change of less than min_delta, will count as no
            improvement.
        patience: Number of epochs with no improvement
            after which training will be stopped.
        verbose: verbosity mode.
        mode: One of `{"auto", "min", "max"}`. In `min` mode,
            training will stop when the quantity
            monitored has stopped decreasing; in `max`
            mode it will stop when the quantity
            monitored has stopped increasing; in `auto`
            mode, the direction is automatically inferred
            from the name of the monitored quantity.
        baseline: Baseline value for the monitored quantity.
            Training will stop if the model doesn't show improvement over the
            baseline.
        restore_best_weights: Whether to restore model weights from
            the epoch with the best value of the monitored quantity.
            If False, the model weights obtained at the last step of
            training are used.
    """
    super(EarlyStopping, self).__init__()

    self.monitor = monitor
    self.patience = patience
    self.verbose = verbose
    self.baseline = baseline
    self.min_delta = abs(min_delta)
    self.wait = 0
    self.stopped_epoch = 0
    self.restore_best_weights = restore_best_weights
    self.best_weights = None

    if mode not in ['auto', 'min', 'max']:
      logging.warning('EarlyStopping mode %s is unknown, '
                      'fallback to auto mode.', mode)
      mode = 'auto'

    if mode == 'min':
      self.monitor_op = np.less
    elif mode == 'max':
      self.monitor_op = np.greater
    else:
      if 'acc' in self.monitor:
        self.monitor_op = np.greater
      else:
        self.monitor_op = np.less

    if self.monitor_op == np.greater:
      self.min_delta *= 1
    else:
      self.min_delta *= -1

  def on_train_begin(self, logs=None):
    # Allow instances to be re-used
    self.wait = 0
    self.stopped_epoch = 0
    if self.baseline is not None:
      self.best = self.baseline
    else:
      self.best = np.Inf if self.monitor_op == np.less else -np.Inf

  def on_epoch_end(self, epoch, logs=None):
    current = self.get_monitor_value(logs)
    if current is None:
      return
    if self.monitor_op(current - self.min_delta, self.best):
      self.best = current
      self.wait = 0
      if self.restore_best_weights:
        self.best_weights = self.model.get_weights()
    else:
      self.wait += 1
      if self.wait >= self.patience:
        self.stopped_epoch = epoch
        self.model.stop_training = True
        if self.restore_best_weights:
          if self.verbose > 0:
            print('Restoring model weights from the end of the best epoch.')
          self.model.set_weights(self.best_weights)

  def on_train_end(self, logs=None):
    if self.stopped_epoch > 0 and self.verbose > 0:
      print('Epoch %05d: early stopping' % (self.stopped_epoch + 1))

  def get_monitor_value(self, logs):
    logs = logs or {}
    monitor_value = logs.get(self.monitor)
    if monitor_value is None:
      logging.warning('Early stopping conditioned on metric `%s` '
                      'which is not available. Available metrics are: %s',
                      self.monitor, ','.join(list(logs.keys())))
    return monitor_value


@keras_export('keras.callbacks.RemoteMonitor')
class RemoteMonitor(Callback):
  """Callback used to stream events to a server.

  Requires the `requests` library.
  Events are sent to `root + '/publish/epoch/end/'` by default. Calls are
  HTTP POST, with a `data` argument which is a
  JSON-encoded dictionary of event data.
  If send_as_json is set to True, the content type of the request will be
  application/json. Otherwise the serialized JSON will be sent within a form.

  Arguments:
      root: String; root url of the target server.
      path: String; path relative to `root` to which the events will be sent.
      field: String; JSON field under which the data will be stored.
          The field is used only if the payload is sent within a form
          (i.e. send_as_json is set to False).
      headers: Dictionary; optional custom HTTP headers.
      send_as_json: Boolean; whether the request should be
          sent as application/json.
  """

  def __init__(self,
               root='http://localhost:9000',
               path='/publish/epoch/end/',
               field='data',
               headers=None,
               send_as_json=False):
    super(RemoteMonitor, self).__init__()

    self.root = root
    self.path = path
    self.field = field
    self.headers = headers
    self.send_as_json = send_as_json

  def on_epoch_end(self, epoch, logs=None):
    if requests is None:
      raise ImportError('RemoteMonitor requires the `requests` library.')
    logs = logs or {}
    send = {}
    send['epoch'] = epoch
    for k, v in logs.items():
      # np.ndarray and np.generic are not scalar types
      # therefore we must unwrap their scalar values and
      # pass to the json-serializable dict 'send'
      if isinstance(v, (np.ndarray, np.generic)):
        send[k] = v.item()
      else:
        send[k] = v
    try:
      if self.send_as_json:
        requests.post(self.root + self.path, json=send, headers=self.headers)
      else:
        requests.post(
            self.root + self.path, {self.field: json.dumps(send)},
            headers=self.headers)
    except requests.exceptions.RequestException:
      logging.warning('Warning: could not reach RemoteMonitor '
                      'root server at ' + str(self.root))


@keras_export('keras.callbacks.LearningRateScheduler')
class LearningRateScheduler(Callback):
  """Learning rate scheduler.

  Arguments:
      schedule: a function that takes an epoch index as input
          (integer, indexed from 0) and returns a new
          learning rate as output (float).
      verbose: int. 0: quiet, 1: update messages.

  ```python
  # This function keeps the learning rate at 0.001 for the first ten epochs
  # and decreases it exponentially after that.
  def scheduler(epoch):
    if epoch < 10:
      return 0.001
    else:
      return 0.001 * tf.math.exp(0.1 * (10 - epoch))

  callback = tf.keras.callbacks.LearningRateScheduler(scheduler)
  model.fit(data, labels, epochs=100, callbacks=[callback],
            validation_data=(val_data, val_labels))
  ```
  """

  def __init__(self, schedule, verbose=0):
    super(LearningRateScheduler, self).__init__()
    self.schedule = schedule
    self.verbose = verbose

  def on_epoch_begin(self, epoch, logs=None):
    if not hasattr(self.model.optimizer, 'lr'):
      raise ValueError('Optimizer must have a "lr" attribute.')
    try:  # new API
      lr = float(K.get_value(self.model.optimizer.lr))
      lr = self.schedule(epoch, lr)
    except TypeError:  # Support for old API for backward compatibility
      lr = self.schedule(epoch)
    if not isinstance(lr, (ops.Tensor, float, np.float32, np.float64)):
      raise ValueError('The output of the "schedule" function '
                       'should be float.')
    if isinstance(lr, ops.Tensor) and not lr.dtype.is_floating:
      raise ValueError('The dtype of Tensor should be float')
    K.set_value(self.model.optimizer.lr, K.get_value(lr))
    if self.verbose > 0:
      print('\nEpoch %05d: LearningRateScheduler reducing learning '
            'rate to %s.' % (epoch + 1, lr))

  def on_epoch_end(self, epoch, logs=None):
    logs = logs or {}
    logs['lr'] = K.get_value(self.model.optimizer.lr)


@keras_export('keras.callbacks.TensorBoard', v1=[])
class TensorBoard(Callback):
  # pylint: disable=line-too-long
  """Enable visualizations for TensorBoard.

  TensorBoard is a visualization tool provided with TensorFlow.

  This callback logs events for TensorBoard, including:

  * Metrics summary plots
  * Training graph visualization
  * Activation histograms
  * Sampled profiling

  If you have installed TensorFlow with pip, you should be able
  to launch TensorBoard from the command line:

  ```sh
  tensorboard --logdir=path_to_your_logs
  ```

  You can find more information about TensorBoard
  [here](https://www.tensorflow.org/get_started/summaries_and_tensorboard).

  Example:
  ```python
  tensorboard_callback = tf.keras.callbacks.TensorBoard(log_dir="./logs")
  model.fit(x_train, y_train, epochs=2, callbacks=[tensorboard_callback])
  #run the tensorboard command to view the visualizations
  ```

  Arguments:
      log_dir: the path of the directory where to save the log files to be
        parsed by TensorBoard.
      histogram_freq: frequency (in epochs) at which to compute activation and
        weight histograms for the layers of the model. If set to 0, histograms
        won't be computed. Validation data (or split) must be specified for
        histogram visualizations.
      write_graph: whether to visualize the graph in TensorBoard. The log file
        can become quite large when write_graph is set to True.
      write_images: whether to write model weights to visualize as image in
        TensorBoard.
      update_freq: `'batch'` or `'epoch'` or integer. When using `'batch'`,
        writes the losses and metrics to TensorBoard after each batch. The same
        applies for `'epoch'`. If using an integer, let's say `1000`, the
        callback will write the metrics and losses to TensorBoard every 1000
        batches. Note that writing too frequently to TensorBoard can slow down
        your training.
      profile_batch: Profile the batch to sample compute characteristics. By
        default, it will profile the second batch. Set profile_batch=0 to
        disable profiling. Must run in TensorFlow eager mode.
      embeddings_freq: frequency (in epochs) at which embedding layers will
        be visualized. If set to 0, embeddings won't be visualized.
      embeddings_metadata: a dictionary which maps layer name to a file name in
        which metadata for this embedding layer is saved. See the
        [details](
          https://www.tensorflow.org/how_tos/embedding_viz/#metadata_optional)
        about metadata files format. In case if the same metadata file is
        used for all embedding layers, string can be passed.

  Raises:
      ValueError: If histogram_freq is set and no validation data is provided.
  """

  # pylint: enable=line-too-long

  def __init__(self,
               log_dir='logs',
               histogram_freq=0,
               write_graph=True,
               write_images=False,
               update_freq='epoch',
               profile_batch=2,
               embeddings_freq=0,
               embeddings_metadata=None,
               **kwargs):
    super(TensorBoard, self).__init__()
    self._validate_kwargs(kwargs)

    self.log_dir = log_dir
    self.histogram_freq = histogram_freq
    self.write_graph = write_graph
    self.write_images = write_images
    if update_freq == 'batch':
      self.update_freq = 1
    else:
      self.update_freq = update_freq
    self.embeddings_freq = embeddings_freq
    self.embeddings_metadata = embeddings_metadata

    self._samples_seen = 0
    self._samples_seen_at_last_write = 0
    self._current_batch = 0

    # A collection of file writers currently in use, to be closed when
    # training ends for this callback. Writers are keyed by the
    # directory name under the root logdir: e.g., "train" or
    # "validation".
    self._train_run_name = 'train'
    self._validation_run_name = 'validation'
    self._writers = {}

    self._profile_batch = profile_batch
    # True when a trace is running.
    self._is_tracing = False

  def _validate_kwargs(self, kwargs):
    """Handle arguments were supported in V1."""
    if kwargs.get('write_grads', False):
      logging.warning('`write_grads` will be ignored in TensorFlow 2.0 '
                      'for the `TensorBoard` Callback.')
    if kwargs.get('batch_size', False):
      logging.warning('`batch_size` is no longer needed in the '
                      '`TensorBoard` Callback and will be ignored '
                      'in TensorFlow 2.0.')
    if kwargs.get('embeddings_layer_names', False):
      logging.warning('`embeddings_layer_names` is not supported in '
                      'TensorFlow 2.0. Instead, all `Embedding` layers '
                      'will be visualized.')
    if kwargs.get('embeddings_data', False):
      logging.warning('`embeddings_data` is not supported in TensorFlow '
                      '2.0. Instead, all `Embedding` variables will be '
                      'visualized.')

    unrecognized_kwargs = set(kwargs.keys()) - {
        'write_grads', 'embeddings_layer_names', 'embeddings_data', 'batch_size'
    }

    # Only allow kwargs that were supported in V1.
    if unrecognized_kwargs:
      raise ValueError('Unrecognized arguments in `TensorBoard` '
                       'Callback: ' + str(unrecognized_kwargs))

  def set_model(self, model):
    """Sets Keras model and writes graph if specified."""
    self.model = model

    # TensorBoard callback involves writing a summary file in a
    # possibly distributed settings.
    self._log_write_dir = distributed_file_utils.write_dirpath(
        self.log_dir, self.model._get_distribution_strategy())  # pylint: disable=protected-access

    with context.eager_mode():
      self._close_writers()
      if self.write_graph:
        with self._get_writer(self._train_run_name).as_default():
          with summary_ops_v2.always_record_summaries():
            if not model.run_eagerly:
              summary_ops_v2.graph(K.get_graph(), step=0)

            summary_writable = (
                self.model._is_graph_network or  # pylint: disable=protected-access
                self.model.__class__.__name__ == 'Sequential')  # pylint: disable=protected-access
            if summary_writable:
              summary_ops_v2.keras_model('keras', self.model, step=0)

    if self.embeddings_freq:
      self._configure_embeddings()

    summary_state = summary_ops_v2._summary_state  # pylint: disable=protected-access
    self._prev_summary_recording = summary_state.is_recording
    self._prev_summary_writer = summary_state.writer
    self._prev_summary_step = summary_state.step

  def _configure_embeddings(self):
    """Configure the Projector for embeddings."""
    # TODO(omalleyt): Add integration tests.
    from tensorflow.python.keras.layers import embeddings
    try:
      from tensorboard.plugins import projector
    except ImportError:
      raise ImportError('Failed to import TensorBoard. Please make sure that '
                        'TensorBoard integration is complete."')
    config = projector.ProjectorConfig()
    for layer in self.model.layers:
      if isinstance(layer, embeddings.Embedding):
        embedding = config.embeddings.add()
        embedding.tensor_name = layer.embeddings.name

        if self.embeddings_metadata is not None:
          if isinstance(self.embeddings_metadata, str):
            embedding.metadata_path = self.embeddings_metadata
          else:
            if layer.name in embedding.metadata_path:
              embedding.metadata_path = self.embeddings_metadata.pop(layer.name)

    if self.embeddings_metadata:
      raise ValueError('Unrecognized `Embedding` layer names passed to '
                       '`keras.callbacks.TensorBoard` `embeddings_metadata` '
                       'argument: ' + str(self.embeddings_metadata.keys()))

    class DummyWriter(object):
      """Dummy writer to conform to `Projector` API."""

      def __init__(self, logdir):
        self.logdir = logdir

      def get_logdir(self):
        return self.logdir

    writer = DummyWriter(self._log_write_dir)
    projector.visualize_embeddings(writer, config)

  def _close_writers(self):
    """Close all remaining open file writers owned by this callback.

    If there are no such file writers, this is a no-op.
    """
    with context.eager_mode():
      for writer in six.itervalues(self._writers):
        writer.close()
      self._writers.clear()

  def _get_writer(self, writer_name):
    """Get a summary writer for the given subdirectory under the logdir.

    A writer will be created if it does not yet exist.

    Arguments:
      writer_name: The name of the directory for which to create or
        retrieve a writer. Should be either `self._train_run_name` or
        `self._validation_run_name`.

    Returns:
      A `SummaryWriter` object.
    """
    if writer_name not in self._writers:
      path = os.path.join(self._log_write_dir, writer_name)
      writer = summary_ops_v2.create_file_writer_v2(path)
      self._writers[writer_name] = writer
    return self._writers[writer_name]

  def _set_default_writer(self, writer_name):
    """Sets the default writer for custom batch-level summaries."""
    if self.update_freq == 'epoch':
      # Writer is only used for custom summaries, which are written
      # batch-by-batch.
      return

    step = self._total_batches_seen[writer_name]

    def _should_record():
      return math_ops.equal(step % self.update_freq, 0)

    summary_state = summary_ops_v2._summary_state  # pylint: disable=protected-access
    summary_state.is_recording = _should_record
    summary_state.writer = self._get_writer(writer_name)
    summary_ops_v2.set_step(step)

  def _init_batch_steps(self):
    """Create the total batch counters."""
    if ops.executing_eagerly_outside_functions():
      # Variables are needed for the `step` value of custom tf.summaries
      # to be updated inside a tf.function.
      self._total_batches_seen = {
          self._train_run_name: variables.Variable(0, dtype='int64'),
          self._validation_run_name: variables.Variable(0, dtype='int64')
      }
    else:
      # Custom tf.summaries are not supported in legacy graph mode.
      self._total_batches_seen = {
          self._train_run_name: 0,
          self._validation_run_name: 0
      }

  def _increment_step(self, writer_name):
    step = self._total_batches_seen[writer_name]
    if isinstance(step, variables.Variable):
      step.assign_add(1)
    else:
      self._total_batches_seen[writer_name] += 1

  def on_train_begin(self, logs=None):
    self._init_batch_steps()
    if self._profile_batch == 1:
      summary_ops_v2.trace_on(graph=True, profiler=True)
      self._is_tracing = True

  def on_test_begin(self, logs=None):
    self._set_default_writer(self._validation_run_name)

  def on_train_batch_end(self, batch, logs=None):
    """Writes scalar summaries for metrics on every training batch.

    Performs profiling if current batch is in profiler_batches.

    Arguments:
      batch: Integer, index of batch within the current epoch.
      logs: Dict. Metric results for this batch.
    """
    if self.update_freq == 'epoch' and self._profile_batch is None:
      return

    # Don't output batch_size and batch number as TensorBoard summaries
    logs = logs or {}
    train_batches = self._total_batches_seen[self._train_run_name]
    if self.update_freq != 'epoch' and batch % self.update_freq == 0:
      self._log_metrics(logs, prefix='batch_', step=train_batches)

    self._increment_step(self._train_run_name)

    if context.executing_eagerly():
      if self._is_tracing:
        self._log_trace()
      elif (not self._is_tracing and
            math_ops.equal(train_batches, self._profile_batch - 1)):
        self._enable_trace()

  def on_test_batch_end(self, batch, logs=None):
    if self.update_freq == 'epoch':
      return
    self._increment_step(self._validation_run_name)

  def on_epoch_begin(self, epoch, logs=None):
    self._set_default_writer(self._train_run_name)

  def on_epoch_end(self, epoch, logs=None):
    """Runs metrics and histogram summaries at epoch end."""
    self._log_metrics(logs, prefix='epoch_', step=epoch)

    if self.histogram_freq and epoch % self.histogram_freq == 0:
      self._log_weights(epoch)

    if self.embeddings_freq and epoch % self.embeddings_freq == 0:
      self._log_embeddings(epoch)

  def on_train_end(self, logs=None):
    if self._is_tracing:
      self._log_trace()
    self._close_writers()

    summary_state = summary_ops_v2._summary_state  # pylint: disable=protected-access
    summary_state.is_recording = self._prev_summary_recording
    summary_state.writer = self._prev_summary_writer
    summary_state.step = self._prev_summary_step

    # Safely remove the unneeded temp files.
    distributed_file_utils.remove_temp_dirpath(
        self.log_dir, self.model._get_distribution_strategy())  # pylint: disable=protected-access

  def _enable_trace(self):
    if context.executing_eagerly():
      summary_ops_v2.trace_on(graph=True, profiler=True)
      self._is_tracing = True

  def _log_trace(self):
    """Logs the trace graph to TensorBoard."""
    if context.executing_eagerly():
      with self._get_writer(self._train_run_name).as_default(), \
          summary_ops_v2.always_record_summaries():
        # TODO(b/126388999): Remove step info in the summary name.
        step = K.get_value(self._total_batches_seen[self._train_run_name])
        summary_ops_v2.trace_export(
            name='batch_%d' % step,
            step=step,
            profiler_outdir=os.path.join(self._log_write_dir, 'train'))
      self._is_tracing = False

  def _log_metrics(self, logs, prefix, step):
    """Writes metrics out as custom scalar summaries.

    Arguments:
        logs: Dict. Keys are scalar summary names, values are NumPy scalars.
        prefix: String. The prefix to apply to the scalar summary names.
        step: Int. The global step to use for TensorBoard.
    """
    if logs is None:
      logs = {}

    # Group metrics by the name of their associated file writer. Values
    # are lists of metrics, as (name, scalar_value) pairs.
    logs_by_writer = {
        self._train_run_name: [],
        self._validation_run_name: [],
    }
    validation_prefix = 'val_'
    for (name, value) in logs.items():
      if name in ('batch', 'size', 'num_steps'):
        # Scrub non-metric items.
        continue
      if name.startswith(validation_prefix):
        name = name[len(validation_prefix):]
        writer_name = self._validation_run_name
      else:
        writer_name = self._train_run_name
      name = prefix + name  # assign batch or epoch prefix
      logs_by_writer[writer_name].append((name, value))

    with context.eager_mode():
      with summary_ops_v2.always_record_summaries():
        for writer_name in logs_by_writer:
          these_logs = logs_by_writer[writer_name]
          if not these_logs:
            # Don't create a "validation" events file if we don't
            # actually have any validation data.
            continue
          writer = self._get_writer(writer_name)
          with writer.as_default():
            for (name, value) in these_logs:
              summary_ops_v2.scalar(name, value, step=step)

  def _log_weights(self, epoch):
    """Logs the weights of the Model to TensorBoard."""
    writer = self._get_writer(self._train_run_name)
    with context.eager_mode(), \
          writer.as_default(), \
          summary_ops_v2.always_record_summaries():
      for layer in self.model.layers:
        for weight in layer.weights:
          weight_name = weight.name.replace(':', '_')
          with ops.init_scope():
            weight = K.get_value(weight)
          summary_ops_v2.histogram(weight_name, weight, step=epoch)
          if self.write_images:
            self._log_weight_as_image(weight, weight_name, epoch)
      writer.flush()

  def _log_weight_as_image(self, weight, weight_name, epoch):
    """Logs a weight as a TensorBoard image."""
    w_img = array_ops.squeeze(weight)
    shape = K.int_shape(w_img)
    if len(shape) == 1:  # Bias case
      w_img = array_ops.reshape(w_img, [1, shape[0], 1, 1])
    elif len(shape) == 2:  # Dense layer kernel case
      if shape[0] > shape[1]:
        w_img = array_ops.transpose(w_img)
        shape = K.int_shape(w_img)
      w_img = array_ops.reshape(w_img, [1, shape[0], shape[1], 1])
    elif len(shape) == 3:  # ConvNet case
      if K.image_data_format() == 'channels_last':
        # Switch to channels_first to display every kernel as a separate
        # image.
        w_img = array_ops.transpose(w_img, perm=[2, 0, 1])
        shape = K.int_shape(w_img)
      w_img = array_ops.reshape(w_img, [shape[0], shape[1], shape[2], 1])

    shape = K.int_shape(w_img)
    # Not possible to handle 3D convnets etc.
    if len(shape) == 4 and shape[-1] in [1, 3, 4]:
      summary_ops_v2.image(weight_name, w_img, step=epoch)

  def _log_embeddings(self, epoch):
    embeddings_ckpt = os.path.join(self._log_write_dir, 'train',
                                   'keras_embedding.ckpt-{}'.format(epoch))
    self.model.save_weights(embeddings_ckpt)


@keras_export('keras.callbacks.ReduceLROnPlateau')
class ReduceLROnPlateau(Callback):
  """Reduce learning rate when a metric has stopped improving.

  Models often benefit from reducing the learning rate by a factor
  of 2-10 once learning stagnates. This callback monitors a
  quantity and if no improvement is seen for a 'patience' number
  of epochs, the learning rate is reduced.

  Example:

  ```python
  reduce_lr = ReduceLROnPlateau(monitor='val_loss', factor=0.2,
                                patience=5, min_lr=0.001)
  model.fit(X_train, Y_train, callbacks=[reduce_lr])
  ```

  Arguments:
      monitor: quantity to be monitored.
      factor: factor by which the learning rate will be reduced. new_lr = lr *
        factor
      patience: number of epochs with no improvement after which learning rate
        will be reduced.
      verbose: int. 0: quiet, 1: update messages.
      mode: one of {auto, min, max}. In `min` mode, lr will be reduced when the
        quantity monitored has stopped decreasing; in `max` mode it will be
        reduced when the quantity monitored has stopped increasing; in `auto`
        mode, the direction is automatically inferred from the name of the
        monitored quantity.
      min_delta: threshold for measuring the new optimum, to only focus on
        significant changes.
      cooldown: number of epochs to wait before resuming normal operation after
        lr has been reduced.
      min_lr: lower bound on the learning rate.
  """

  def __init__(self,
               monitor='val_loss',
               factor=0.1,
               patience=10,
               verbose=0,
               mode='auto',
               min_delta=1e-4,
               cooldown=0,
               min_lr=0,
               **kwargs):
    super(ReduceLROnPlateau, self).__init__()

    self.monitor = monitor
    if factor >= 1.0:
      raise ValueError('ReduceLROnPlateau ' 'does not support a factor >= 1.0.')
    if 'epsilon' in kwargs:
      min_delta = kwargs.pop('epsilon')
      logging.warning('`epsilon` argument is deprecated and '
                      'will be removed, use `min_delta` instead.')
    self.factor = factor
    self.min_lr = min_lr
    self.min_delta = min_delta
    self.patience = patience
    self.verbose = verbose
    self.cooldown = cooldown
    self.cooldown_counter = 0  # Cooldown counter.
    self.wait = 0
    self.best = 0
    self.mode = mode
    self.monitor_op = None
    self._reset()

  def _reset(self):
    """Resets wait counter and cooldown counter.
    """
    if self.mode not in ['auto', 'min', 'max']:
      logging.warning('Learning Rate Plateau Reducing mode %s is unknown, '
                      'fallback to auto mode.', self.mode)
      self.mode = 'auto'
    if (self.mode == 'min' or
        (self.mode == 'auto' and 'acc' not in self.monitor)):
      self.monitor_op = lambda a, b: np.less(a, b - self.min_delta)
      self.best = np.Inf
    else:
      self.monitor_op = lambda a, b: np.greater(a, b + self.min_delta)
      self.best = -np.Inf
    self.cooldown_counter = 0
    self.wait = 0

  def on_train_begin(self, logs=None):
    self._reset()

  def on_epoch_end(self, epoch, logs=None):
    logs = logs or {}
    logs['lr'] = K.get_value(self.model.optimizer.lr)
    current = logs.get(self.monitor)
    if current is None:
      logging.warning('Reduce LR on plateau conditioned on metric `%s` '
                      'which is not available. Available metrics are: %s',
                      self.monitor, ','.join(list(logs.keys())))

    else:
      if self.in_cooldown():
        self.cooldown_counter -= 1
        self.wait = 0

      if self.monitor_op(current, self.best):
        self.best = current
        self.wait = 0
      elif not self.in_cooldown():
        self.wait += 1
        if self.wait >= self.patience:
          old_lr = float(K.get_value(self.model.optimizer.lr))
          if old_lr > self.min_lr:
            new_lr = old_lr * self.factor
            new_lr = max(new_lr, self.min_lr)
            K.set_value(self.model.optimizer.lr, new_lr)
            if self.verbose > 0:
              print('\nEpoch %05d: ReduceLROnPlateau reducing learning '
                    'rate to %s.' % (epoch + 1, new_lr))
            self.cooldown_counter = self.cooldown
            self.wait = 0

  def in_cooldown(self):
    return self.cooldown_counter > 0


@keras_export('keras.callbacks.CSVLogger')
class CSVLogger(Callback):
  """Callback that streams epoch results to a csv file.

  Supports all values that can be represented as a string,
  including 1D iterables such as np.ndarray.

  Example:

  ```python
  csv_logger = CSVLogger('training.log')
  model.fit(X_train, Y_train, callbacks=[csv_logger])
  ```

  Arguments:
      filename: filename of the csv file, e.g. 'run/log.csv'.
      separator: string used to separate elements in the csv file.
      append: True: append if file exists (useful for continuing
          training). False: overwrite existing file,
  """

  def __init__(self, filename, separator=',', append=False):
    self.sep = separator
    self.filename = filename
    self.append = append
    self.writer = None
    self.keys = None
    self.append_header = True
    if six.PY2:
      self.file_flags = 'b'
      self._open_args = {}
    else:
      self.file_flags = ''
      self._open_args = {'newline': '\n'}
    super(CSVLogger, self).__init__()

  def on_train_begin(self, logs=None):
    if self.append:
      if file_io.file_exists(self.filename):
        with open(self.filename, 'r' + self.file_flags) as f:
          self.append_header = not bool(len(f.readline()))
      mode = 'a'
    else:
      mode = 'w'
    self.csv_file = io.open(self.filename,
                            mode + self.file_flags,
                            **self._open_args)

  def on_epoch_end(self, epoch, logs=None):
    logs = logs or {}

    def handle_value(k):
      is_zero_dim_ndarray = isinstance(k, np.ndarray) and k.ndim == 0
      if isinstance(k, six.string_types):
        return k
      elif isinstance(k, collections_abc.Iterable) and not is_zero_dim_ndarray:
        return '"[%s]"' % (', '.join(map(str, k)))
      else:
        return k

    if self.keys is None:
      self.keys = sorted(logs.keys())

    if self.model.stop_training:
      # We set NA so that csv parsers do not fail for this last epoch.
      logs = dict([(k, logs[k]) if k in logs else (k, 'NA') for k in self.keys])

    if not self.writer:

      class CustomDialect(csv.excel):
        delimiter = self.sep

      fieldnames = ['epoch'] + self.keys
      if six.PY2:
        fieldnames = [unicode(x) for x in fieldnames]

      self.writer = csv.DictWriter(
          self.csv_file,
          fieldnames=fieldnames,
          dialect=CustomDialect)
      if self.append_header:
        self.writer.writeheader()

    row_dict = collections.OrderedDict({'epoch': epoch})
    row_dict.update((key, handle_value(logs[key])) for key in self.keys)
    self.writer.writerow(row_dict)
    self.csv_file.flush()

  def on_train_end(self, logs=None):
    self.csv_file.close()
    self.writer = None


@keras_export('keras.callbacks.LambdaCallback')
class LambdaCallback(Callback):
  r"""Callback for creating simple, custom callbacks on-the-fly.

  This callback is constructed with anonymous functions that will be called
  at the appropriate time. Note that the callbacks expects positional
  arguments, as:

   - `on_epoch_begin` and `on_epoch_end` expect two positional arguments:
      `epoch`, `logs`
   - `on_batch_begin` and `on_batch_end` expect two positional arguments:
      `batch`, `logs`
   - `on_train_begin` and `on_train_end` expect one positional argument:
      `logs`

  Arguments:
      on_epoch_begin: called at the beginning of every epoch.
      on_epoch_end: called at the end of every epoch.
      on_batch_begin: called at the beginning of every batch.
      on_batch_end: called at the end of every batch.
      on_train_begin: called at the beginning of model training.
      on_train_end: called at the end of model training.

  Example:

  ```python
  # Print the batch number at the beginning of every batch.
  batch_print_callback = LambdaCallback(
      on_batch_begin=lambda batch,logs: print(batch))

  # Stream the epoch loss to a file in JSON format. The file content
  # is not well-formed JSON but rather has a JSON object per line.
  import json
  json_log = open('loss_log.json', mode='wt', buffering=1)
  json_logging_callback = LambdaCallback(
      on_epoch_end=lambda epoch, logs: json_log.write(
          json.dumps({'epoch': epoch, 'loss': logs['loss']}) + '\n'),
      on_train_end=lambda logs: json_log.close()
  )

  # Terminate some processes after having finished model training.
  processes = ...
  cleanup_callback = LambdaCallback(
      on_train_end=lambda logs: [
          p.terminate() for p in processes if p.is_alive()])

  model.fit(...,
            callbacks=[batch_print_callback,
                       json_logging_callback,
                       cleanup_callback])
  ```
  """

  def __init__(self,
               on_epoch_begin=None,
               on_epoch_end=None,
               on_batch_begin=None,
               on_batch_end=None,
               on_train_begin=None,
               on_train_end=None,
               **kwargs):
    super(LambdaCallback, self).__init__()
    self.__dict__.update(kwargs)
    if on_epoch_begin is not None:
      self.on_epoch_begin = on_epoch_begin
    else:
      self.on_epoch_begin = lambda epoch, logs: None
    if on_epoch_end is not None:
      self.on_epoch_end = on_epoch_end
    else:
      self.on_epoch_end = lambda epoch, logs: None
    if on_batch_begin is not None:
      self.on_batch_begin = on_batch_begin
    else:
      self.on_batch_begin = lambda batch, logs: None
    if on_batch_end is not None:
      self.on_batch_end = on_batch_end
    else:
      self.on_batch_end = lambda batch, logs: None
    if on_train_begin is not None:
      self.on_train_begin = on_train_begin
    else:
      self.on_train_begin = lambda logs: None
    if on_train_end is not None:
      self.on_train_end = on_train_end
    else:
      self.on_train_end = lambda logs: None