# 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=invalid-name
"""Save and restore variables."""
from __future__ import absolute_import
from __future__ import division
from __future__ import print_function

import collections
import os.path
import re
import time
import uuid

import numpy as np
import six

from google.protobuf import text_format

from tensorflow.core.protobuf import meta_graph_pb2
from tensorflow.core.protobuf import saver_pb2
from tensorflow.python.client import session
from tensorflow.python.eager import context
from tensorflow.python.framework import constant_op
from tensorflow.python.framework import device as pydev
from tensorflow.python.framework import errors
from tensorflow.python.framework import meta_graph
from tensorflow.python.framework import ops
from tensorflow.python.lib.io import file_io
from tensorflow.python.ops import array_ops
from tensorflow.python.ops import control_flow_ops
from tensorflow.python.ops import gen_io_ops
from tensorflow.python.ops import io_ops
from tensorflow.python.ops import resource_variable_ops
from tensorflow.python.ops import state_ops
from tensorflow.python.ops import string_ops
from tensorflow.python.ops import variables
from tensorflow.python.platform import gfile
from tensorflow.python.platform import tf_logging as logging
from tensorflow.python.training import training_util
from tensorflow.python.training.checkpoint_state_pb2 import CheckpointState
from tensorflow.python.util import compat


# Op names which identify variable reads which should be saved.
_VARIABLE_OPS = set(["Variable",
                     "VariableV2",
                     "AutoReloadVariable",
                     "VarHandleOp",
                     "ReadVariableOp"])


def _set_cpu0(device_string):
  """Creates a new device string based on `device_string` but using /CPU:0.

  If the device is already on /CPU:0, this is a no-op.

  Args:
    device_string: A device string.

  Returns:
    A device string.
  """
  parsed_device = pydev.DeviceSpec.from_string(device_string)
  parsed_device.device_type = "CPU"
  parsed_device.device_index = 0
  return parsed_device.to_string()


class BaseSaverBuilder(object):
  """Base class for Savers.

  Can be extended to create different Ops.
  """

  class SaveSpec(object):
    """Class used to describe tensor slices that need to be saved."""

    def __init__(self, tensor, slice_spec, name):
      """Creates a `SaveSpec` object.

      Args:
        tensor: the tensor to save or callable that produces a tensor to save.
        slice_spec: the slice to be saved. See `Variable.SaveSliceInfo`.
        name: the name to save the tensor under.
      """
      self._tensor = tensor
      self.slice_spec = slice_spec
      self.name = name

    @property
    def tensor(self):
      return self._tensor() if callable(self._tensor) else self._tensor

  class SaveableObject(object):
    """Base class for saving and restoring saveable objects."""

    def __init__(self, op, specs, name):
      """Creates a `SaveableObject` object.

      Args:
        op: the "producer" object that this class wraps; it produces a list of
          tensors to save.  E.g., a "Variable" object saving its backing tensor.
        specs: a list of SaveSpec, each element of which describes one tensor to
          save under this object.
        name: the name to save the object under.
      """
      self.op = op
      self.specs = specs
      self.name = name
      # The device of this saveable. All tensors must be on the same device.
      self.device = specs[0].tensor.device

    def restore(self, restored_tensors, restored_shapes):
      """Restores this object from 'restored_tensors'.

      Args:
        restored_tensors: the tensors that were loaded from a checkpoint
        restored_shapes: the shapes this object should conform to after
          restore, or None.

      Returns:
        An operation that restores the state of the object.

      Raises:
        ValueError: If the object cannot be restored using the provided
          parameters.
      """
      # pylint: disable=unused-argument
      raise ValueError("Calling an abstract method.")

  class VariableSaveable(SaveableObject):
    """SaveableObject implementation that handles Variables."""

    def __init__(self, var, slice_spec, name):
      spec = BaseSaverBuilder.SaveSpec(var, slice_spec, name)
      super(BaseSaverBuilder.VariableSaveable, self).__init__(var, [spec], name)

    def restore(self, restored_tensors, restored_shapes):
      restored_tensor = restored_tensors[0]
      if restored_shapes is not None:
        restored_tensor = array_ops.reshape(restored_tensor, restored_shapes[0])
      return state_ops.assign(
          self.op,
          restored_tensor,
          validate_shape=restored_shapes is None and
          self.op.get_shape().is_fully_defined())

  class ResourceVariableSaveable(SaveableObject):
    """SaveableObject implementation that handles ResourceVariables."""

    def __init__(self, var, slice_spec, name):
      self._var_device = var.device
      if isinstance(var, ops.Tensor):
        self.handle_op = var.op.inputs[0]
        tensor = var
      elif isinstance(var, resource_variable_ops.ResourceVariable):

        def _read_variable_closure(v):
          def f():
            with ops.device(v.device):
              x = v.read_value()
            with ops.device("/device:CPU:0"):
              return array_ops.identity(x)
          return f

        self.handle_op = var.handle
        tensor = _read_variable_closure(var)
      else:
        raise ValueError(
            "Saveable is neither a resource variable nor a read operation."
            " Got: %s" % repr(var))
      spec = BaseSaverBuilder.SaveSpec(tensor, slice_spec, name)
      super(BaseSaverBuilder.ResourceVariableSaveable, self).__init__(
          var, [spec], name)

    def restore(self, restored_tensors, restored_shapes):
      restored_tensor = restored_tensors[0]
      if restored_shapes is not None:
        restored_tensor = array_ops.reshape(restored_tensor, restored_shapes[0])
      # Copy the restored tensor to the variable's device.
      with ops.device(self._var_device):
        restored_tensor = array_ops.identity(restored_tensor)
      return resource_variable_ops.assign_variable_op(
          self.handle_op, restored_tensor)

  def __init__(self, write_version=saver_pb2.SaverDef.V2):
    self._write_version = write_version

  def save_op(self, filename_tensor, saveables):
    """Create an Op to save 'saveables'.

    This is intended to be overridden by subclasses that want to generate
    different Ops.

    Args:
      filename_tensor: String Tensor.
      saveables: A list of BaseSaverBuilder.SaveableObject objects.

    Returns:
      An Operation that save the variables.

    Raises:
      RuntimeError: (implementation detail) if "self._write_version" is an
        unexpected value.
    """
    # pylint: disable=protected-access
    tensor_names = []
    tensors = []
    tensor_slices = []
    for saveable in saveables:
      for spec in saveable.specs:
        tensor_names.append(spec.name)
        tensors.append(spec.tensor)
        tensor_slices.append(spec.slice_spec)
    if self._write_version == saver_pb2.SaverDef.V1:
      return io_ops._save(
          filename=filename_tensor,
          tensor_names=tensor_names,
          tensors=tensors,
          tensor_slices=tensor_slices)
    elif self._write_version == saver_pb2.SaverDef.V2:
      # "filename_tensor" is interpreted *NOT AS A FILENAME*, but as a prefix
      # of a V2 checkpoint: e.g. "/fs/train/ckpt-<step>/tmp/worker<i>-<step>".
      return io_ops.save_v2(filename_tensor, tensor_names, tensor_slices,
                            tensors)
    else:
      raise RuntimeError("Unexpected write_version: " + self._write_version)

  # pylint: disable=unused-argument
  def restore_op(self, filename_tensor, saveable, preferred_shard):
    """Create ops to restore 'saveable'.

    This is intended to be overridden by subclasses that want to generate
    different Ops.

    Args:
      filename_tensor: String Tensor.
      saveable: A BaseSaverBuilder.SaveableObject object.
      preferred_shard: Int.  Shard to open first when loading a sharded file.

    Returns:
      A list of Tensors resulting from reading 'saveable' from
        'filename'.
    """
    # pylint: disable=protected-access
    tensors = []
    for spec in saveable.specs:
      tensors.append(
          io_ops.restore_v2(
              filename_tensor,
              [spec.name],
              [spec.slice_spec],
              [spec.tensor.dtype])[0])

    return tensors
  # pylint: enable=unused-argument

  def sharded_filename(self, filename_tensor, shard, num_shards):
    """Append sharding information to a filename.

    Args:
      filename_tensor: A string tensor.
      shard: Integer.  The shard for the filename.
      num_shards: An int Tensor for the number of shards.

    Returns:
      A string tensor.
    """
    # pylint: disable=protected-access
    return gen_io_ops._sharded_filename(filename_tensor, shard, num_shards)

  def _AddSaveOps(self, filename_tensor, saveables):
    """Add ops to save variables that are on the same shard.

    Args:
      filename_tensor: String Tensor.
      saveables: A list of SaveableObject objects.

    Returns:
      A tensor with the filename used to save.
    """
    save = self.save_op(filename_tensor, saveables)
    return control_flow_ops.with_dependencies([save], filename_tensor)

  def _AddShardedSaveOpsForV2(self, checkpoint_prefix, per_device):
    """Add ops to save the params per shard, for the V2 format.

    Note that the sharded save procedure for the V2 format is different from
    V1: there is a special "merge" step that merges the small metadata produced
    from each device.

    Args:
      checkpoint_prefix: scalar String Tensor.  Interpreted *NOT AS A
        FILENAME*, but as a prefix of a V2 checkpoint;
      per_device: A list of (device, BaseSaverBuilder.VarToSave) pairs, as
        returned by _GroupByDevices().

    Returns:
      An op to save the variables, which, when evaluated, returns the prefix
        "<user-fed prefix>" only and does not include the sharded spec suffix.
    """
    # IMPLEMENTATION DETAILS: most clients should skip.
    #
    # Suffix for any well-formed "checkpoint_prefix", when sharded.
    # Transformations:
    # * Users pass in "save_path" in save() and restore().  Say "myckpt".
    # * checkpoint_prefix gets fed <save_path><_SHARDED_SUFFIX>.
    #
    # Example:
    #   During runtime, a temporary directory is first created, which contains
    #   files
    #
    #     <train dir>/myckpt_temp/
    #        part-?????-of-?????{.index, .data-00000-of-00001}
    #
    #   Before .save() finishes, they will be (hopefully, atomically) renamed to
    #
    #     <train dir>/
    #        myckpt{.index, .data-?????-of-?????}
    #
    # Users only need to interact with the user-specified prefix, which is
    # "<train dir>/myckpt" in this case.  Save() and Restore() work with the
    # prefix directly, instead of any physical pathname.  (On failure and
    # subsequent restore, an outdated and orphaned temporary directory can be
    # safely removed.)
    _SHARDED_SUFFIX = "_temp_%s/part" % uuid.uuid4().hex
    tmp_checkpoint_prefix = string_ops.string_join(
        [checkpoint_prefix, _SHARDED_SUFFIX])

    num_shards = len(per_device)
    sharded_saves = []
    sharded_prefixes = []
    num_shards_tensor = constant_op.constant(num_shards, name="num_shards")
    last_device = None
    for shard, (device, saveables) in enumerate(per_device):
      last_device = device
      with ops.device(device):
        sharded_filename = self.sharded_filename(tmp_checkpoint_prefix, shard,
                                                 num_shards_tensor)
        sharded_prefixes.append(sharded_filename)
        sharded_saves.append(self._AddSaveOps(sharded_filename, saveables))

    with ops.control_dependencies([x.op for x in sharded_saves]):
      # Co-locates the merge step with the last device.
      with ops.device(last_device):
        # V2 format write path consists of a metadata merge step.  Once merged,
        # attempts to delete the temporary directory, "<user-fed prefix>_temp".
        merge_step = gen_io_ops.merge_v2_checkpoints(
            sharded_prefixes, checkpoint_prefix, delete_old_dirs=True)
        with ops.control_dependencies([merge_step]):
          # Returns the prefix "<user-fed prefix>" only.  DOES NOT include the
          # sharded spec suffix.
          return array_ops.identity(checkpoint_prefix)

  def _AddShardedSaveOps(self, filename_tensor, per_device):
    """Add ops to save the params per shard.

    Args:
      filename_tensor: a scalar String Tensor.
      per_device: A list of (device, BaseSaverBuilder.SaveableObject) pairs, as
        returned by _GroupByDevices().

    Returns:
      An op to save the variables.
    """
    if self._write_version == saver_pb2.SaverDef.V2:
      return self._AddShardedSaveOpsForV2(filename_tensor, per_device)

    num_shards = len(per_device)
    sharded_saves = []
    num_shards_tensor = constant_op.constant(num_shards, name="num_shards")
    for shard, (device, saveables) in enumerate(per_device):
      with ops.device(device):
        sharded_filename = self.sharded_filename(filename_tensor, shard,
                                                 num_shards_tensor)
        sharded_saves.append(self._AddSaveOps(sharded_filename, saveables))
    # Return the sharded name for the save path.
    with ops.control_dependencies([x.op for x in sharded_saves]):
      # pylint: disable=protected-access
      return gen_io_ops._sharded_filespec(filename_tensor, num_shards_tensor)

  def _AddRestoreOps(self,
                     filename_tensor,
                     saveables,
                     restore_sequentially,
                     reshape,
                     preferred_shard=-1,
                     name="restore_all"):
    """Add operations to restore saveables.

    Args:
      filename_tensor: Tensor for the path of the file to load.
      saveables: A list of SaveableObject objects.
      restore_sequentially: True if we want to restore variables sequentially
        within a shard.
      reshape: True if we want to reshape loaded tensors to the shape of
        the corresponding variable.
      preferred_shard: Shard to open first when loading a sharded file.
      name: Name for the returned op.

    Returns:
      An Operation that restores the variables.
    """
    assign_ops = []
    for saveable in saveables:
      restore_control_inputs = assign_ops[-1:] if restore_sequentially else []
      # Load and optionally reshape on the CPU, as string tensors are not
      # available on the GPU.
      # TODO(touts): Re-enable restore on GPU when we can support annotating
      # string tensors as "HostMemory" inputs.
      with ops.device(_set_cpu0(saveable.device) if saveable.device else None):
        with ops.control_dependencies(restore_control_inputs):
          tensors = self.restore_op(filename_tensor, saveable, preferred_shard)
          shapes = None
          if reshape:
            # Compute the shapes, let the restore op decide if and how to do
            # the reshape.
            shapes = []
            for spec in saveable.specs:
              v = spec.tensor
              shape = v.get_shape()
              if not shape.is_fully_defined():
                shape = array_ops.shape(v)
              shapes.append(shape)
          assign_ops.append(saveable.restore(tensors, shapes))

      # Create a Noop that has control dependencies from all the updates.
    return control_flow_ops.group(*assign_ops, name=name)

  def _AddShardedRestoreOps(self, filename_tensor, per_device,
                            restore_sequentially, reshape):
    """Add Ops to restore variables from multiple devices.

    Args:
      filename_tensor: Tensor for the path of the file to load.
      per_device: A list of (device, SaveableObject) pairs, as
        returned by _GroupByDevices().
      restore_sequentially: True if we want to restore variables sequentially
        within a shard.
      reshape: True if we want to reshape loaded tensors to the shape of
        the corresponding variable.

    Returns:
      An Operation that restores the variables.
    """
    sharded_restores = []
    for shard, (device, saveables) in enumerate(per_device):
      with ops.device(device):
        sharded_restores.append(
            self._AddRestoreOps(
                filename_tensor,
                saveables,
                restore_sequentially,
                reshape,
                preferred_shard=shard,
                name="restore_shard"))
    return control_flow_ops.group(*sharded_restores, name="restore_all")

  @staticmethod
  def _IsVariable(v):
    return isinstance(v, ops.Tensor) and v.op.type in _VARIABLE_OPS

  def _GroupByDevices(self, saveables):
    """Group Variable tensor slices per device.

    TODO(touts): Make sure that all the devices found are on different
    job/replica/task/cpu|gpu.  It would be bad if 2 were on the same device.
    It can happen if the devices are unspecified.

    Args:
      saveables: A list of BaseSaverBuilder.SaveableObject objects.

    Returns:
      A list of tuples: (device_name, BaseSaverBuilder.SaveableObject) tuples.
      The list is sorted by ascending device_name.

    Raises:
      ValueError: If the tensors of a saveable are on different devices.
    """
    per_device = collections.defaultdict(lambda: [])
    for saveable in saveables:
      canonical_device = set(
          pydev.canonical_name(spec.tensor.device) for spec in saveable.specs)
      if len(canonical_device) != 1:
        raise ValueError("All tensors of a saveable object must be "
                         "on the same device: %s" % saveable.name)
      per_device[canonical_device.pop()].append(saveable)
    return sorted(per_device.items(), key=lambda t: t[0])

  @staticmethod
  def OpListToDict(op_list):
    """Create a dictionary of names to operation lists.

    Args:
      op_list: A list, tuple, or set of Variables or SaveableObjects.

    Returns:
      A dictionary of names to the operations that must be saved under
      that name.  Variables with save_slice_info are grouped together under the
      same key in no particular order.

    Raises:
      TypeError: If the type of op_list or its elements is not supported.
      ValueError: If at least two saveables share the same name.
    """
    if not isinstance(op_list, (list, tuple, set)):
      raise TypeError("Variables to save should be passed in a dict or a "
                      "list: %s" % op_list)
    op_list = set(op_list)
    names_to_saveables = {}
    # pylint: disable=protected-access
    for var in op_list:
      if isinstance(var, BaseSaverBuilder.SaveableObject):
        names_to_saveables[var.name] = var
      elif isinstance(var, variables.PartitionedVariable):
        if var.name in names_to_saveables:
          raise ValueError("At least two variables have the same name: %s" %
                           var.name)
        names_to_saveables[var.name] = var
      elif isinstance(var, variables.Variable) and var._save_slice_info:
        name = var._save_slice_info.full_name
        if name in names_to_saveables:
          if not isinstance(names_to_saveables[name], list):
            raise ValueError("Mixing slices and non-slices with the same name: "
                             "%s" % name)
          names_to_saveables[name].append(var)
        else:
          names_to_saveables[name] = [var]
      else:
        if context.in_graph_mode():
          var = ops.internal_convert_to_tensor(var, as_ref=True)
          if not BaseSaverBuilder._IsVariable(var):
            raise TypeError("Variable to save is not a Variable: %s" % var)
          if var.op.type == "ReadVariableOp":
            name = var.op.inputs[0].op.name
          else:
            name = var.op.name
          if name in names_to_saveables:
            raise ValueError("At least two variables have the same name: %s" %
                             name)
          names_to_saveables[name] = var
        else:
          if not isinstance(var, resource_variable_ops.ResourceVariable):
            raise ValueError("Can only save/restore ResourceVariable eager "
                             "mode is enabled, type: %s." % type(var))
          names_to_saveables[var._shared_name] = var

      # pylint: enable=protected-access
    return names_to_saveables

  def _ValidateAndSliceInputs(self, names_to_saveables):
    """Returns the variables and names that will be used for a Saver.

    Args:
      names_to_saveables: A dict (k, v) where k is the name of an operation and
         v is an operation to save or a BaseSaverBuilder.Saver.

    Returns:
      A list of BaseSaverBuilder.SaveableObject objects.

    Raises:
      TypeError: If any of the keys are not strings or any of the
        values are not one of Tensor or Variable or a checkpointable operation.
      ValueError: If the same operation is given in more than one value
        (this also applies to slices of SlicedVariables).
    """
    if not isinstance(names_to_saveables, dict):
      names_to_saveables = BaseSaverBuilder.OpListToDict(names_to_saveables)

    saveables = []
    seen_ops = set()
    for name in sorted(names_to_saveables.keys()):
      if not isinstance(name, six.string_types):
        raise TypeError(
            "names_to_saveables must be a dict mapping string names to "
            "checkpointable operations. Name is not a string: %s" % name)
      op = names_to_saveables[name]
      if isinstance(op, BaseSaverBuilder.SaveableObject):
        self._AddSaveable(saveables, seen_ops, op)
      elif isinstance(op, (list, tuple, variables.PartitionedVariable)):
        if isinstance(op, variables.PartitionedVariable):
          op = list(op)
        # A set of slices.
        slice_name = None
        # pylint: disable=protected-access
        for variable in op:
          if not isinstance(variable, variables.Variable):
            raise ValueError("Slices must all be Variables: %s" % variable)
          if not variable._save_slice_info:
            raise ValueError("Slices must all be slices: %s" % variable)
          if slice_name is None:
            slice_name = variable._save_slice_info.full_name
          elif slice_name != variable._save_slice_info.full_name:
            raise ValueError(
                "Slices must all be from the same tensor: %s != %s" %
                (slice_name, variable._save_slice_info.full_name))
          if variable.op.type in ["Variable", "VariableV2",
                                  "AutoReloadVariable"]:
            saveable = BaseSaverBuilder.VariableSaveable(
                variable, variable._save_slice_info.spec, name)
          else:
            saveable = BaseSaverBuilder.ResourceVariableSaveable(
                variable, variable._save_slice_info.spec, name)
          self._AddSaveable(saveables, seen_ops, saveable)
        # pylint: enable=protected-access
      else:
        # A variable or tensor.
        if context.in_eager_mode():
          if not isinstance(op, resource_variable_ops.ResourceVariable):
            raise ValueError("Can only save/restore ResourceVariable eager "
                             "mode is enabled, type: %s." % type(op))
          saveable = BaseSaverBuilder.ResourceVariableSaveable(op, "", name)
        else:
          variable = ops.internal_convert_to_tensor(op, as_ref=True)
          if not BaseSaverBuilder._IsVariable(variable):
            raise TypeError("names_to_saveables must be a dict mapping string "
                            "names to Tensors/Variables. Not a variable: %s" %
                            variable)
          if variable.op.type in ["Variable", "VariableV2",
                                  "AutoReloadVariable"]:
            saveable = BaseSaverBuilder.VariableSaveable(variable, "", name)
          else:
            saveable = BaseSaverBuilder.ResourceVariableSaveable(
                variable, "", name)
        self._AddSaveable(saveables, seen_ops, saveable)
    return saveables

  def _AddSaveable(self, saveables, seen_ops, saveable):
    """Adds the saveable to the saveables list.

    Args:
      saveables: List to append the SaveableObject to.
      seen_ops: Set of the ops of the saveables already processed.  Used to
        check that each saveable is only saved once.
      saveable: The saveable.

    Raises:
      ValueError: If the saveable has already been processed.
    """
    if saveable.op in seen_ops:
      raise ValueError("The same saveable will be restored with two names: %s" %
                       saveable.name)
    saveables.append(saveable)
    seen_ops.add(saveable.op)

  def build(self,
            names_to_saveables,
            reshape=False,
            sharded=False,
            max_to_keep=5,
            keep_checkpoint_every_n_hours=10000.0,
            name=None,
            restore_sequentially=False,
            filename="model"):
    """Builds save/restore graph nodes or runs save/restore in eager mode.

    Args:
      names_to_saveables: A dictionary mapping name to a Variable or
        SaveableObject. Each name will be associated with the
        corresponding variable in the checkpoint.
      reshape: If True, allow restoring parameters from a checkpoint
        that where the parameters have a different shape.  This is
        only needed when you try to restore from a Dist-Belief checkpoint,
        and only some times.
      sharded: If True, shard the checkpoints, one per device that has
        Variable nodes.
      max_to_keep: Maximum number of checkpoints to keep.  As new checkpoints
        are created, old ones are deleted.  If None or 0, no checkpoints are
        deleted from the filesystem but only the last one is kept in the
        `checkpoint` file.  Presently the number is only roughly enforced.  For
        example in case of restarts more than max_to_keep checkpoints may be
        kept.
      keep_checkpoint_every_n_hours: How often checkpoints should be kept.
        Defaults to 10,000 hours.
      name: String.  Optional name to use as a prefix when adding operations.
      restore_sequentially: A Bool, which if true, causes restore of different
        variables to happen sequentially within each device.
      filename: If known at graph construction time, filename used for variable
        loading/saving. If None, then the default name "model" will be used.

    Returns:
      A SaverDef proto.

    Raises:
      TypeError: If 'names_to_saveables' is not a dictionary mapping string
        keys to variable Tensors.
      ValueError: If any of the keys or values in 'names_to_saveables' is not
        unique.
    """
    return self._build_internal(
        names_to_saveables=names_to_saveables,
        reshape=reshape,
        sharded=sharded,
        max_to_keep=max_to_keep,
        keep_checkpoint_every_n_hours=keep_checkpoint_every_n_hours,
        name=name,
        restore_sequentially=restore_sequentially,
        filename=filename)

  def _build_internal(self,
                      names_to_saveables,
                      reshape=False,
                      sharded=False,
                      max_to_keep=5,
                      keep_checkpoint_every_n_hours=10000.0,
                      name=None,
                      restore_sequentially=False,
                      filename="model",
                      build_save=True,
                      build_restore=True):
    """build() with option to only perform save and restore."""
    if context.in_graph_mode() and (not build_save or not build_restore):
      raise ValueError("Graph mode needs to build save and restore together.")

    saveables = self._ValidateAndSliceInputs(names_to_saveables)
    if max_to_keep is None:
      max_to_keep = 0

    with ops.name_scope(name, "save",
                        [saveable.op for saveable in saveables]) as name:
      # Add the Constant string tensor for the filename.
      filename_tensor = constant_op.constant(filename or "model")

      # Add the save ops.
      if sharded:
        per_device = self._GroupByDevices(saveables)
        if build_save:
          save_tensor = self._AddShardedSaveOps(filename_tensor, per_device)
        if build_restore:
          restore_op = self._AddShardedRestoreOps(filename_tensor, per_device,
                                                  restore_sequentially, reshape)
      else:
        if build_save:
          save_tensor = self._AddSaveOps(filename_tensor, saveables)
        if build_restore:
          restore_op = self._AddRestoreOps(filename_tensor, saveables,
                                           restore_sequentially, reshape)

    # In the following use case, it's possible to have restore_ops be called
    # something else:
    # - Build inference graph and export a meta_graph.
    # - Import the inference meta_graph
    # - Extend the inference graph to a train graph.
    # - Export a new meta_graph.
    # Now the second restore_op will be called "restore_all_1".
    # As such, comment out the assert for now until we know whether supporting
    # such usage model makes sense.
    #
    # assert restore_op.name.endswith("restore_all"), restore_op.name
    if context.in_graph_mode():
      return saver_pb2.SaverDef(
          filename_tensor_name=filename_tensor.name,
          save_tensor_name=save_tensor.name,
          restore_op_name=restore_op.name,
          max_to_keep=max_to_keep,
          sharded=sharded,
          keep_checkpoint_every_n_hours=keep_checkpoint_every_n_hours,
          version=self._write_version)
    else:
      # Store the tensor values to the tensor_names.
      save_tensor_name = save_tensor.numpy() if build_save else ""
      return saver_pb2.SaverDef(
          filename_tensor_name=filename_tensor.numpy(),
          save_tensor_name=save_tensor_name,
          restore_op_name="",
          max_to_keep=max_to_keep,
          sharded=sharded,
          keep_checkpoint_every_n_hours=keep_checkpoint_every_n_hours,
          version=self._write_version)


def _get_saver_or_default():
  """Returns the saver from SAVERS collection, or creates a default one.

  This method is used by other members of the training module, such as
  `Scaffold`, or `CheckpointSaverHook`.

  Returns:
    `Saver`.

  Raises:
    RuntimeError: If the SAVERS collection already has more than one items.
  """
  collection_key = ops.GraphKeys.SAVERS
  savers = ops.get_collection(collection_key)
  if savers:
    if 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))
    return savers[0]
  saver = Saver(sharded=True, allow_empty=True)
  if saver is not None:
    ops.add_to_collection(collection_key, saver)
  return saver


def _GetCheckpointFilename(save_dir, latest_filename):
  """Returns a filename for storing the CheckpointState.

  Args:
    save_dir: The directory for saving and restoring checkpoints.
    latest_filename: Name of the file in 'save_dir' that is used
      to store the CheckpointState.

  Returns:
    The path of the file that contains the CheckpointState proto.
  """
  if latest_filename is None:
    latest_filename = "checkpoint"
  return os.path.join(save_dir, latest_filename)


def generate_checkpoint_state_proto(save_dir,
                                    model_checkpoint_path,
                                    all_model_checkpoint_paths=None):
  """Generates a checkpoint state proto.

  Args:
    save_dir: Directory where the model was saved.
    model_checkpoint_path: The checkpoint file.
    all_model_checkpoint_paths: List of strings.  Paths to all not-yet-deleted
      checkpoints, sorted from oldest to newest.  If this is a non-empty list,
      the last element must be equal to model_checkpoint_path.  These paths
      are also saved in the CheckpointState proto.

  Returns:
    CheckpointState proto with model_checkpoint_path and
    all_model_checkpoint_paths updated to either absolute paths or
    relative paths to the current save_dir.
  """
  if all_model_checkpoint_paths is None:
    all_model_checkpoint_paths = []

  if (not all_model_checkpoint_paths or
      all_model_checkpoint_paths[-1] != model_checkpoint_path):
    logging.info("%s is not in all_model_checkpoint_paths. Manually adding it.",
                 model_checkpoint_path)
    all_model_checkpoint_paths.append(model_checkpoint_path)

  # Relative paths need to be rewritten to be relative to the "save_dir"
  # if model_checkpoint_path already contains "save_dir".
  if not os.path.isabs(save_dir):
    if not os.path.isabs(model_checkpoint_path):
      model_checkpoint_path = os.path.relpath(model_checkpoint_path, save_dir)
    for i in range(len(all_model_checkpoint_paths)):
      p = all_model_checkpoint_paths[i]
      if not os.path.isabs(p):
        all_model_checkpoint_paths[i] = os.path.relpath(p, save_dir)

  coord_checkpoint_proto = CheckpointState(
      model_checkpoint_path=model_checkpoint_path,
      all_model_checkpoint_paths=all_model_checkpoint_paths)

  return coord_checkpoint_proto


def update_checkpoint_state(save_dir,
                            model_checkpoint_path,
                            all_model_checkpoint_paths=None,
                            latest_filename=None):
  """Updates the content of the 'checkpoint' file.

  This updates the checkpoint file containing a CheckpointState
  proto.

  Args:
    save_dir: Directory where the model was saved.
    model_checkpoint_path: The checkpoint file.
    all_model_checkpoint_paths: List of strings.  Paths to all not-yet-deleted
      checkpoints, sorted from oldest to newest.  If this is a non-empty list,
      the last element must be equal to model_checkpoint_path.  These paths
      are also saved in the CheckpointState proto.
    latest_filename: Optional name of the checkpoint file.  Default to
      'checkpoint'.

  Raises:
    RuntimeError: If any of the model checkpoint paths conflict with the file
      containing CheckpointSate.
  """
  _update_checkpoint_state(
      save_dir=save_dir,
      model_checkpoint_path=model_checkpoint_path,
      all_model_checkpoint_paths=all_model_checkpoint_paths,
      latest_filename=latest_filename,
      save_relative_paths=False)


def _update_checkpoint_state(save_dir,
                             model_checkpoint_path,
                             all_model_checkpoint_paths=None,
                             latest_filename=None,
                             save_relative_paths=False):
  """Updates the content of the 'checkpoint' file.

  This updates the checkpoint file containing a CheckpointState
  proto.

  Args:
    save_dir: Directory where the model was saved.
    model_checkpoint_path: The checkpoint file.
    all_model_checkpoint_paths: List of strings.  Paths to all not-yet-deleted
      checkpoints, sorted from oldest to newest.  If this is a non-empty list,
      the last element must be equal to model_checkpoint_path.  These paths
      are also saved in the CheckpointState proto.
    latest_filename: Optional name of the checkpoint file.  Default to
      'checkpoint'.
    save_relative_paths: If `True`, will write relative paths to the checkpoint
      state file.

  Raises:
    RuntimeError: If any of the model checkpoint paths conflict with the file
      containing CheckpointSate.
  """
  # Writes the "checkpoint" file for the coordinator for later restoration.
  coord_checkpoint_filename = _GetCheckpointFilename(save_dir, latest_filename)
  if save_relative_paths:
    if os.path.isabs(model_checkpoint_path):
      rel_model_checkpoint_path = os.path.relpath(
          model_checkpoint_path, save_dir)
    else:
      rel_model_checkpoint_path = model_checkpoint_path
    rel_all_model_checkpoint_paths = []
    for p in all_model_checkpoint_paths:
      if os.path.isabs(p):
        rel_all_model_checkpoint_paths.append(os.path.relpath(p, save_dir))
      else:
        rel_all_model_checkpoint_paths.append(p)
    ckpt = generate_checkpoint_state_proto(
        save_dir,
        rel_model_checkpoint_path,
        all_model_checkpoint_paths=rel_all_model_checkpoint_paths)
  else:
    ckpt = generate_checkpoint_state_proto(
        save_dir,
        model_checkpoint_path,
        all_model_checkpoint_paths=all_model_checkpoint_paths)

  if coord_checkpoint_filename == ckpt.model_checkpoint_path:
    raise RuntimeError("Save path '%s' conflicts with path used for "
                       "checkpoint state.  Please use a different save path." %
                       model_checkpoint_path)

  # Preventing potential read/write race condition by *atomically* writing to a
  # file.
  file_io.atomic_write_string_to_file(coord_checkpoint_filename,
                                      text_format.MessageToString(ckpt))


def get_checkpoint_state(checkpoint_dir, latest_filename=None):
  """Returns CheckpointState proto from the "checkpoint" file.

  If the "checkpoint" file contains a valid CheckpointState
  proto, returns it.

  Args:
    checkpoint_dir: The directory of checkpoints.
    latest_filename: Optional name of the checkpoint file.  Default to
      'checkpoint'.

  Returns:
    A CheckpointState if the state was available, None
    otherwise.

  Raises:
    ValueError: if the checkpoint read doesn't have model_checkpoint_path set.
  """
  ckpt = None
  coord_checkpoint_filename = _GetCheckpointFilename(checkpoint_dir,
                                                     latest_filename)
  f = None
  try:
    # Check that the file exists before opening it to avoid
    # many lines of errors from colossus in the logs.
    if file_io.file_exists(coord_checkpoint_filename):
      file_content = file_io.read_file_to_string(
          coord_checkpoint_filename)
      ckpt = CheckpointState()
      text_format.Merge(file_content, ckpt)
      if not ckpt.model_checkpoint_path:
        raise ValueError("Invalid checkpoint state loaded from %s",
                         checkpoint_dir)
      # For relative model_checkpoint_path and all_model_checkpoint_paths,
      # prepend checkpoint_dir.
      if not os.path.isabs(ckpt.model_checkpoint_path):
        ckpt.model_checkpoint_path = os.path.join(checkpoint_dir,
                                                  ckpt.model_checkpoint_path)
      for i in range(len(ckpt.all_model_checkpoint_paths)):
        p = ckpt.all_model_checkpoint_paths[i]
        if not os.path.isabs(p):
          ckpt.all_model_checkpoint_paths[i] = os.path.join(checkpoint_dir, p)
  except errors.OpError as e:
    # It's ok if the file cannot be read
    logging.warning("%s: %s", type(e).__name__, e)
    logging.warning("%s: Checkpoint ignored", coord_checkpoint_filename)
    return None
  except text_format.ParseError as e:
    logging.warning("%s: %s", type(e).__name__, e)
    logging.warning("%s: Checkpoint ignored", coord_checkpoint_filename)
    return None
  finally:
    if f:
      f.close()
  return ckpt


class Saver(object):
  """Saves and restores variables.

  See @{$variables$Variables}
  for an overview of variables, saving and restoring.

  The `Saver` class adds ops to save and restore variables to and from
  *checkpoints*.  It also provides convenience methods to run these ops.

  Checkpoints are binary files in a proprietary format which map variable names
  to tensor values.  The best way to examine the contents of a checkpoint is to
  load it using a `Saver`.

  Savers can automatically number checkpoint filenames with a provided counter.
  This lets you keep multiple checkpoints at different steps while training a
  model.  For example you can number the checkpoint filenames with the training
  step number.  To avoid filling up disks, savers manage checkpoint files
  automatically. For example, they can keep only the N most recent files, or
  one checkpoint for every N hours of training.

  You number checkpoint filenames by passing a value to the optional
  `global_step` argument to `save()`:

  ```python
  saver.save(sess, 'my-model', global_step=0) ==> filename: 'my-model-0'
  ...
  saver.save(sess, 'my-model', global_step=1000) ==> filename: 'my-model-1000'
  ```

  Additionally, optional arguments to the `Saver()` constructor let you control
  the proliferation of checkpoint files on disk:

  * `max_to_keep` indicates the maximum number of recent checkpoint files to
    keep.  As new files are created, older files are deleted.  If None or 0,
    all checkpoint files are kept.  Defaults to 5 (that is, the 5 most recent
    checkpoint files are kept.)

  * `keep_checkpoint_every_n_hours`: In addition to keeping the most recent
    `max_to_keep` checkpoint files, you might want to keep one checkpoint file
    for every N hours of training.  This can be useful if you want to later
    analyze how a model progressed during a long training session.  For
    example, passing `keep_checkpoint_every_n_hours=2` ensures that you keep
    one checkpoint file for every 2 hours of training.  The default value of
    10,000 hours effectively disables the feature.

  Note that you still have to call the `save()` method to save the model.
  Passing these arguments to the constructor will not save variables
  automatically for you.

  A training program that saves regularly looks like:

  ```python
  ...
  # Create a saver.
  saver = tf.train.Saver(...variables...)
  # Launch the graph and train, saving the model every 1,000 steps.
  sess = tf.Session()
  for step in xrange(1000000):
      sess.run(..training_op..)
      if step % 1000 == 0:
          # Append the step number to the checkpoint name:
          saver.save(sess, 'my-model', global_step=step)
  ```

  In addition to checkpoint files, savers keep a protocol buffer on disk with
  the list of recent checkpoints. This is used to manage numbered checkpoint
  files and by `latest_checkpoint()`, which makes it easy to discover the path
  to the most recent checkpoint. That protocol buffer is stored in a file named
  'checkpoint' next to the checkpoint files.

  If you create several savers, you can specify a different filename for the
  protocol buffer file in the call to `save()`.
  """

  def __init__(self,
               var_list=None,
               reshape=False,
               sharded=False,
               max_to_keep=5,
               keep_checkpoint_every_n_hours=10000.0,
               name=None,
               restore_sequentially=False,
               saver_def=None,
               builder=None,
               defer_build=False,
               allow_empty=False,
               write_version=saver_pb2.SaverDef.V2,
               pad_step_number=False,
               save_relative_paths=False,
               filename=None):
    """Creates a `Saver`.

    The constructor adds ops to save and restore variables.

    `var_list` specifies the variables that will be saved and restored. It can
    be passed as a `dict` or a list:

    * A `dict` of names to variables: The keys are the names that will be
      used to save or restore the variables in the checkpoint files.
    * A list of variables: The variables will be keyed with their op name in
      the checkpoint files.

    For example:

    ```python
    v1 = tf.Variable(..., name='v1')
    v2 = tf.Variable(..., name='v2')

    # Pass the variables as a dict:
    saver = tf.train.Saver({'v1': v1, 'v2': v2})

    # Or pass them as a list.
    saver = tf.train.Saver([v1, v2])
    # Passing a list is equivalent to passing a dict with the variable op names
    # as keys:
    saver = tf.train.Saver({v.op.name: v for v in [v1, v2]})
    ```

    The optional `reshape` argument, if `True`, allows restoring a variable from
    a save file where the variable had a different shape, but the same number
    of elements and type.  This is useful if you have reshaped a variable and
    want to reload it from an older checkpoint.

    The optional `sharded` argument, if `True`, instructs the saver to shard
    checkpoints per device.

    Args:
      var_list: A list of `Variable`/`SaveableObject`, or a dictionary mapping
        names to `SaveableObject`s. If `None`, defaults to the list of all
        saveable objects.
      reshape: If `True`, allows restoring parameters from a checkpoint
        where the variables have a different shape.
      sharded: If `True`, shard the checkpoints, one per device.
      max_to_keep: Maximum number of recent checkpoints to keep.
        Defaults to 5.
      keep_checkpoint_every_n_hours: How often to keep checkpoints.
        Defaults to 10,000 hours.
      name: String.  Optional name to use as a prefix when adding operations.
      restore_sequentially: A `Bool`, which if true, causes restore of different
        variables to happen sequentially within each device.  This can lower
        memory usage when restoring very large models.
      saver_def: Optional `SaverDef` proto to use instead of running the
        builder. This is only useful for specialty code that wants to recreate
        a `Saver` object for a previously built `Graph` that had a `Saver`.
        The `saver_def` proto should be the one returned by the
        `as_saver_def()` call of the `Saver` that was created for that `Graph`.
      builder: Optional `SaverBuilder` to use if a `saver_def` was not provided.
        Defaults to `BaseSaverBuilder()`.
      defer_build: If `True`, defer adding the save and restore ops to the
        `build()` call. In that case `build()` should be called before
        finalizing the graph or using the saver.
      allow_empty: If `False` (default) raise an error if there are no
        variables in the graph. Otherwise, construct the saver anyway and make
        it a no-op.
      write_version: controls what format to use when saving checkpoints.  It
        also affects certain filepath matching logic.  The V2 format is the
        recommended choice: it is much more optimized than V1 in terms of
        memory required and latency incurred during restore.  Regardless of
        this flag, the Saver is able to restore from both V2 and V1 checkpoints.
      pad_step_number: if True, pads the global step number in the checkpoint
        filepaths to some fixed width (8 by default).  This is turned off by
        default.
      save_relative_paths: If `True`, will write relative paths to the
        checkpoint state file. This is needed if the user wants to copy the
        checkpoint directory and reload from the copied directory.
      filename: If known at graph construction time, filename used for variable
        loading/saving.

    Raises:
      TypeError: If `var_list` is invalid.
      ValueError: If any of the keys or values in `var_list` are not unique.
    """
    if defer_build and var_list:
      raise ValueError(
          "If `var_list` is provided then build cannot be deferred. "
          "Either set defer_build=False or var_list=None.")
    if context.in_eager_mode() and var_list is None:
      raise ValueError(
          "When eager execution is enabled, `var_list` must specify a list of "
          "variables to save")
    self._var_list = var_list
    self._reshape = reshape
    self._sharded = sharded
    self._max_to_keep = max_to_keep
    self._keep_checkpoint_every_n_hours = keep_checkpoint_every_n_hours
    self._name = name
    self._restore_sequentially = restore_sequentially
    self.saver_def = saver_def
    self._builder = builder
    self._is_built = False
    self._allow_empty = allow_empty
    self._is_empty = None
    self._write_version = write_version
    self._pad_step_number = pad_step_number
    self._filename = filename
    if not defer_build and context.in_graph_mode():
      self.build()
    if self.saver_def:
      self._check_saver_def()
      self._write_version = self.saver_def.version
    self._save_relative_paths = save_relative_paths

  def build(self):
    if context.in_eager_mode():
      raise ValueError("Use save/restore instead of build in eager mode.")
    self._build(self._filename, build_save=True, build_restore=True)

  def _build_eager(self, checkpoint_path, build_save, build_restore):
    self._build(
        checkpoint_path, build_save=build_save, build_restore=build_restore)

  def _build(self, checkpoint_path, build_save, build_restore):
    """Builds saver_def."""
    if context.in_graph_mode():
      if self._is_built:
        return
      self._is_built = True

    if not self.saver_def or context.in_eager_mode():
      if self._builder is None:
        self._builder = BaseSaverBuilder(self._write_version)
      if self._var_list is None:
        # pylint: disable=protected-access
        self._var_list = variables._all_saveable_objects()
      if not self._var_list:
        if self._allow_empty:
          self._is_empty = True
          return
        else:
          raise ValueError("No variables to save")
      self._is_empty = False

      self.saver_def = self._builder._build_internal(  # pylint: disable=protected-access
          self._var_list,
          reshape=self._reshape,
          sharded=self._sharded,
          max_to_keep=self._max_to_keep,
          keep_checkpoint_every_n_hours=self._keep_checkpoint_every_n_hours,
          name=self._name,
          restore_sequentially=self._restore_sequentially,
          filename=checkpoint_path,
          build_save=build_save, build_restore=build_restore)
    elif self.saver_def and self._name:
      # Since self._name is used as a name_scope by builder(), we are
      # overloading the use of this field to represent the "import_scope" as
      # well.
      self.saver_def.filename_tensor_name = ops.prepend_name_scope(
          self.saver_def.filename_tensor_name, self._name)
      self.saver_def.save_tensor_name = ops.prepend_name_scope(
          self.saver_def.save_tensor_name, self._name)
      self.saver_def.restore_op_name = ops.prepend_name_scope(
          self.saver_def.restore_op_name, self._name)

    self._check_saver_def()
    # Updates next checkpoint time.
    self._next_checkpoint_time = (
        time.time() + self.saver_def.keep_checkpoint_every_n_hours * 3600)
    self._last_checkpoints = []
    self._checkpoints_to_be_deleted = []

  def _check_saver_def(self):
    if not isinstance(self.saver_def, saver_pb2.SaverDef):
      raise ValueError("saver_def must be a saver_pb2.SaverDef: %s" %
                       self.saver_def)
    if context.in_graph_mode():
      if not self.saver_def.save_tensor_name:
        raise ValueError("saver_def must specify the save_tensor_name: %s" %
                         str(self.saver_def))
      if not self.saver_def.restore_op_name:
        raise ValueError("saver_def must specify the restore_op_name: %s" %
                         str(self.saver_def))

  def _CheckpointFilename(self, p):
    """Returns the checkpoint filename given a `(filename, time)` pair.

    Args:
      p: (filename, time) pair.

    Returns:
      Checkpoint file name.
    """
    name, _ = p
    return name

  def _MetaGraphFilename(self, checkpoint_filename, meta_graph_suffix="meta"):
    """Returns the meta graph filename.

    Args:
      checkpoint_filename: Name of the checkpoint file.
      meta_graph_suffix: Suffix for `MetaGraphDef` file. Defaults to 'meta'.

    Returns:
      MetaGraph file name.
    """
    # If the checkpoint_filename is sharded, the checkpoint_filename could
    # be of format model.ckpt-step#-?????-of-shard#. For example,
    # model.ckpt-123456-?????-of-00005, or model.ckpt-123456-00001-of-00002.
    basename = re.sub(r"-[\d\?]+-of-\d+$", "", checkpoint_filename)
    meta_graph_filename = ".".join([basename, meta_graph_suffix])
    return meta_graph_filename

  def _RecordLastCheckpoint(self, latest_save_path):
    """Manages the list of the latest checkpoints."""
    if not self.saver_def.max_to_keep:
      return
    # Remove first from list if the same name was used before.
    for p in self._last_checkpoints:
      if latest_save_path == self._CheckpointFilename(p):
        self._last_checkpoints.remove(p)
    # Append new path to list
    self._last_checkpoints.append((latest_save_path, time.time()))

    # If more than max_to_keep, remove oldest.
    if len(self._last_checkpoints) > self.saver_def.max_to_keep:
      self._checkpoints_to_be_deleted.append(self._last_checkpoints.pop(0))

  def _MaybeDeleteOldCheckpoints(self, meta_graph_suffix="meta"):
    """Deletes old checkpoints if necessary.

    `self._checkpoints_to_be_deleted` is going to contain checkpoints that are
    over `max_to_keep`.  They are going to be deleted.  If
    `keep_checkpoint_every_n_hours` was specified, keep an additional checkpoint
    every `N` hours. For example, if `N` is 0.5, an additional checkpoint is
    kept for every 0.5 hours of training; if `N` is 10, an additional
    checkpoint is kept for every 10 hours of training.

    Args:
      meta_graph_suffix: Suffix for `MetaGraphDef` file. Defaults to 'meta'.
    """
    if self._checkpoints_to_be_deleted:
      p = self._checkpoints_to_be_deleted.pop(0)
      # Do not delete the file if we keep_checkpoint_every_n_hours is set and we
      # have reached N hours of training.
      should_keep = p[1] > self._next_checkpoint_time
      if should_keep:
        self._next_checkpoint_time += (
            self.saver_def.keep_checkpoint_every_n_hours * 3600)
        return

      # Otherwise delete the files.
      try:
        checkpoint_prefix = self._CheckpointFilename(p)
        self._delete_file_if_exists(
            self._MetaGraphFilename(checkpoint_prefix, meta_graph_suffix))
        if self.saver_def.version == saver_pb2.SaverDef.V2:
          # V2 has a metadata file and some data files.
          self._delete_file_if_exists(checkpoint_prefix + ".index")
          self._delete_file_if_exists(checkpoint_prefix +
                                      ".data-?????-of-?????")
        else:
          # V1, Legacy.  Exact match on the data file.
          self._delete_file_if_exists(checkpoint_prefix)
      except Exception as e:  # pylint: disable=broad-except
        logging.warning("Ignoring: %s", str(e))

  def _delete_file_if_exists(self, filespec):
    for pathname in file_io.get_matching_files(filespec):
      file_io.delete_file(pathname)

  def as_saver_def(self):
    """Generates a `SaverDef` representation of this saver.

    Returns:
      A `SaverDef` proto.
    """
    return self.saver_def

  def to_proto(self, export_scope=None):
    """Converts this `Saver` to a `SaverDef` protocol buffer.

    Args:
      export_scope: Optional `string`. Name scope to remove.

    Returns:
      A `SaverDef` protocol buffer.
    """
    if export_scope is None:
      return self.saver_def

    if not (self.saver_def.filename_tensor_name.startswith(export_scope) and
            self.saver_def.save_tensor_name.startswith(export_scope) and
            self.saver_def.restore_op_name.startswith(export_scope)):
      return None

    saver_def = saver_pb2.SaverDef()
    saver_def.CopyFrom(self.saver_def)
    saver_def.filename_tensor_name = ops.strip_name_scope(
        saver_def.filename_tensor_name, export_scope)
    saver_def.save_tensor_name = ops.strip_name_scope(
        saver_def.save_tensor_name, export_scope)
    saver_def.restore_op_name = ops.strip_name_scope(
        saver_def.restore_op_name, export_scope)
    return saver_def

  @staticmethod
  def from_proto(saver_def, import_scope=None):
    """Returns a `Saver` object created from `saver_def`.

    Args:
      saver_def: a `SaverDef` protocol buffer.
      import_scope: Optional `string`. Name scope to use.

    Returns:
      A `Saver` built from saver_def.
    """
    return Saver(saver_def=saver_def, name=import_scope)

  @property
  def last_checkpoints(self):
    """List of not-yet-deleted checkpoint filenames.

    You can pass any of the returned values to `restore()`.

    Returns:
      A list of checkpoint filenames, sorted from oldest to newest.
    """
    return list(self._CheckpointFilename(p) for p in self._last_checkpoints)

  def set_last_checkpoints(self, last_checkpoints):
    """DEPRECATED: Use set_last_checkpoints_with_time.

    Sets the list of old checkpoint filenames.

    Args:
      last_checkpoints: A list of checkpoint filenames.

    Raises:
      AssertionError: If last_checkpoints is not a list.
    """
    assert isinstance(last_checkpoints, list)
    # We use a timestamp of +inf so that this checkpoint will never be
    # deleted.  This is both safe and backwards compatible to a previous
    # version of the code which used s[1] as the "timestamp".
    self._last_checkpoints = [(s, np.inf) for s in last_checkpoints]

  def set_last_checkpoints_with_time(self, last_checkpoints_with_time):
    """Sets the list of old checkpoint filenames and timestamps.

    Args:
      last_checkpoints_with_time: A list of tuples of checkpoint filenames and
        timestamps.

    Raises:
      AssertionError: If last_checkpoints_with_time is not a list.
    """
    assert isinstance(last_checkpoints_with_time, list)
    self._last_checkpoints = last_checkpoints_with_time

  def recover_last_checkpoints(self, checkpoint_paths):
    """Recovers the internal saver state after a crash.

    This method is useful for recovering the "self._last_checkpoints" state.

    Globs for the checkpoints pointed to by `checkpoint_paths`.  If the files
    exist, use their mtime as the checkpoint timestamp.

    Args:
      checkpoint_paths: a list of checkpoint paths.
    """
    mtimes = get_checkpoint_mtimes(checkpoint_paths)
    self.set_last_checkpoints_with_time(list(zip(checkpoint_paths, mtimes)))

  def save(self,
           sess,
           save_path,
           global_step=None,
           latest_filename=None,
           meta_graph_suffix="meta",
           write_meta_graph=True,
           write_state=True):
    """Saves variables.

    This method runs the ops added by the constructor for saving variables.
    It requires a session in which the graph was launched.  The variables to
    save must also have been initialized.

    The method returns the path of the newly created checkpoint file.  This
    path can be passed directly to a call to `restore()`.

    Args:
      sess: A Session to use to save the variables. None in eager mode.
      save_path: String.  Path to the checkpoint filename.  If the saver is
        `sharded`, this is the prefix of the sharded checkpoint filename.
      global_step: If provided the global step number is appended to
        `save_path` to create the checkpoint filename. The optional argument
        can be a `Tensor`, a `Tensor` name or an integer.
      latest_filename: Optional name for the protocol buffer file that will
        contains the list of most recent checkpoint filenames.  That file,
        kept in the same directory as the checkpoint files, is automatically
        managed by the saver to keep track of recent checkpoints.  Defaults to
        'checkpoint'.
      meta_graph_suffix: Suffix for `MetaGraphDef` file. Defaults to 'meta'.
      write_meta_graph: `Boolean` indicating whether or not to write the meta
        graph file.
      write_state: `Boolean` indicating whether or not to write the
        `CheckpointStateProto`.

    Returns:
      A string: path at which the variables were saved.  If the saver is
        sharded, this string ends with: '-?????-of-nnnnn' where 'nnnnn'
        is the number of shards created.
      If the saver is empty, returns None.

    Raises:
      TypeError: If `sess` is not a `Session`.
      ValueError: If `latest_filename` contains path components, or if it
        collides with `save_path`.
      RuntimeError: If save and restore ops weren't built.
    """
    if not self._is_built and context.in_graph_mode():
      raise RuntimeError(
          "`build()` should be called before save if defer_build==True")
    if latest_filename is None:
      latest_filename = "checkpoint"
    if self._write_version != saver_pb2.SaverDef.V2:
      logging.warning("*******************************************************")
      logging.warning("TensorFlow's V1 checkpoint format has been deprecated.")
      logging.warning("Consider switching to the more efficient V2 format:")
      logging.warning("   `tf.train.Saver(write_version=tf.train.SaverDef.V2)`")
      logging.warning("now on by default.")
      logging.warning("*******************************************************")

    if os.path.split(latest_filename)[0]:
      raise ValueError("'latest_filename' must not contain path components")

    if global_step is not None:
      if not isinstance(global_step, compat.integral_types):
        global_step = training_util.global_step(sess, global_step)
      checkpoint_file = "%s-%d" % (save_path, global_step)
      if self._pad_step_number:
        # Zero-pads the step numbers, so that they are sorted when listed.
        checkpoint_file = "%s-%s" % (save_path, "{:08d}".format(global_step))
    else:
      checkpoint_file = save_path
      if os.path.basename(
          save_path) == latest_filename and not self._sharded:
        # Guard against collision between data file and checkpoint state file.
        raise ValueError(
            "'latest_filename' collides with 'save_path': '%s' and '%s'" %
            (latest_filename, save_path))

    if (context.in_graph_mode() and
        not isinstance(sess, session.SessionInterface)):
      raise TypeError("'sess' must be a Session; %s" % sess)

    save_path_parent = os.path.dirname(save_path)
    if not self._is_empty:
      try:
        if context.in_graph_mode():
          model_checkpoint_path = sess.run(
              self.saver_def.save_tensor_name,
              {self.saver_def.filename_tensor_name: checkpoint_file})
        else:
          self._build_eager(
              checkpoint_file, build_save=True, build_restore=False)
          model_checkpoint_path = self.saver_def.save_tensor_name

        model_checkpoint_path = compat.as_str(model_checkpoint_path)
        if write_state:
          self._RecordLastCheckpoint(model_checkpoint_path)
          _update_checkpoint_state(
              save_dir=save_path_parent,
              model_checkpoint_path=model_checkpoint_path,
              all_model_checkpoint_paths=self.last_checkpoints,
              latest_filename=latest_filename,
              save_relative_paths=self._save_relative_paths)
          self._MaybeDeleteOldCheckpoints(meta_graph_suffix=meta_graph_suffix)
      except (errors.FailedPreconditionError, errors.NotFoundError) as exc:
        if not gfile.IsDirectory(save_path_parent):
          exc = ValueError(
              "Parent directory of {} doesn't exist, can't save.".format(
                  save_path))
        raise exc

    if write_meta_graph:
      meta_graph_filename = self._MetaGraphFilename(
          checkpoint_file, meta_graph_suffix=meta_graph_suffix)
      if context.in_graph_mode():
        with sess.graph.as_default():
          self.export_meta_graph(meta_graph_filename)

    if self._is_empty:
      return None
    else:
      return model_checkpoint_path

  def export_meta_graph(self,
                        filename=None,
                        collection_list=None,
                        as_text=False,
                        export_scope=None,
                        clear_devices=False,
                        clear_extraneous_savers=False):
    """Writes `MetaGraphDef` to save_path/filename.

    Args:
      filename: Optional meta_graph filename including the path.
      collection_list: List of string keys to collect.
      as_text: If `True`, writes the meta_graph as an ASCII proto.
      export_scope: Optional `string`. Name scope to remove.
      clear_devices: Whether or not to clear the device field for an `Operation`
        or `Tensor` during export.
      clear_extraneous_savers: Remove any Saver-related information from the
        graph (both Save/Restore ops and SaverDefs) that are not associated
        with this Saver.

    Returns:
      A `MetaGraphDef` proto.
    """
    return export_meta_graph(
        filename=filename,
        graph_def=ops.get_default_graph().as_graph_def(add_shapes=True),
        saver_def=self.saver_def,
        collection_list=collection_list,
        as_text=as_text,
        export_scope=export_scope,
        clear_devices=clear_devices,
        clear_extraneous_savers=clear_extraneous_savers)

  def restore(self, sess, save_path):
    """Restores previously saved variables.

    This method runs the ops added by the constructor for restoring variables.
    It requires a session in which the graph was launched.  The variables to
    restore do not have to have been initialized, as restoring is itself a way
    to initialize variables.

    The `save_path` argument is typically a value previously returned from a
    `save()` call, or a call to `latest_checkpoint()`.

    Args:
      sess: A `Session` to use to restore the parameters. None in eager mode.
      save_path: Path where parameters were previously saved.

    Raises:
      ValueError: If save_path is None.
    """
    if self._is_empty:
      return
    if save_path is None:
      raise ValueError("Can't load save_path when it is None.")
    logging.info("Restoring parameters from %s", save_path)
    if context.in_graph_mode():
      sess.run(self.saver_def.restore_op_name,
               {self.saver_def.filename_tensor_name: save_path})
    else:
      self._build_eager(save_path, build_save=False, build_restore=True)

  @staticmethod
  def _add_collection_def(meta_graph_def, key, export_scope=None):
    """Adds a collection to MetaGraphDef protocol buffer.

    Args:
      meta_graph_def: MetaGraphDef protocol buffer.
      key: One of the GraphKeys or user-defined string.
      export_scope: Optional `string`. Name scope to remove.
    """
    meta_graph.add_collection_def(meta_graph_def, key,
                                  export_scope=export_scope)


def _prefix_to_checkpoint_path(prefix, format_version):
  """Returns the pathname of a checkpoint file, given the checkpoint prefix.

  For V1 checkpoint, simply returns the prefix itself (the data file).  For V2,
  returns the pathname to the index file.

  Args:
    prefix: a string, the prefix of a checkpoint.
    format_version: the checkpoint format version that corresponds to the
      prefix.
  Returns:
    The pathname of a checkpoint file, taking into account the checkpoint
      format version.
  """
  if format_version == saver_pb2.SaverDef.V2:
    return prefix + ".index"  # The index file identifies a checkpoint.
  return prefix  # Just the data file.


def latest_checkpoint(checkpoint_dir, latest_filename=None):
  """Finds the filename of latest saved checkpoint file.

  Args:
    checkpoint_dir: Directory where the variables were saved.
    latest_filename: Optional name for the protocol buffer file that
      contains the list of most recent checkpoint filenames.
      See the corresponding argument to `Saver.save()`.

  Returns:
    The full path to the latest checkpoint or `None` if no checkpoint was found.
  """
  # Pick the latest checkpoint based on checkpoint state.
  ckpt = get_checkpoint_state(checkpoint_dir, latest_filename)
  if ckpt and ckpt.model_checkpoint_path:
    # Look for either a V2 path or a V1 path, with priority for V2.
    v2_path = _prefix_to_checkpoint_path(ckpt.model_checkpoint_path,
                                         saver_pb2.SaverDef.V2)
    v1_path = _prefix_to_checkpoint_path(ckpt.model_checkpoint_path,
                                         saver_pb2.SaverDef.V1)
    if file_io.get_matching_files(v2_path) or file_io.get_matching_files(
        v1_path):
      return ckpt.model_checkpoint_path
    else:
      logging.error("Couldn't match files for checkpoint %s",
                    ckpt.model_checkpoint_path)
  return None


def import_meta_graph(meta_graph_or_file, clear_devices=False,
                      import_scope=None, **kwargs):
  """Recreates a Graph saved in a `MetaGraphDef` proto.

  This function takes a `MetaGraphDef` protocol buffer as input. If
  the argument is a file containing a `MetaGraphDef` protocol buffer ,
  it constructs a protocol buffer from the file content. The function
  then adds all the nodes from the `graph_def` field to the
  current graph, recreates all the collections, and returns a saver
  constructed from the `saver_def` field.

  In combination with `export_meta_graph()`, this function can be used to

  * Serialize a graph along with other Python objects such as `QueueRunner`,
    `Variable` into a `MetaGraphDef`.

  * Restart training from a saved graph and checkpoints.

  * Run inference from a saved graph and checkpoints.

  ```Python
  ...
  # Create a saver.
  saver = tf.train.Saver(...variables...)
  # Remember the training_op we want to run by adding it to a collection.
  tf.add_to_collection('train_op', train_op)
  sess = tf.Session()
  for step in xrange(1000000):
      sess.run(train_op)
      if step % 1000 == 0:
          # Saves checkpoint, which by default also exports a meta_graph
          # named 'my-model-global_step.meta'.
          saver.save(sess, 'my-model', global_step=step)
  ```

  Later we can continue training from this saved `meta_graph` without building
  the model from scratch.

  ```Python
  with tf.Session() as sess:
    new_saver = tf.train.import_meta_graph('my-save-dir/my-model-10000.meta')
    new_saver.restore(sess, 'my-save-dir/my-model-10000')
    # tf.get_collection() returns a list. In this example we only want the
    # first one.
    train_op = tf.get_collection('train_op')[0]
    for step in xrange(1000000):
      sess.run(train_op)
  ```

  NOTE: Restarting training from saved `meta_graph` only works if the
  device assignments have not changed.

  Args:
    meta_graph_or_file: `MetaGraphDef` protocol buffer or filename (including
      the path) containing a `MetaGraphDef`.
    clear_devices: Whether or not to clear the device field for an `Operation`
      or `Tensor` during import.
    import_scope: Optional `string`. Name scope to add. Only used when
      initializing from protocol buffer.
    **kwargs: Optional keyed arguments.

  Returns:
    A saver constructed from `saver_def` in `MetaGraphDef` or None.

    A None value is returned if no variables exist in the `MetaGraphDef`
    (i.e., there are no variables to restore).
  """  # pylint: disable=g-doc-exception
  if context.in_eager_mode():
    raise ValueError("Exporting/importing meta graphs is not supported when "
                     "eager execution is enabled. No graph exists when eager "
                     "execution is enabled.")
  if not isinstance(meta_graph_or_file, meta_graph_pb2.MetaGraphDef):
    meta_graph_def = meta_graph.read_meta_graph_file(meta_graph_or_file)
  else:
    meta_graph_def = meta_graph_or_file

  meta_graph.import_scoped_meta_graph(meta_graph_def,
                                      clear_devices=clear_devices,
                                      import_scope=import_scope,
                                      **kwargs)
  if meta_graph_def.HasField("saver_def"):
    return Saver(saver_def=meta_graph_def.saver_def, name=import_scope)
  else:
    if variables._all_saveable_objects():  # pylint: disable=protected-access
      # Return the default saver instance for all graph variables.
      return Saver()
    else:
      # If no graph variables exist, then a Saver cannot be constructed.
      logging.info("Saver not created because there are no variables in the"
                   " graph to restore")
      return None


def export_meta_graph(filename=None,
                      meta_info_def=None,
                      graph_def=None,
                      saver_def=None,
                      collection_list=None,
                      as_text=False,
                      graph=None,
                      export_scope=None,
                      clear_devices=False,
                      clear_extraneous_savers=False,
                      **kwargs):
  """Returns `MetaGraphDef` proto. Optionally writes it to filename.

  This function exports the graph, saver, and collection objects into
  `MetaGraphDef` protocol buffer with the intention of it being imported
  at a later time or location to restart training, run inference, or be
  a subgraph.

  Args:
    filename: Optional filename including the path for writing the
      generated `MetaGraphDef` protocol buffer.
    meta_info_def: `MetaInfoDef` protocol buffer.
    graph_def: `GraphDef` protocol buffer.
    saver_def: `SaverDef` protocol buffer.
    collection_list: List of string keys to collect.
    as_text: If `True`, writes the `MetaGraphDef` as an ASCII proto.
    graph: The `Graph` to import into. If `None`, use the default graph.
    export_scope: Optional `string`. Name scope under which to extract
      the subgraph. The scope name will be striped from the node definitions
      for easy import later into new name scopes. If `None`, the whole graph
      is exported. graph_def and export_scope cannot both be specified.
    clear_devices: Whether or not to clear the device field for an `Operation`
      or `Tensor` during export.
    clear_extraneous_savers: Remove any Saver-related information from the
        graph (both Save/Restore ops and SaverDefs) that are not associated
        with the provided SaverDef.
    **kwargs: Optional keyed arguments.

  Returns:
    A `MetaGraphDef` proto.

  Raises:
    ValueError: When the `GraphDef` is larger than 2GB.
  """
  if context.in_eager_mode():
    raise ValueError("Exporting/importing meta graphs is not supported when "
                     "eager execution is enabled. No graph exists when eager "
                     "execution is enabled.")
  meta_graph_def, _ = meta_graph.export_scoped_meta_graph(
      filename=filename,
      meta_info_def=meta_info_def,
      graph_def=graph_def,
      saver_def=saver_def,
      collection_list=collection_list,
      as_text=as_text,
      graph=graph,
      export_scope=export_scope,
      clear_devices=clear_devices,
      clear_extraneous_savers=clear_extraneous_savers,
      **kwargs)
  return meta_graph_def


def checkpoint_exists(checkpoint_prefix):
  """Checks whether a V1 or V2 checkpoint exists with the specified prefix.

  This is the recommended way to check if a checkpoint exists, since it takes
  into account the naming difference between V1 and V2 formats.

  Args:
    checkpoint_prefix: the prefix of a V1 or V2 checkpoint, with V2 taking
      priority.  Typically the result of `Saver.save()` or that of
      `tf.train.latest_checkpoint()`, regardless of sharded/non-sharded or
      V1/V2.
  Returns:
    A bool, true iff a checkpoint referred to by `checkpoint_prefix` exists.
  """
  pathname = _prefix_to_checkpoint_path(checkpoint_prefix,
                                        saver_pb2.SaverDef.V2)
  if file_io.get_matching_files(pathname):
    return True
  elif file_io.get_matching_files(checkpoint_prefix):
    return True
  else:
    return False


def get_checkpoint_mtimes(checkpoint_prefixes):
  """Returns the mtimes (modification timestamps) of the checkpoints.

  Globs for the checkpoints pointed to by `checkpoint_prefixes`.  If the files
  exist, collect their mtime.  Both V2 and V1 checkpoints are considered, in
  that priority.

  This is the recommended way to get the mtimes, since it takes into account
  the naming difference between V1 and V2 formats.

  Args:
    checkpoint_prefixes: a list of checkpoint paths, typically the results of
      `Saver.save()` or those of `tf.train.latest_checkpoint()`, regardless of
      sharded/non-sharded or V1/V2.
  Returns:
    A list of mtimes (in microseconds) of the found checkpoints.
  """
  mtimes = []

  def match_maybe_append(pathname):
    fnames = file_io.get_matching_files(pathname)
    if fnames:
      mtimes.append(file_io.stat(fnames[0]).mtime_nsec / 1e9)
      return True
    return False

  for checkpoint_prefix in checkpoint_prefixes:
    # Tries V2's metadata file first.
    pathname = _prefix_to_checkpoint_path(checkpoint_prefix,
                                          saver_pb2.SaverDef.V2)
    if match_maybe_append(pathname):
      continue
    # Otherwise, tries V1, where the prefix is the complete pathname.
    match_maybe_append(checkpoint_prefix)

  return mtimes


ops.register_proto_function(
    ops.GraphKeys.SAVERS,
    proto_type=saver_pb2.SaverDef,
    to_proto=Saver.to_proto,
    from_proto=Saver.from_proto)
