# 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.
# ==============================================================================
"""Control Flow Operations.

See the [autograph](https://www.tensorflow.org/guide/autograph) guide.
"""
# pylint: disable=g-bad-name
import abc

from tensorflow.core.framework import attr_value_pb2
from tensorflow.core.protobuf import control_flow_pb2
from tensorflow.python.eager import context
from tensorflow.python.framework import composite_tensor
from tensorflow.python.framework import constant_op
from tensorflow.python.framework import dtypes
from tensorflow.python.framework import indexed_slices
from tensorflow.python.framework import ops
from tensorflow.python.framework import tensor as tensor_lib
from tensorflow.python.framework import tensor_shape
from tensorflow.python.framework import tensor_util
from tensorflow.python.framework import type_spec
from tensorflow.python.ops import array_ops
from tensorflow.python.ops import control_flow_util as util
from tensorflow.python.ops import gen_array_ops
from tensorflow.python.ops import gen_control_flow_ops
from tensorflow.python.ops import math_ops
from tensorflow.python.ops import tensor_array_ops
# go/tf-wildcard-import
# pylint: disable=wildcard-import,undefined-variable
from tensorflow.python.ops.gen_control_flow_ops import *
# pylint: enable=wildcard-import
from tensorflow.python.util import compat
from tensorflow.python.util import dispatch
from tensorflow.python.util import nest
from tensorflow.python.util import variable_utils
from tensorflow.python.util.tf_export import tf_export


# We override the 'tuple' for a control flow op, so we keep python's
# existing 'tuple' for later use in this module.
_basetuple = tuple


# pylint: disable=protected-access


def _Identity(tensor, name=None):
  """Return a tensor with the same shape and contents as the input tensor.

  Args:
    tensor: A Tensor.
    name: A name for this operation (optional).

  Returns:
    A Tensor with the same type and value as the input Tensor.
  """
  tensor = ops.internal_convert_to_tensor_or_composite(tensor, as_ref=True)
  # TODO(b/246438937): Remove this when we expand ResourceVariables into
  # dt_resource tensors.
  tensor = variable_utils.convert_variables_to_tensors(tensor)
  if isinstance(tensor, tensor_lib.Tensor):
    if tensor.dtype._is_ref_dtype:  # pylint: disable=protected-access
      return gen_array_ops.ref_identity(tensor, name=name)
    else:
      return array_ops.identity(tensor, name=name)
  elif isinstance(tensor, composite_tensor.CompositeTensor):
    return nest.map_structure(_Identity, tensor, expand_composites=True)
  else:
    raise TypeError("'tensor' must be a Tensor or CompositeTensor. "
                    f"Received: {type(tensor)}.")


def _NextIteration(tensor, name=None):
  tensor = ops.internal_convert_to_tensor_or_composite(tensor, as_ref=True)
  if isinstance(tensor, tensor_lib.Tensor):
    if tensor.dtype._is_ref_dtype:  # pylint: disable=protected-access
      return ref_next_iteration(tensor, name=name)
    else:
      return next_iteration(tensor, name=name)
  elif isinstance(tensor, composite_tensor.CompositeTensor):
    return nest.map_structure(_NextIteration, tensor, expand_composites=True)
  else:
    raise TypeError("'tensor' must be a Tensor or CompositeTensor. "
                    f"Received: {type(tensor)}.")


def _Enter(tensor,
           frame_name,
           is_constant=False,
           parallel_iterations=10,
           use_ref=True,
           use_input_shape=True,
           name=None):
  """Creates or finds a child frame, and makes `tensor` available to it.

  The unique `frame_name` is used by the `Executor` to identify frames. If
  `is_constant` is true, `tensor` is a constant in the child frame; otherwise
  it may be changed in the child frame. At most `parallel_iterations`
  iterations are run in parallel in the child frame.

  Args:
    tensor: The tensor to be made available to the child frame.
    frame_name: The name of the child frame.
    is_constant: If true, the output is constant within the child frame.
    parallel_iterations: The number of iterations allowed to run in parallel.
    use_ref: If true, use ref_enter if tensor is of ref type.
    use_input_shape: If true, set the result's shape based on tensor's shape.
    name: A name for this operation (optional).

  Returns:
    The same tensor as `tensor`.

  Raises:
    ValueError: If any tensor in `tensor` has a less specific shape
      than its corresponding shape in `shape_invariant`.
  """
  tensor = ops.internal_convert_to_tensor_or_composite(tensor, as_ref=True)
  if isinstance(tensor, tensor_lib.Tensor):
    if tensor.dtype._is_ref_dtype and use_ref:  # pylint: disable=protected-access
      result = gen_control_flow_ops.ref_enter(
          tensor, frame_name, is_constant, parallel_iterations, name=name)
    else:
      result = gen_control_flow_ops.enter(
          tensor, frame_name, is_constant, parallel_iterations, name=name)
    if use_input_shape:
      result.set_shape(tensor.get_shape())
    return result
  elif isinstance(tensor, composite_tensor.CompositeTensor):

    def enter_component(t):
      return _Enter(t, frame_name, is_constant, parallel_iterations, use_ref,
                    use_input_shape)

    return nest.map_structure(enter_component, tensor, expand_composites=True)
  else:
    raise TypeError("'tensor' must be a Tensor or CompositeTensor. "
                    f"Received: {type(tensor)}.")


def exit(tensor, name=None):  # pylint: disable=redefined-builtin
  """Exits the current frame to its parent frame.

  Exit makes its input `tensor` available to the parent frame.

  Args:
    tensor: The tensor to be made available to the parent frame.
    name: A name for this operation (optional).

  Returns:
    The same tensor as `tensor`.
  """
  tensor = ops.internal_convert_to_tensor_or_composite(tensor, as_ref=True)
  if isinstance(tensor, tensor_lib.Tensor):
    if tensor.dtype._is_ref_dtype:  # pylint: disable=protected-access
      return gen_control_flow_ops.ref_exit(tensor, name)
    else:
      return gen_control_flow_ops._exit(tensor, name)
  elif isinstance(tensor, composite_tensor.CompositeTensor):
    return nest.map_structure(exit, tensor, expand_composites=True)
  else:
    raise TypeError("'tensor' must be a Tensor or CompositeTensor. "
                    f"Received: {type(tensor)}.")


def switch(data, pred, dtype=None, name=None):
  """Forwards `data` to an output determined by `pred`.

  If `pred` is false, the `data` input is forwarded to the first output.
  Otherwise, the data goes to the second output.

  This op handles `Tensor`s and `IndexedSlices`.

  Args:
    data: The tensor to be forwarded to the appropriate output.
    pred: A scalar that specifies which output port will receive data.
    dtype: Optional element type for the returned tensor. If missing, the type
      is inferred from the type of `value`.
    name: A name for this operation (optional).

  Returns:
    `(output_false, output_true)`: If `pred` is true, data will be forwarded
    to `output_true`, otherwise it goes to `output_false`.
  """
  with ops.name_scope(name, "Switch", [data, pred]) as name:
    data = ops.internal_convert_to_tensor_or_composite(
        data, dtype=dtype, name="data", as_ref=True)
    pred = ops.convert_to_tensor(pred, name="pred")
    if isinstance(data, tensor_lib.Tensor):
      return gen_control_flow_ops.switch(data, pred, name=name)
    else:
      if not isinstance(data, composite_tensor.CompositeTensor):
        raise TypeError(
            "'data' must be a Tensor or CompositeTensor. "
            f"Received: {type(data)}.")
      tensors = nest.flatten(data, expand_composites=True)
      mapped = [gen_control_flow_ops.switch(tensor, pred) for tensor in tensors]
      mapped_f, mapped_t = zip(*mapped)
      return (nest.pack_sequence_as(data, mapped_f, expand_composites=True),
              nest.pack_sequence_as(data, mapped_t, expand_composites=True))


def _SwitchRefOrTensor(data, pred, name="Switch"):
  """Forwards `data` to an output determined by `pred`.

  If `pred` is false, the `data` input is forwarded to the first output.
  Otherwise, the data goes to the second output.

  This op handles `Tensor`s and `IndexedSlices`.

  Args:
    data: The tensor to be forwarded to the appropriate output.
    pred: A scalar that specifies which output port will receive data.
    name: A name for this operation (optional).

  Returns:
    `(output_false, output_true)`: If `pred` is true, data will be forwarded to
    `output_true`, otherwise it goes to `output_false`.

  Raises:
    TypeError: if data is not a Tensor or IndexedSlices
  """
  data = ops.convert_to_tensor_or_composite(data, name="data")
  # NOTE(vrv): ops.colocate_with(data, ignore_existing=True) below
  # addresses the following scenario.
  #
  # Assume you execute Optimizer.apply_gradients() in a branch of a cond().
  #
  # 1. The update op is created inside a `with ops.colocate(var):` block
  #
  # 2. Some tensor `data` is captured and a switch is created in a
  #    `with ops.colocate_with(data):` block.
  #
  # with ops.colocate_with(var):
  #  with ops.colocate_with(data):
  #    op = ...
  #
  # var and data may be pinned to different devices, so we want to ops
  # created within ops.colocate_with(data) to ignore the existing stack.
  with ops.colocate_with(data, ignore_existing=True):
    if isinstance(data, tensor_lib.Tensor):
      if data.dtype._is_ref_dtype:  # pylint: disable=protected-access
        return ref_switch(data, pred, name=name)
    return switch(data, pred, name=name)


def merge(inputs, name=None):
  """Returns the value of an available element of `inputs`.

  This op tests each of the tensors in `inputs` in turn to determine if any of
  them is available. If it finds an available tensor, it returns it and its
  index in `inputs`.

  It is an error if more than one tensor in `inputs` is available. If no tensor
  in `inputs` is available, the returned tensor and index are not set.

  This op handles both `Tensor`s and `IndexedSlices`. If inputs has a mix of
  `Tensor`s and `IndexedSlices`, all inputs are converted to IndexedSlices
  before merging.

  Args:
    inputs: The input tensors, at most one of which is available.
    name: A name for this operation (optional).

  Returns:
    A tuple containing the chosen input tensor and its index in `inputs`.

  Raises:
    ValueError: If any of the inputs is None, or inputs are IndexedSlices and
      some but not all have a dense_shape property.
  """
  if any(inp is None for inp in inputs):
    raise ValueError("At least one of the merge inputs is None: %s" % inputs)
  with ops.name_scope(name, "Merge", inputs) as name:
    inputs = [
        ops.internal_convert_to_tensor_or_composite(inp, as_ref=True)
        for inp in inputs
    ]
    if all(isinstance(v, tensor_lib.Tensor) for v in inputs):
      if all(v.dtype._is_ref_dtype for v in inputs):  # pylint: disable=protected-access
        return gen_control_flow_ops.ref_merge(inputs, name)
      else:
        return gen_control_flow_ops.merge(inputs, name)
    else:
      # If there is a mix of tensors and indexed slices, then convert the
      # tensors to indexed slices.
      if all(
          isinstance(v, (indexed_slices.IndexedSlices, tensor_lib.Tensor))
          for v in inputs):
        inputs = math_ops._as_indexed_slices_list(inputs, optimize=False)

      for v in inputs:
        if not isinstance(v, composite_tensor.CompositeTensor):
          raise TypeError("Type %s not supported" % type(v))

      for v in inputs[1:]:
        nest.assert_same_structure(inputs[0], v, expand_composites=True)

      flat_inputs = [nest.flatten(v, expand_composites=True) for v in inputs]
      merged_results = [
          gen_control_flow_ops.merge(component)
          for component in zip(*flat_inputs)
      ]
      flat_merged = [tensor for (tensor, _) in merged_results]
      chosen_index = merged_results[0][1]
      merged_inputs = nest.pack_sequence_as(
          inputs[0], flat_merged, expand_composites=True)
      return (merged_inputs, chosen_index)


def _convert_tensorarray_to_flow(tensor_or_tensor_array):
  if isinstance(tensor_or_tensor_array, tensor_array_ops.TensorArray):
    return tensor_or_tensor_array.flow
  else:
    return tensor_or_tensor_array


def _convert_flow_to_tensorarray(tensor_or_tensor_array, tensor_or_flow):
  if isinstance(tensor_or_tensor_array, tensor_array_ops.TensorArray):
    return tensor_array_ops.build_ta_with_new_flow(tensor_or_tensor_array,
                                                   tensor_or_flow)
  else:
    return tensor_or_flow


def _convert_to_tensor_or_composite_or_tensorarray(var):
  if isinstance(var, tensor_array_ops.TensorArray):
    return var
  return ops.convert_to_tensor_or_composite(var)


# TODO(xjun): replace this with is_subtype_of after it is landed.
def _ShapeLessThanOrEqual(shape1, shape2):
  if shape2.dims is None:
    return True
  if shape1.ndims != shape2.ndims:
    return False
  for dim1, dim2 in zip(shape1.dims, shape2.dims):
    if dim2.value is not None and dim1.value != dim2.value:
      return False
  return True


def _shape_invariant_to_type_spec(var, shape=None):
  """Converts a shape invariant to a TypeSpec.

  If `var` is a TensorArray, it will first be converted to its flow.

  Args:
    var: The tensor, tensor array or composite tensor whose shape is described
      by the shape invariant.
    shape: A `TypeSpec` or `TensorShape`.  If `shape` is already a `TypeSpec`,
      then it is simply returned as-is.

  Returns:
    A `TypeSpec` for `var`, consistent with the given shape.

  Raises:
    TypeError: If `shape` is a TypeSpec and not compatible with `var`.
    TypeError: If `shape` is not None, a TypeSpec, or a TensorShape.
    TypeError: If `shape` is a TensorShape, `var` is a CompositeTensor, and
      `var` doesn't implement the `_shape_invariant_to_type_spec` method.
  """
  var = _convert_tensorarray_to_flow(var)
  if shape is None:
    return type_spec.type_spec_from_value(var)
  elif isinstance(shape, type_spec.TypeSpec):
    if not shape.is_compatible_with(var):
      raise TypeError("TypeSpec %r is not compatible with %r" % (shape, var))
    return shape
  elif not isinstance(shape, tensor_shape.TensorShape):
    raise TypeError(
        "'shape' must be one of TypeSpec, TensorShape or None. "
        f"Received: {type(shape)}")

  if isinstance(var, tensor_lib.Tensor):
    return tensor_lib.TensorSpec(shape, var.dtype)
  else:
    try:
      return var._shape_invariant_to_type_spec(shape)  # pylint: disable=protected-access
    except NotImplementedError as e:
      raise TypeError(
          f"To describe or constrain a {type(var).__name__}, use a "
          f"{type(var._type_spec).__name__} instead of a TensorShape.") from e  # pylint: disable=protected-access


def _EnforceShapeInvariant(merge_var, next_var):
  """Check if the shapes of the loops variables are invariants.

  Args:
    merge_var: The tensor representing the initial values of the loop
      variables.
    next_var: The tensor representing the values of the loop variables
      after one loop iteration.

  Raises:
    ValueError: If any tensor in `merge_var` has a more specific shape than
      its corresponding tensor in `next_var`.
  """
  if isinstance(merge_var, tensor_lib.Tensor):
    m_shape = merge_var.get_shape()
    n_shape = next_var.get_shape()
    if not _ShapeLessThanOrEqual(n_shape, m_shape):
      enter = merge_var.op.inputs[0].op
      assert util.IsLoopEnter(enter)
      input_t = enter.inputs[0]
      raise ValueError(
          "Input tensor '%s' enters the loop with shape %s, but has shape %s "
          "after one iteration. To allow the shape to vary across iterations, "
          "use the `shape_invariants` argument of tf.while_loop to specify a "
          "less-specific shape." % (input_t.name, input_t.shape, n_shape))
  else:
    raise TypeError("'merge_var' must be a Tensor. "
                    f"Received: {type(merge_var)}.")


def _AddNextAndBackEdge(m, v, enforce_shape_invariant=True):
  """Add NextIteration and back edge from v to m."""
  if isinstance(m, tensor_lib.Tensor):
    v = ops.convert_to_tensor(v)
    v = _NextIteration(v)
    if enforce_shape_invariant:
      # Make sure the shapes of loop outputs are correct. We do this before
      # calling _update_input, which will raise a less-helpful error message if
      # the types don't match.
      # TODO(skyewm): call this for other cases below (needs testing)
      _EnforceShapeInvariant(m, v)
    m.op._update_input(1, v)  # pylint: disable=protected-access
  elif isinstance(m, composite_tensor.CompositeTensor):
    # pylint: disable=protected-access
    def update_component(m_component, v_component):
      m_component.op._update_input(1, v_component)

    if isinstance(m, indexed_slices.IndexedSlices):
      v = math_ops._as_indexed_slices(v, optimize=False)
    # pylint: enable=protected-access
    v = _NextIteration(v)
    return nest.map_structure(update_component, m, v, expand_composites=True)
  else:
    raise TypeError("'m' must be a Tensor or CompositeTensor. "
                    f"Received: {type(m)}.")
  return v


class ControlFlowContext(metaclass=abc.ABCMeta):
  """The base class for control flow context.

  The usage pattern is a sequence of (Enter, Exit) followed by a final
  ExitResult.

  We maintain the following state for control flow contexts during graph
  construction:
   1. graph has _control_flow_context: the current context used to
      construct new nodes. Changed by ctxt.Enter() and ctxt.Exit()
   2. op has _control_flow_context: the context to which the op belongs.
      Set at the time the op is created. Immutable.
   3. A ControlFlowContext has _outer_context: the context in which this
      context is created. Set at the time a context is created. Immutable.
   4. A ControlFlowContext has _context_stack.
      Pushed and popped by ctxt.Enter() and ctxt.Exit()
  """

  def __init__(self, values_def=None, import_scope=None):
    self._nested_contexts = []
    self._outer_context = ops.get_default_graph()._get_control_flow_context()
    if self._outer_context:
      self._outer_context._nested_contexts.append(self)  # pylint: disable=protected-access
    self._context_stack = []
    if values_def:
      self._init_values_from_proto(values_def, import_scope=import_scope)
    else:
      # The names of tensors that have been already seen in this context.
      self._values = set()
      # The keys are the names of tensors referenced by but external to this
      # context. Each value is the Tensor that should be used by this context to
      # access the key value (e.g. a switch output guarding a cond input value).
      self._external_values = {}

  def _init_values_from_proto(self, values_def, import_scope=None):
    """Initializes values and external_values from `ValuesDef` protocol buffer.

    Args:
      values_def: `ValuesDef` protocol buffer.
      import_scope: Optional `string`. Name scope to add.
    """
    assert isinstance(values_def, control_flow_pb2.ValuesDef)
    self._values = set(
        ops.prepend_name_scope(value, import_scope)
        for value in values_def.values)
    g = ops.get_default_graph()
    self._external_values = {}
    for k, v in values_def.external_values.items():
      k = ops.prepend_name_scope(k, import_scope)
      self._external_values[k] = g.as_graph_element(
          ops.prepend_name_scope(v, import_scope))
    op_names = set([
        op.split(":")[0]
        for op in self._values - set(self._external_values.keys())
    ])
    for op in op_names:
      # pylint: disable=protected-access
      g.as_graph_element(op)._set_control_flow_context(self)
      # pylint: enable=protected-access

  @property
  def name(self):
    return self._name

  @property
  def outer_context(self):
    """Return the context containing this context."""
    return self._outer_context

  @property
  def grad_state(self):
    raise NotImplementedError("Abstract method")

  @property
  def back_prop(self):
    raise NotImplementedError("Abstract method")

  @abc.abstractmethod
  def to_control_flow_context_def(self, context_def, export_scope=None):
    """Serializes this into `context_def`.

    Args:
      context_def: a `ControlFlowContextDef` protocol buffer.
      export_scope: Optional `string`. Name scope to remove.
    """
    raise NotImplementedError("Abstract method")

  def _to_values_def(self, export_scope=None):
    """Converts the values to a `ValuesDef` protocol buffer.

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

    Returns:
      A `ValuesDef` protocol buffer.
    """
    values_def = control_flow_pb2.ValuesDef()
    values_def.values.extend(
        [ops.strip_name_scope(v, export_scope) for v in sorted(self._values)])
    for k, v in self._external_values.items():
      k = ops.strip_name_scope(k, export_scope)
      values_def.external_values[k] = ops.strip_name_scope(v.name, export_scope)
    return values_def

  def AddName(self, name):
    self._values.add(name)

  # pylint: disable=protected-access
  def Enter(self):
    """Enter this control flow context."""
    graph = ops.get_default_graph()
    self._context_stack.append(graph._get_control_flow_context())
    graph._set_control_flow_context(self)

  def Exit(self):
    """Exit this control flow context."""
    graph = ops.get_default_graph()
    last_context = self._context_stack.pop()
    graph._set_control_flow_context(last_context)

  def EnterGradientColocation(self, op: ops.Operation, gradient_uid):
    """Start building a gradient colocated with an op."""
    if self._outer_context:
      self._outer_context.EnterGradientColocation(op, gradient_uid)

  def ExitGradientColocation(self, op: ops.Operation, gradient_uid):
    """Start building a gradient colocated with an op."""
    if self._outer_context:
      self._outer_context.ExitGradientColocation(op, gradient_uid)

  def ExitResult(self, result):
    """Make a list of tensors available in the outer context."""
    if self._outer_context:
      def fn(x):
        self._outer_context.AddName(x.name)
        return x
      nest.map_structure(fn, result, expand_composites=True)

  def GetWhileContext(self):
    """Return the while context containing this context."""
    if self._outer_context:
      return self._outer_context.GetWhileContext()
    return None

  def _RemoveExternalControlEdges(self, op: ops.Operation):
    """Remove any external control dependency on this op."""
    while_ctxt = self.GetWhileContext()
    # A control input of `op` is internal if it is in the same while
    # loop context as the enclosing while loop context of self.
    if while_ctxt is None:
      internal_control_inputs, external_control_inputs = op.control_inputs, []
    else:
      internal_control_inputs, external_control_inputs = [], []
      for x in op.control_inputs:
        ctxt = util.GetOutputContext(x)
        if ctxt is not None and ctxt.GetWhileContext() == while_ctxt:
          internal_control_inputs.append(x)
        else:
          external_control_inputs.append(x)
    if len(internal_control_inputs) != len(op.control_inputs):
      # TODO(mdan): perhaps there should be a replace_control_inputs()
      op._remove_all_control_inputs()
      op._add_control_inputs(internal_control_inputs)
    return internal_control_inputs, external_control_inputs

  # pylint: enable=protected-access

  def AddInnerOp(self, op: ops.Operation):
    """Notifies a scope about an operator added to an inner scope."""
    if self._outer_context:
      self._outer_context.AddInnerOp(op)

  def GetControlPivot(self):
    """Returns the pivot node for this context, or None."""
    return None

  def IsWhileContext(self):
    return False

  def IsCondContext(self):
    return False

  def IsXLAContext(self):
    return False

  def __str__(self):
    return self.name


class CondContext(ControlFlowContext):
  """The context for the conditional construct."""

  def __init__(self,
               pred=None,
               pivot=None,
               branch=None,
               name="cond_text",
               context_def=None,
               import_scope=None):
    """Creates a `CondContext`.

    Args:
      pred: The `boolean` tensor for the conditional predicate.
      pivot: The predicate tensor in this branch.
      branch: 0 or 1 representing this branch.
      name: Name of the `CondContext` python object.
      context_def: Optional `ContextDef` protocol buffer to initialize the
        `CondContext` object from.
      import_scope: Optional `string`. Name scope to add. Only used when
        initialing from protocol buffer.
    """
    self._name = ops.get_default_graph().unique_name(name)

    if context_def:
      self._init_from_proto(context_def, import_scope=import_scope)
    else:
      # Initializes the default fields.
      ControlFlowContext.__init__(self)
      self._pred = pred  # The boolean tensor for the cond predicate
      self._pivot = pivot  # The predicate tensor in this branch
      self._branch = branch  # 0 or 1 representing this branch

      # Values considered to have been already seen in this context. pred is not
      # included in this context.
      self._values.add(pred.name)
      self._external_values[pred.name] = pred
      self._values.add(pivot.name)
      pivot.op._set_control_flow_context(self)  # pylint: disable=protected-access

  def _init_from_proto(self, context_def, import_scope=None):
    """Creates a new `CondContext` from protocol buffer.

    Args:
      context_def: `CondContextDef` protocol buffer.
      import_scope: Optional `string`. Name scope to add.
    """
    assert isinstance(context_def, control_flow_pb2.CondContextDef)
    # Create from context_def.
    g = ops.get_default_graph()
    self._name = ops.prepend_name_scope(context_def.context_name, import_scope)
    self._pred = g.as_graph_element(
        ops.prepend_name_scope(context_def.pred_name, import_scope))
    self._pivot = g.as_graph_element(
        ops.prepend_name_scope(context_def.pivot_name, import_scope))
    self._branch = context_def.branch
    super(CondContext, self).__init__(
        values_def=context_def.values_def, import_scope=import_scope)

  @property
  def pred(self):
    return self._pred

  @property
  def pivot(self):
    return self._pivot

  @property
  def branch(self):
    return self._branch

  @property
  def grad_state(self):
    if self.GetWhileContext():
      return self.GetWhileContext().grad_state
    return None

  @property
  def back_prop(self):
    if self.GetWhileContext():
      return self.GetWhileContext().back_prop
    return False

  def GetControlPivot(self):
    return self._pivot

  def to_proto(self, export_scope=None):
    """Converts a `CondContext` to a `CondContextDef` protocol buffer.

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

    Returns:
      A `CondContextDef` protocol buffer.
    """
    if (export_scope is None or self.name.startswith(export_scope)):
      context_def = control_flow_pb2.CondContextDef()
      context_def.context_name = ops.strip_name_scope(self.name, export_scope)
      context_def.pred_name = ops.strip_name_scope(self._pred.name,
                                                   export_scope)
      context_def.pivot_name = ops.strip_name_scope(self._pivot.name,
                                                    export_scope)
      context_def.branch = self._branch
      context_def.values_def.MergeFrom(
          super(CondContext, self)._to_values_def(export_scope))
      for nested in self._nested_contexts:
        nested_def = context_def.nested_contexts.add()
        nested.to_control_flow_context_def(nested_def)

      return context_def
    else:
      return None

  @staticmethod
  def from_proto(context_def, import_scope=None):
    """Returns a `CondContext` object created from `context_def`."""
    ret = CondContext(context_def=context_def, import_scope=import_scope)

    ret.Enter()
    for nested_def in context_def.nested_contexts:
      from_control_flow_context_def(nested_def, import_scope=import_scope)
    ret.Exit()
    return ret

  def to_control_flow_context_def(self, context_def, export_scope=None):
    context_def.cond_ctxt.CopyFrom(self.to_proto(export_scope=export_scope))

  def AddValue(self, val):
    """Add `val` to the current context and its outer context recursively."""
    if val.name in self._values:
      # Use the real value if it comes from outer context. This is needed in
      # particular for nested conds.
      result = self._external_values.get(val.name)
      result = val if result is None else result
    else:
      result = val
      self._values.add(val.name)
      if self._outer_context:
        result = self._outer_context.AddValue(val)
        self._values.add(result.name)
        self._external_values[result.name] = result
      with ops.control_dependencies(None):
        result = _SwitchRefOrTensor(result, self._pred)[self._branch]
        if self._outer_context:
          self._outer_context.AddInnerOp(result.op)

      result.op.graph.prevent_fetching(result.op)
      # pylint: disable=protected-access
      result.op._set_control_flow_context(self)
      # pylint: enable=protected-access

      # Mark Switch output as seen by this context and any outer contexts,
      # just like what we do for normal op outputs in _AddOpInternal() below.
      ctxt = self
      while ctxt is not None:
        # pylint: disable=protected-access
        ctxt._values.add(result.name)
        ctxt = ctxt._outer_context
        # pylint: enable=protected-access

      self._external_values[val.name] = result
    return result

  def AddOp(self, op: ops.Operation):
    self._AddOpInternal(op)

  def _AddOpInternal(self, op: ops.Operation):
    """Add `op` to the current context."""
    if not op.inputs:
      # If we're in a while loop, remove any control inputs from outside the
      # loop.
      self._RemoveExternalControlEdges(op)

      if not any(
          util.OpInContext(input_op, self) for input_op in op.control_inputs):
        # pylint: disable=protected-access
        op._add_control_input(self._pivot.op)
        # pylint: enable=protected-access
    else:
      # Make each input to 'op' available in this CondContext. If an input is
      # already part of this context there's nothing to do, but if it's
      # external, AddValue() will handle adding the appropriate Switch node and
      # other bookkeeping.
      for index in range(len(op.inputs)):
        x = op.inputs[index]
        if op.type == "Merge" and x.op.type == "NextIteration":
          # Edge case: if we're importing a while loop inside this CondContext,
          # AddValue() will not correctly handle the NextIteration inputs to
          # Merge node. The problem is that the NextIteration should also be
          # part of this context, but if we're importing it won't have been
          # processed and added to the context yet, so AddValue() will try to
          # add a Switch which results in an invalid graph. Instead, we use the
          # NextIteration input as-is here, and it will eventually be added to
          # the context via AddOp().
          real_x = x
        else:
          real_x = self.AddValue(x)
        if real_x != x:
          # pylint: disable=protected-access
          op._update_input(index, real_x)
          # pylint: enable=protected-access
      # Remove any external control dependency on this op.
      self._RemoveExternalControlEdges(op)
      # pylint: disable=protected-access
      if op.graph._is_function(op.type) or op.type == "SymbolicGradient":
        op._add_control_input(self._pivot.op)
      # pylint: enable=protected-access

    # Mark op's outputs as seen by this context and any outer contexts.
    output_names = [x.name for x in op.outputs]
    ctxt = self
    while ctxt is not None:
      # pylint: disable=protected-access
      ctxt._values.update(output_names)
      ctxt = ctxt._outer_context
      # pylint: enable=protected-access

    if self._outer_context or not util.IsLoopExit(op):
      op.graph.prevent_fetching(op)

    if self._outer_context:
      self._outer_context.AddInnerOp(op)

  def _ProcessOutputTensor(self, val):
    """Process an output tensor of a conditional branch."""
    real_val = val
    if val.name not in self._values:
      # Handle the special case of lambda: x
      self._values.add(val.name)
      if self._outer_context:
        real_val = self._outer_context.AddValue(val)
        self._values.add(real_val.name)
        self._external_values[real_val.name] = real_val
      real_val = _SwitchRefOrTensor(real_val, self._pred)[self._branch]
      self._external_values[val.name] = real_val
    else:
      external_val = self._external_values.get(val.name)
      if external_val is not None:
        real_val = external_val
    return real_val

  def _BuildCondTensor(self, v):
    if isinstance(v, ops.Operation):
      # Use pivot as the proxy for this op.
      return with_dependencies([v], self._pivot)
    else:
      v = nest.map_structure(
          _convert_tensorarray_to_flow, v, expand_composites=True)
      return self._ProcessOutputTensor(ops.convert_to_tensor(v))

  def BuildCondBranch(self, fn):
    """Add the subgraph defined by fn() to the graph."""
    pre_summaries = ops.get_collection(ops.GraphKeys._SUMMARY_COLLECTION)  # pylint: disable=protected-access
    original_result = fn()
    post_summaries = ops.get_collection(ops.GraphKeys._SUMMARY_COLLECTION)  # pylint: disable=protected-access
    if len(post_summaries) > len(pre_summaries):
      new_summaries = post_summaries[len(pre_summaries):]
      summary_ref = ops.get_collection_ref(ops.GraphKeys._SUMMARY_COLLECTION)  # pylint: disable=protected-access
      summary_ref[:] = pre_summaries
      with ops.control_dependencies(new_summaries):
        if original_result is None:
          return no_op(), None
        elif not isinstance(original_result, ops.Operation):
          original_result = variable_utils.convert_variables_to_tensors(
              original_result)
          original_result = nest.map_structure(
              array_ops.identity, original_result, expand_composites=True)
    if original_result is None:
      return None, None

    original_result = variable_utils.convert_variables_to_tensors(
        original_result)
    result = nest.map_structure(
        self._BuildCondTensor, original_result, expand_composites=True)
    if not isinstance(result, (list, _basetuple)):
      result = [result]
    return original_result, result

  def IsCondContext(self):
    return True


# pylint: enable=g-doc-args
# pylint: enable=redefined-outer-name


def _resource_safe_shape(t):
  """Returns the shape of t or the variable it points to."""
  if t.dtype == dtypes.resource:
    while t.op.inputs:
      t = t.op.inputs[0]
    return tensor_shape.TensorShape(t.op.get_attr("shape"))
  return array_ops.shape_internal(t, optimize=False)


# TODO(yuanbyu): Consider having a unified notion of context for
# not only conditionals and loops but also control dependency and
# subgraphs.
class WhileContext(ControlFlowContext):
  """The context for the loop construct."""

  def __init__(self,
               maximum_iterations=None,
               parallel_iterations=10,
               back_prop=True,
               swap_memory=False,
               name="while_context",
               grad_state=None,
               context_def=None,
               import_scope=None):
    """"Creates a `WhileContext`.

    Args:
      maximum_iterations: Optional upper bound on number of loop iterations.
      parallel_iterations: The number of iterations allowed to run in parallel.
      back_prop: Whether backprop is enabled for this while loop.
      swap_memory: Whether GPU-CPU memory swap is enabled for this loop.
      name: Optional name prefix for the returned tensors.
      grad_state: The gradient loop state.
      context_def: Optional `WhileContextDef` protocol buffer to initialize the
        `Whilecontext` python object from.
      import_scope: Optional `string`. Name scope to add. Only used when
        initialing from protocol buffer.
    """
    if context_def:
      self._init_from_proto(context_def, import_scope=import_scope)
    else:
      ControlFlowContext.__init__(self)
      self._init_from_args(maximum_iterations, parallel_iterations, back_prop,
                           swap_memory, name)
    # The gradient loop state.
    self._grad_state = grad_state

  def _init_from_args(self, maximum_iterations, parallel_iterations, back_prop,
                      swap_memory, name):
    """Creates a new `WhileContext` from arguments.

    Args:
      maximum_iterations: Optional upper bound on number of loop iterations.
      parallel_iterations: The number of iterations allowed to run in parallel.
      back_prop: Whether backprop is enabled for this while loop.
      swap_memory: Whether GPU-CPU memory swap is enabled for this loop.
      name: Optional name prefix for the returned tensors.

    Raises:
      ValueError: If `parallel_iterations` has invalid value.
    """
    if not isinstance(parallel_iterations, int) or (parallel_iterations <= 0):
      raise ValueError("'parallel_iterations' must be a positive integer: "
                       "%s" % parallel_iterations)
    self._name = ops.get_default_graph().unique_name(name)
    self._maximum_iterations = maximum_iterations
    self._parallel_iterations = parallel_iterations
    self._back_prop = back_prop
    self._swap_memory = swap_memory
    # We use this node to control constants created by the pred lambda.
    self._pivot_for_pred = None
    # We use this node to control constants created by the body lambda.
    self._pivot_for_body = None
    # The boolean tensor for loop termination condition. Used in code
    # generation for gradient computation
    self._pivot = None
    # The list of exit tensors for loop variables.
    self._loop_exits = []
    # The list of enter tensors for loop variables.
    self._loop_enters = []
    self._graph = ops.get_default_graph()

  def _init_from_proto(self, context_def, import_scope=None):
    """Creates a new `WhileContext` from protocol buffer.

    Args:
      context_def: `WhileContextDef` protocol buffer.
      import_scope: Optional `string`. Name scope to add.
    """
    assert isinstance(context_def, control_flow_pb2.WhileContextDef)
    # Create from context_def.
    g = ops.get_default_graph()
    self._name = ops.prepend_name_scope(context_def.context_name, import_scope)
    if context_def.maximum_iterations_name:
      self._maximum_iterations = g.as_graph_element(
          ops.prepend_name_scope(context_def.maximum_iterations_name,
                                 import_scope))
    else:
      self._maximum_iterations = None
    self._parallel_iterations = context_def.parallel_iterations
    self._back_prop = context_def.back_prop
    self._swap_memory = context_def.swap_memory
    self._pivot_for_pred = g.as_graph_element(
        ops.prepend_name_scope(context_def.pivot_for_pred_name, import_scope))
    # We use this node to control constants created by the body lambda.
    self._pivot_for_body = g.as_graph_element(
        ops.prepend_name_scope(context_def.pivot_for_body_name, import_scope))
    # The boolean tensor for loop termination condition. Used in code
    # generation for gradient computation.
    self._pivot = g.as_graph_element(
        ops.prepend_name_scope(context_def.pivot_name, import_scope))
    # The list of exit tensors for loop variables.
    self._loop_exits = [
        g.as_graph_element(ops.prepend_name_scope(exit_name, import_scope))
        for exit_name in context_def.loop_exit_names
    ]
    # The list of enter tensors for loop variables.
    self._loop_enters = [
        g.as_graph_element(ops.prepend_name_scope(enter_name, import_scope))
        for enter_name in context_def.loop_enter_names
    ]
    super(WhileContext, self).__init__(
        values_def=context_def.values_def, import_scope=import_scope)

    # import_scope causes self.name to be different from the original serialized
    # context's name. Rewrite "frame_name" attrs with the new name.
    if import_scope:
      for tensor_name in self._values:
        op = g.as_graph_element(tensor_name).op
        if util.IsLoopEnter(op):
          # pylint: disable=protected-access
          op._set_attr("frame_name",
                       attr_value_pb2.AttrValue(s=compat.as_bytes(self.name)))
          # pylint: enable=protected-access
    self._graph = ops.get_default_graph()

  @property
  def maximum_iterations(self):
    """The maximum number of iterations that will be executed."""
    return self._maximum_iterations

  @property
  def parallel_iterations(self):
    """The number of iterations allowed to run in parallel."""
    return self._parallel_iterations

  @property
  def back_prop(self):
    """True iff backprop is enabled for this while loop."""
    return self._back_prop

  @property
  def swap_memory(self):
    """True iff GPU-CPU memory swap is enabled for this while loop."""
    return self._swap_memory

  @property
  def pivot(self):
    """The boolean tensor representing the loop termination condition."""
    return self._pivot

  @property
  def loop_enters(self):
    """The list of enter tensors for loop variables."""
    return self._loop_enters

  @property
  def loop_exits(self):
    """The list of exit tensors for loop variables."""
    return self._loop_exits

  @property
  def grad_state(self):
    """The gradient loop state."""
    return self._grad_state

  def to_proto(self, export_scope=None):
    """Converts a `WhileContext` to a `WhileContextDef` protocol buffer.

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

    Returns:
      A `WhileContextDef` protocol buffer.
    """
    if (export_scope is None or self.name.startswith(export_scope)):
      context_def = control_flow_pb2.WhileContextDef()
      context_def.context_name = ops.strip_name_scope(self.name, export_scope)
      context_def.parallel_iterations = self._parallel_iterations
      if self._maximum_iterations is not None:
        context_def.maximum_iterations_name = ops.strip_name_scope(
            self._maximum_iterations.name, export_scope)
      context_def.back_prop = self._back_prop
      context_def.swap_memory = self._swap_memory
      context_def.pivot_for_pred_name = ops.strip_name_scope(
          self._pivot_for_pred.name, export_scope)
      context_def.pivot_for_body_name = ops.strip_name_scope(
          self._pivot_for_body.name, export_scope)
      context_def.pivot_name = ops.strip_name_scope(self._pivot.name,
                                                    export_scope)
      context_def.loop_exit_names.extend([
          ops.strip_name_scope(l.name, export_scope) for l in self._loop_exits
      ])
      context_def.loop_enter_names.extend([
          ops.strip_name_scope(l.name, export_scope) for l in self._loop_enters
      ])
      context_def.values_def.MergeFrom(
          super(WhileContext, self)._to_values_def(export_scope=export_scope))
      for nested in self._nested_contexts:
        nested_def = context_def.nested_contexts.add()
        nested.to_control_flow_context_def(nested_def)

      return context_def
    else:
      return None

  def to_control_flow_context_def(self, context_def, export_scope=None):
    context_def.while_ctxt.CopyFrom(self.to_proto(export_scope=export_scope))

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

    Args:
      context_def: A `WhileContextDef` protocol buffer.
      import_scope: Optional `string`. Name scope to add.

    Returns:
      A `WhileContext` Python object.
    """
    ret = WhileContext(context_def=context_def, import_scope=import_scope)
    ret.Enter()
    for nested_def in context_def.nested_contexts:
      from_control_flow_context_def(nested_def, import_scope=import_scope)
    ret.Exit()
    return ret

  def GetWhileContext(self):
    return self

  def GetControlPivot(self):
    if self._pivot_for_body is not None:
      return self._pivot_for_body
    return self._pivot_for_pred

  def AddValue(self, val):
    """Add `val` to the current context and its outer context recursively."""
    result = val
    new_value = val.name not in self._values
    # Don't treat ops in this context as new values. Usually all known values
    # are in self._values, except when we're importing a while loop inside this
    # WhileContext. Since there's a cycle in this case, `val` may be part of the
    # imported while loop but not yet processed by this context and added to
    # self._values in _AddOpInternal. We only want to process external input
    # tensors to the while loop here.
    new_value &= val.op._control_flow_context is not self  # pylint: disable=protected-access
    if new_value:
      self._values.add(val.name)

      # If we are in a grad context and val is from its forward context,
      # use GetRealValue(), which adds the logic to save the history of
      # val in forward.
      grad_ctxt = ops.get_default_graph()._get_control_flow_context()
      if grad_ctxt:
        grad_ctxt = grad_ctxt.GetWhileContext()
        if grad_ctxt.grad_state:
          forward_ctxt = util.GetWhileContext(val.op)
          if util.IsLoopExit(val.op):
            forward_ctxt = forward_ctxt.outer_context
            if forward_ctxt:
              forward_ctxt = forward_ctxt.GetWhileContext()
          if forward_ctxt == grad_ctxt.grad_state.forward_context:
            real_val = grad_ctxt.grad_state.GetRealValue(val)
            self._external_values[val.name] = real_val
            return real_val

      if self._outer_context is not None:
        result = self._outer_context.AddValue(val)
      # Create an Enter to make `result` known to this loop context.
      with ops.control_dependencies(None):
        enter = _Enter(
            result,
            self._name,
            is_constant=True,
            parallel_iterations=self._parallel_iterations)
        enter.graph.prevent_feeding(enter)
        if self._outer_context:
          self._outer_context.AddInnerOp(enter.op)
      # Fix the control inputs and control flow context of these enter ops.
      self._FixControlInputsAndContext([enter])

      # Add `enter` in this context.
      self._values.add(enter.name)
      self._external_values[val.name] = enter
      result = enter
    else:
      actual_val = self._external_values.get(val.name)
      if actual_val is not None:
        result = actual_val
    return result

  def AddOp(self, op: ops.Operation):
    """Add `op` to the current context."""
    # For a reduction op, if op is in a grad context and its input is from
    # its forward context, moving op to the forward context means we would
    # store the tensor after the reduction as opposed to the tensor before
    # reduction, and therefore could significantly reduce memory consumption.
    # For now, we do this only for a few ops.
    #
    # If in XLA context, do not move constant ops to forward pass as pushing to
    # and popping from a stack removes the constant property of an op and breaks
    # XLA compilation, which requires certain inputs to be constant for certain
    # ops.
    if not util.IsInXLAContext(op) and op.type in {"Shape", "Size", "Rank"}:
      grad_ctxt = ops.get_default_graph()._get_control_flow_context()
      if grad_ctxt:
        grad_ctxt = grad_ctxt.GetWhileContext()
        if grad_ctxt.grad_state:
          op_input_forward_ctxt = util.GetWhileContext(op.inputs[0].op)
          if op_input_forward_ctxt == grad_ctxt.grad_state.forward_context:
            op_input_ctxt = op.inputs[0].op._get_control_flow_context()
            op._set_control_flow_context(op_input_ctxt)
            op_input_ctxt._AddOpInternal(op)
            return
    self._AddOpInternal(op)

  #  pylint: disable=g-doc-args
  def _AddOpInternal(self, op: ops.Operation):
    """Add `op` to the current context.

    We move any external control dependencies of the op to the loop pivot, to
    ensure they get executed.
    """
    # This is needed to prevent frame mismatch errors where there are Const
    # nodes inside tf.function in v1 while_loop and inlining is turned on.
    if op.type in ["PartitionedCall", "StatefulPartitionedCall"]:
      op._add_control_input(self.GetControlPivot().op)  # pylint: disable=protected-access
    if not op.inputs:
      # Remove any external control dependency on this op
      control_inputs, external_inputs = self._RemoveExternalControlEdges(op)
      # Add a control edge from the control pivot to this op.
      if not control_inputs:
        # pylint: disable=protected-access
        op._add_control_input(self.GetControlPivot().op)
        # pylint: enable=protected-access
      for x in op.outputs:
        self._values.add(x.name)
    else:
      for index in range(len(op.inputs)):
        x = op.inputs[index]
        real_x = self.AddValue(x)
        if real_x != x:
          op._update_input(index, real_x)  # pylint: disable=protected-access
      # Remove any external control dependency on this op.
      _, external_inputs = self._RemoveExternalControlEdges(op)
      # Add a control dependency to prevent loop invariants from
      # enabling ops that should not be executed.
      self._MaybeAddControlDependency(op)
      for x in op.outputs:
        self._values.add(x.name)
    if external_inputs:
      # Use an identity to pull control inputs as data inputs. Note that we
      # ignore ops which don't have outputs. TODO(apassos): fix that
      with ops.control_dependencies(None):
        self.Enter()
        external_inputs = [
            array_ops.identity(x.outputs[0]).op
            for x in external_inputs
            if x.outputs
        ]
        self.Exit()
      op._add_control_inputs(external_inputs)  # pylint: disable=protected-access
    if self._outer_context or not util.IsLoopExit(op):
      op.graph.prevent_fetching(op)
      for x in op.outputs:
        op.graph.prevent_feeding(x)

    if self._outer_context:
      self._outer_context.AddInnerOp(op)

  def _MaybeAddControlDependency(self, op: ops.Operation):
    """Add a control input to the op if it only depends on loop invariants."""

    def _IsOpFree(op):
      """Determines if `op` needs a control dependency."""
      if op.control_inputs:
        return False
      # pylint: disable=protected-access
      if op.graph._is_function(op.type) or op.type == "SymbolicGradient":
        return True
      # pylint: enable=protected-access
      for x in op.inputs:
        if not util.IsLoopConstantEnter(x.op):
          return False
      return True

    if _IsOpFree(op):
      # pylint: disable=protected-access
      op._add_control_input(self.GetControlPivot().op)
      # pylint: enable=protected-access

  def AddForwardLoopCounter(self, outer_grad_state):
    """Adds a loop that counts the number of iterations.

    This is added to the forward loop at the time when we start to
    create the loop for backprop gradient computation. Called in
    the outer context of this forward context.

    The pseudocode is:
      `n = 0; while (_pivot) { n++; }`

    Note that a control dependency is added to `n` to ensure the correct
    execution order of stack push ops.

    Args:
      outer_grad_state: The outer grad state. None if not nested.

    Returns:
      The number of iterations taken by the forward loop and the loop index.
    """
    n = constant_op.constant(0, name="f_count")
    if outer_grad_state is not None:
      # Force the stack pushes of i-th execution of an inner loop to be ordered
      # before the pushes of (i+1)-th execution of the same inner loop.
      outer_add_op = outer_grad_state.forward_index.op.inputs[0].op
      n.op._add_control_input(outer_add_op)  # pylint: disable=protected-access

    self.Enter()
    self.AddName(n.name)
    enter_n = _Enter(
        n,
        self._name,
        is_constant=False,
        parallel_iterations=self._parallel_iterations,
        name="f_count")
    self.loop_enters.append(enter_n)

    merge_n = merge([enter_n, enter_n])[0]
    switch_n = switch(merge_n, self._pivot)

    index = math_ops.add(switch_n[1], 1)
    next_n = _NextIteration(index)
    merge_n.op._update_input(1, next_n)

    total_iterations = exit(switch_n[0], name="f_count")
    self.loop_exits.append(total_iterations)
    self.ExitResult([total_iterations])
    self.Exit()
    return total_iterations, next_n

  def AddBackpropLoopCounter(self, count, outer_grad_state):
    """Add the backprop loop that controls the iterations.

    This is added to the backprop loop. It is used to control the loop
    termination of the backprop loop. Called in the outer context of
    this grad context.

    The pseudocode is:
      `n = count; while (n >= 1) { n--; }`

    Note that a control dependency is added to `final_zero` to ensure the
    correct execution order of stack pop ops.

    Args:
      count: The number of iterations for backprop.
      outer_grad_state: The outer grad state. None if not nested.

    Returns:
      The loop index.
    """
    in_separate_functions = count.graph is not ops.get_default_graph()
    if in_separate_functions:
      # Brings the count into this graph
      count = array_ops.identity(count)
    else:
      # TODO(apassos) XLA expects this constant to be created outside the loop,
      # so doing that for now.
      one = constant_op.constant(1, name="b_count")

    self.Enter()
    self.AddName(count.name)
    enter_count = _Enter(
        count,
        self._name,
        is_constant=False,
        parallel_iterations=self._parallel_iterations,
        name="b_count")
    self.loop_enters.append(enter_count)

    merge_count = merge([enter_count, enter_count])[0]
    self._pivot_for_pred = merge_count

    if in_separate_functions:
      one = constant_op.constant(1, name="b_count")
    pred = math_ops.greater_equal(merge_count, one)
    self._pivot = loop_cond(pred, name="b_count")
    switch_count = switch(merge_count, self._pivot)

    index = math_ops.subtract(switch_count[1], one)
    self._pivot_for_body = index
    next_count = _NextIteration(index)
    merge_count.op._update_input(1, next_count)

    final_zero = exit(switch_count[0], name="b_count")
    self.loop_exits.append(final_zero)
    if outer_grad_state is not None:
      # Force the stack pops of i-th execution of an inner loop to be ordered
      # before the pops of (i+1)-th execution of the same inner loop.
      # pylint: disable=protected-access
      outer_grad_state.grad_sync._add_control_input(final_zero.op)
      # pylint: enable=protected-access

    self.ExitResult([final_zero])
    self.Exit()
    return next_count

  def AddBackpropAccumulator(self, op: ops.Operation, grad):
    """Add an accumulation loop for every loop invariant.

    This is added to the backprop loop. It is used to accumulate partial
    gradients within each loop iteration. Called when in the gradient while
    context.

    The pseudocode is:
      ```
      acc = 0.0;
      while (_pivot) {
        acc += grad;
      }
      ```

    Args:
      op: The Enter op for a loop invariant.
      grad: The partial gradient of an iteration for a loop invariant.

    Returns:
      The gradient for a loop invariant.
    """
    self.Exit()
    # Create a zeros tensor with the right shape for acc. If we don't
    # know the full shape statically, we will have to get the shape
    # dynamically from the forward inference. Getting the shape right
    # for the zeros is only needed for the base case when the loop exits
    # without running any iterations.
    shape = grad.get_shape()
    if shape.is_fully_defined():
      if self.outer_context:
        self.outer_context.Enter()
      acc = constant_op.constant(0, grad.dtype, shape=shape, name="b_acc")
      if self.outer_context:
        self.outer_context.Exit()
    else:
      value = op.inputs[0]
      if (isinstance(self.outer_context, WhileContext) and
          self.outer_context.grad_state is not None):
        # We are in a nested while loop.
        forward_ctxt = self.grad_state.forward_context
        forward_ctxt.outer_context.Enter()
        zeros_shape = array_ops.shape_internal(value, optimize=False)
        forward_ctxt.outer_context.Exit()
        outer_grad_state = self.grad_state.outer_grad_state
        history_zeros_shape = outer_grad_state.AddForwardAccumulator(
            zeros_shape)
        self.outer_context.Enter()
        real_shape = outer_grad_state.AddBackpropAccumulatedValue(
            history_zeros_shape, zeros_shape)
        acc = array_ops.zeros(real_shape, grad.dtype)
        self.outer_context.Exit()
      else:
        if self.outer_context:
          self.outer_context.Enter()
        zeros_shape = array_ops.shape_internal(value, optimize=False)
        acc = array_ops.zeros(zeros_shape, grad.dtype)
        if self.outer_context:
          self.outer_context.Exit()

    self.Enter()
    self.AddName(acc.name)
    enter_acc = _Enter(
        acc,
        self._name,
        is_constant=False,
        parallel_iterations=self._parallel_iterations,
        name="b_acc")
    self.loop_enters.append(enter_acc)

    merge_acc = merge([enter_acc, enter_acc], name="b_acc")[0]
    switch_acc_false, switch_acc_true = switch(merge_acc, self._pivot)

    add_acc = math_ops.add(switch_acc_true, grad)
    next_acc = _NextIteration(add_acc)
    merge_acc.op._update_input(1, next_acc)  # pylint: disable=protected-access

    result_acc = exit(switch_acc_false, name="b_acc")
    self.loop_exits.append(result_acc)
    self.ExitResult([result_acc])
    return result_acc

  def AddBackpropIndexedSlicesAccumulator(self, op: ops.Operation, grad):
    """This is used for accumulating gradients that are IndexedSlices.

    This is essentially the equivalent of AddBackpropAccumulator but optimized
    for things like updating embeddings from within a while loop.

    Args:
      op: The Enter op for a loop invariant.
      grad: The partial gradients represented as an IndexedSlices.

    Returns:
      The accumulated IndexedSlices gradient of the loop invariant.
    """
    values = grad.values
    indices = grad.indices
    dense_shape = grad.dense_shape

    self.Exit()
    if self.outer_context:
      self.outer_context.Enter()
    if values.get_shape().is_fully_defined():
      values_shape = tensor_shape.TensorShape([tensor_shape.Dimension(1)] +
                                              values.get_shape().dims[1:])
      if self.outer_context:
        self.outer_context.Enter()
      values_acc = constant_op.constant(
          0, values.dtype, shape=values_shape, name="b_acc")
      if self.outer_context:
        self.outer_context.Exit()
    else:
      values_shape = _resource_safe_shape(op.inputs[0])[1:]
      values_shape = array_ops.concat([[1], values_shape], 0)
      values_acc = array_ops.zeros(values_shape, dtype=values.dtype)
    indices_acc = constant_op.constant([0], indices.dtype)
    shape_acc = None
    if dense_shape is not None:
      if dense_shape.get_shape().is_fully_defined():
        if self.outer_context:
          self.outer_context.Enter()
        shape_acc = constant_op.constant(
            0, dense_shape.dtype, shape=dense_shape.get_shape())
        if self.outer_context:
          self.outer_context.Exit()
      else:
        shape_acc = array_ops.zeros_like(
            array_ops.shape_internal(
                op.inputs[0], optimize=False, out_type=dense_shape.dtype),
            optimize=False)

    if self.outer_context:
      self.outer_context.Exit()

    self.Enter()
    self.AddName(values_acc.name)
    self.AddName(indices_acc.name)
    init_acc = [indices_acc, values_acc]
    if shape_acc is not None:
      self.AddName(shape_acc.name)
      init_acc.append(shape_acc)

    # Set use_input_shape=False since the accumulator tensors will grow in
    # size. If use_input_shape=True, the _update_input call below will result in
    # incompatible shapes.
    enter_acc = [
        _Enter(
            x,
            self._name,
            is_constant=False,
            parallel_iterations=self._parallel_iterations,
            use_input_shape=False,
            name="b_acc") for x in init_acc
    ]
    # Manually set appropriate partial shapes.
    enter_acc[0].set_shape([None])
    if values_acc.shape.dims is not None:
      enter_acc[1].set_shape([None] + values_acc.shape.as_list()[1:])
    self.loop_enters.extend(enter_acc)

    merge_acc = [merge([x, x], name="b_acc")[0] for x in enter_acc]
    switch_acc = [switch(x, self._pivot) for x in merge_acc]

    # The actual accumulation.
    acc_indexed_slices = [
        array_ops.concat([xa[1], xv], 0)
        for xa, xv in zip(switch_acc[:2], [indices, values])
    ]
    if shape_acc is not None:
      # For the shape we just keep the maximum
      acc_indexed_slices.append(math_ops.maximum(dense_shape, switch_acc[2][1]))

    next_acc = [_NextIteration(x) for x in acc_indexed_slices]
    for xm, xn in zip(merge_acc, next_acc):
      xm.op._update_input(1, xn)  # pylint: disable=protected-access

    exit_acc = [exit(x[0], name="b_acc") for x in switch_acc]
    self.loop_exits.extend(exit_acc)

    self.ExitResult(exit_acc)
    return indexed_slices.IndexedSlices(
        indices=exit_acc[0],
        values=exit_acc[1],
        dense_shape=exit_acc[2] if shape_acc is not None else None)

  def _InitializeValues(self, values):
    """Makes the values known to this context."""
    self._values = set()
    for x in values:
      if isinstance(x, tensor_lib.Tensor):
        self._values.add(x.name)
      else:
        raise TypeError("'values' must be a list of Tensors. "
                        f"Received: {type(x)}.")

  def _BuildLoop(self, pred, body, flat_orig_loop_vars, flat_loop_vars,
                 loop_vars_signature):
    """Core: Add the loop termination condition and body to the graph."""
    flat_shape_invariants = nest.map_structure(
        lambda spec: spec.shape,
        nest.flatten(loop_vars_signature, expand_composites=True))

    # Let the context know the loop variables so the loop variables
    # would be added in the outer contexts properly.
    self._InitializeValues(flat_loop_vars)
    if self._outer_context:
      real_vars = [self._outer_context.AddValue(x) for x in flat_loop_vars]
    else:
      real_vars = flat_loop_vars

    enter_vars = []
    with ops.control_dependencies(None):
      for real_var, shape_invariant in zip(real_vars, flat_shape_invariants):
        enter_var = _Enter(
            real_var,
            self._name,
            is_constant=False,
            parallel_iterations=self._parallel_iterations,
            use_input_shape=False)

        if _ShapeLessThanOrEqual(real_var.get_shape(), shape_invariant):
          enter_var.set_shape(shape_invariant)
        else:
          raise ValueError(
              f"The shape invariant specified for {real_var.name} is not "
              "compatible with the initial shape of the loop variable. It "
              f"enters the loop with shape {real_var.get_shape()}, but the "
              f"specified shape invariant is {shape_invariant}.")

        enter_var.graph.prevent_feeding(enter_var)
        if self._outer_context:
          self._outer_context.AddInnerOp(enter_var.op)
        enter_vars.append(enter_var)

    # Finds the closest enclosing non-None control pivot.
    outer_context = self._outer_context
    control_pivot = None
    while outer_context is not None and control_pivot is None:
      control_pivot = outer_context.GetControlPivot()
      # pylint: disable=protected-access
      outer_context = outer_context._outer_context
      # pylint: enable=protected-access

    if control_pivot is not None:
      for var in enter_vars:
        if util.IsLoopConstantEnter(var.op.inputs[0].op):
          # pylint: disable=protected-access
          var.op._add_control_input(control_pivot.op)
          # pylint: enable=protected-access

    # Fix the control inputs and control flow context of these enter ops.
    self._FixControlInputsAndContext(enter_vars)
    self._InitializeValues(enter_vars)
    self._loop_enters = enter_vars

    merge_vars = [merge([x, x])[0] for x in enter_vars]
    self._pivot_for_pred = merge_vars[0]

    merge_vars_with_tensorarrays = nest.map_structure(
        _convert_flow_to_tensorarray, flat_orig_loop_vars, merge_vars)
    # Build the graph for pred.
    packed_vars = nest.pack_sequence_as(
        structure=loop_vars_signature,
        flat_sequence=merge_vars_with_tensorarrays,
        expand_composites=True)
    c = ops.convert_to_tensor(pred(*packed_vars))
    self._pivot = loop_cond(c, name="LoopCond")
    switch_vars = [_SwitchRefOrTensor(x, self._pivot) for x in merge_vars]

    # Build the graph for body.
    vars_for_body = [_Identity(x[1]) for x in switch_vars]
    self._pivot_for_body = vars_for_body[0]
    # Convert TensorArray flow variables inside the context back into
    # their associated TensorArrays for calling the body.
    vars_for_body_with_tensorarrays = nest.map_structure(
        _convert_flow_to_tensorarray, flat_orig_loop_vars, vars_for_body)
    packed_vars_for_body = nest.pack_sequence_as(
        structure=loop_vars_signature,
        flat_sequence=vars_for_body_with_tensorarrays,
        expand_composites=True)
    pre_summaries = ops.get_collection(ops.GraphKeys._SUMMARY_COLLECTION)  # pylint: disable=protected-access
    body_result = body(*packed_vars_for_body)
    post_summaries = ops.get_collection(ops.GraphKeys._SUMMARY_COLLECTION)  # pylint: disable=protected-access
    if not nest.is_nested(body_result):
      body_result = [body_result]
    if len(post_summaries) > len(pre_summaries):
      new_summaries = post_summaries[len(pre_summaries):]
      summary_ref = ops.get_collection_ref(ops.GraphKeys._SUMMARY_COLLECTION)  # pylint: disable=protected-access
      summary_ref[:] = pre_summaries
      with ops.control_dependencies(new_summaries):

        def map_fn(x):
          # TODO(apassos) figure out how to trigger with tensor arrays as well
          if isinstance(x, tensor_array_ops.TensorArray):
            return x
          return array_ops.identity(x)

        body_result = nest.map_structure(
            map_fn, body_result, expand_composites=True)

    body_result = variable_utils.convert_variables_to_tensors(body_result)
    # Compare the structure types of input and output of body.
    # For backwards compatibility, the first layer is forced to a list
    # during this comparison, because inputs are typically lists and
    # outputs of the body are typically tuples.
    nest.assert_same_structure(
        list(packed_vars_for_body), list(body_result), expand_composites=True)

    # Store body_result to keep track of TensorArrays returned by body
    original_body_result = body_result
    # Convert TensorArrays returned by body into their flow variables
    result = nest.map_structure(
        _convert_tensorarray_to_flow,
        nest.flatten(body_result, expand_composites=True),
        expand_composites=True)
    result = ops.convert_n_to_tensor_or_composite(result)

    # Add NextIteration and the back edges to complete the loop.
    if len(merge_vars) != len(result):
      raise ValueError("Number of inputs and outputs of 'body' must match "
                       f"'loop_vars'. Got {len(merge_vars)} for the number of "
                       f"inputs/outputs, and {len(result)} for 'loop_vars'.")
    next_vars = []
    for m, v in zip(merge_vars, result):
      next_vars.append(_AddNextAndBackEdge(m, v))

    # Add the exit ops.
    exit_vars = [exit(x[0]) for x in switch_vars]
    self._loop_exits = exit_vars

    # Exit the loop.
    self.ExitResult(exit_vars)

    return original_body_result, exit_vars

  def BuildLoop(self, pred, body, loop_vars, shape_invariants,
                return_same_structure):
    """Add the loop termination condition and body to the graph."""

    # Keep flat_orig_loop_vars to identify which are TensorArrays
    flat_orig_loop_vars = nest.flatten(loop_vars, expand_composites=True)

    loop_vars = nest.map_structure(
        _convert_to_tensor_or_composite_or_tensorarray, loop_vars)
    # Convert TensorArrays to their flow variables
    flat_loop_vars = nest.map_structure(
        _convert_tensorarray_to_flow,
        nest.flatten(loop_vars, expand_composites=True))

    if shape_invariants is not None:
      loop_vars_signature = nest.map_structure(
          _shape_invariant_to_type_spec, loop_vars, shape_invariants)
    else:
      loop_vars_signature = nest.map_structure(
          _shape_invariant_to_type_spec, loop_vars)

    try:
      self.Enter()
      # _BuildLoop calls _update_input in several places. _mutation_lock()
      # ensures a Session.run call cannot occur between creating and mutating
      # new ops.
      with ops.get_default_graph()._mutation_lock():  # pylint: disable=protected-access
        original_body_result, exit_vars = self._BuildLoop(
            pred, body, flat_orig_loop_vars, flat_loop_vars,
            loop_vars_signature)
    finally:
      self.Exit()

    flat_result = nest.flatten(original_body_result, expand_composites=True)
    # Convert TensorArray flow variables outside the context back into
    # their associated TensorArrays for returning to caller.
    exit_vars_with_tensorarrays = nest.map_structure(
        _convert_flow_to_tensorarray, flat_result, exit_vars)

    packed_exit_vars = nest.pack_sequence_as(
        structure=original_body_result,
        flat_sequence=exit_vars_with_tensorarrays,
        expand_composites=True)

    if return_same_structure:
      return packed_exit_vars
    else:
      return packed_exit_vars[0] if len(exit_vars) == 1 else packed_exit_vars

  def _FixControlInputsAndContext(self, enters):
    graph = ops.get_default_graph()
    # pylint: disable=protected-access
    for e in enters:
      if isinstance(e, tensor_lib.Tensor):
        xs = [e]
      else:
        raise TypeError("'enters' must be a list of Tensors. "
                        f"Received: {type(e)}.")
      for x in xs:
        inp_op = x.op.inputs[0].op
        control_inputs = graph._control_dependencies_for_inputs([inp_op])
        outer_control_inputs = []
        for op in control_inputs:
          # We need to keep control inputs that are in any ancestor
          # ControlFlowContext, and within outer WhileContext.
          keep_as_control_input = True
          op_ctxt = util.GetOutputContext(op)
          outer_ctxt = self.outer_context
          outer_while_context = (None if outer_ctxt is None else
                                 outer_ctxt.GetWhileContext())
          while outer_ctxt != op_ctxt:
            if outer_ctxt is None or outer_ctxt == outer_while_context:
              keep_as_control_input = False
              break
            outer_ctxt = outer_ctxt.outer_context
          if keep_as_control_input:
            outer_control_inputs.append(op)
        x.op._set_control_flow_context(self)
        x.op._add_control_inputs(outer_control_inputs)
        graph._record_op_seen_by_control_dependencies(x.op)
    # pylint: enable=protected-access

  def IsWhileContext(self):
    return True


# pylint: enable=redefined-outer-name


def _AsTensorList(x, p):
  """Return x as a list of Tensors or IndexedSlices.

  For entries of `x` that are Operations, this returns an Identity of `p`
  with a dependency on the operation.

  Args:
    x: A Tensor/IndexedSlices/Operation or a list or tuple of them.
    p: A Tensor to return for entries in `x` that are Operations.

  Returns:
    A list of Tensors or IndexedSlices.
  """
  if not isinstance(x, (list, _basetuple)):
    x = [x]

  l = []
  for v in x:
    if isinstance(v, ops.Operation):
      v = with_dependencies([v], p)
    v = ops.convert_to_tensor_or_composite(v)
    if isinstance(v, tensor_lib.Tensor):
      l.append(array_ops.identity(v))
    else:
      l.append(
          indexed_slices.IndexedSlices(
              array_ops.identity(v.values), array_ops.identity(v.indices)))
  return l


def _CheckResults(a, b):
  assert len(a) == len(b), (
      "Values returned by a() and b() must have the same length.")
  for x, y in zip(a, b):
    assert x.dtype == y.dtype, (
        "Values returned by a() [%s] and b() [%s] must have "
        "the same type: %s, %s." % (x.name, y.name, x.dtype.name, y.dtype.name))


def with_dependencies(dependencies, output_tensor, name=None):
  """Produces the content of `output_tensor` only after `dependencies`.

  In some cases, a user may want the output of an operation to be
  consumed externally only after some other dependencies have run
  first. This function ensures returns `output_tensor`, but only after all
  operations in `dependencies` have run. Note that this means that there is
  no guarantee that `output_tensor` will be evaluated after any `dependencies`
  have run.

  See also `tf.tuple` and `tf.group`.

  Args:
    dependencies: Iterable of operations to run before this op finishes.
    output_tensor: A `Tensor` or `IndexedSlices` that will be returned.
    name: (Optional) A name for this operation.

  Returns:
    Same as `output_tensor`.

  Raises:
    TypeError: if `output_tensor` is not a `Tensor` or `IndexedSlices`.
  """
  if context.executing_eagerly():
    return output_tensor
  with ops.name_scope(name, "control_dependency",
                      list(dependencies) + [output_tensor]) as name:
    with ops.colocate_with(output_tensor):
      with ops.control_dependencies(dependencies):
        output_tensor = ops.convert_to_tensor_or_composite(output_tensor)
        if isinstance(output_tensor, indexed_slices.IndexedSlices):
          return indexed_slices.IndexedSlices(
              _Identity(output_tensor.values, name=name), output_tensor.indices,
              output_tensor.dense_shape)
        else:
          return _Identity(output_tensor, name=name)


def _GroupControlDeps(dev, deps, name=None):
  with ops.control_dependencies(deps):
    if dev is None:
      return no_op(name=name)
    else:
      with ops.device(dev):
        return no_op(name=name)


# TODO(touts): Accept "inputs" as a list.
@tf_export("group")
def group(*inputs, **kwargs):
  """Create an op that groups multiple operations.

  When this op finishes, all ops in `inputs` have finished. This op has no
  output.

  Note: *In TensorFlow 2 with eager and/or Autograph, you should not require
  this method, as ops execute in the expected order thanks to automatic control
  dependencies.* Only use `tf.group` when working with v1
  `tf.Graph` code.

  When operating in a v1-style graph context, ops are not executed in the same
  order as specified in the code; TensorFlow will attempt to execute ops in
  parallel or in an order convenient to the result it is computing.  `tf.group`
  allows you to request that one or more results finish before execution
  continues.

  `tf.group` creates a single op (of type `NoOp`), and then adds appropriate
  control dependencies.  Thus, `c = tf.group(a, b)` will compute the same graph
  as this:

      with tf.control_dependencies([a, b]):
          c = tf.no_op()

  See also `tf.tuple` and
  `tf.control_dependencies`.

  Args:
    *inputs: Zero or more tensors to group.
    name: A name for this operation (optional).

  Returns:
    An Operation that executes all its inputs.

  Raises:
    ValueError: If an unknown keyword argument is provided.
  """
  if context.executing_eagerly():
    return None
  name = kwargs.pop("name", None)
  if kwargs:
    raise ValueError("Unknown keyword arguments: " + ", ".join(kwargs.keys()))
  with ops.name_scope(name, "group_deps", inputs) as name:
    # Grouping no inputs means do nothing
    if not inputs:
      return no_op(name=name)

    # Sorts *inputs according to their devices.
    ops_on_device = {}  # device -> operations specified on the device.
    for inp in nest.flatten(inputs, expand_composites=True):
      if not hasattr(inp, "device"):
        raise TypeError("'inputs' should be zero or more (nested) Tensors. "
                        f"Received '{inp}' with type '{type(inp)}'.")
      dev = inp.device
      if dev in ops_on_device:
        ops_on_device[dev].append(inp)
      else:
        ops_on_device[dev] = [inp]
    if len(ops_on_device) == 1:
      # 1-level tree. The root node is the returned NoOp node.
      (dev, deps), = ops_on_device.items()
      return _GroupControlDeps(dev, deps, name=name)

    # 2-level tree. The root node is the returned NoOp node.
    # deps contains 1 NoOp node for each device.
    deps = []

    def device_key(dev):
      """A sort key that allows None to be compared to strings."""
      return "" if dev is None else dev

    for dev in sorted(ops_on_device, key=device_key):
      deps.append(_GroupControlDeps(dev, ops_on_device[dev]))

    with ops.control_dependencies(deps):
      return no_op(name=name)


@tf_export("tuple", v1=[])
@dispatch.add_dispatch_support
def tuple_v2(tensors, control_inputs=None, name=None):
  """Groups tensors together.

  The returned tensors have the same value as the input tensors, but they
  are computed only after all the input tensors have been computed.

  Note: *In TensorFlow 2 with eager and/or Autograph, you should not require
  this method, as ops execute in the expected order thanks to automatic control
  dependencies.* Only use `tf.tuple` when working with v1 `tf.Graph` code.

  See also `tf.group` and `tf.control_dependencies`.

  Example:
  >>> with tf.Graph().as_default():
  ...   with tf.compat.v1.Session() as sess:
  ...     v = tf.Variable(0.0)
  ...     a = tf.constant(1.0)
  ...     sess.run(tf.compat.v1.global_variables_initializer())
  ...     for i in range(5):
  ...       update_op = v.assign_add(1.0)
  ...       b = a + v
  ...       res_b = sess.run(b)
  ...       res_v = sess.run(v)
  ...       print(res_v)
  0.0
  0.0
  0.0
  0.0
  0.0

  >>> with tf.Graph().as_default():
  ...   with tf.compat.v1.Session() as sess:
  ...     v = tf.Variable(0.0)
  ...     a = tf.constant(1.0)
  ...     sess.run(tf.compat.v1.global_variables_initializer())
  ...     for i in range(5):
  ...       update_op = v.assign_add(1.0)
  ...       calc = [a + v]
  ...       # `tf.tuple` ensures `update_op` is run before `b`
  ...       b = tf.tuple(calc, [tf.group(update_op)])
  ...       res_b = sess.run(b)
  ...       res_v = sess.run(v)
  ...       print(res_v)
  1.0
  2.0
  3.0
  4.0
  5.0


  Args:
    tensors: A list of `Tensor`s or `IndexedSlices`, some entries can be `None`.
    control_inputs: List of additional ops to finish before returning.
    name: (optional) A name to use as a `name_scope` for the operation.

  Returns:
    Same as `tensors`.

  Raises:
    ValueError: If `tensors` does not contain any `Tensor` or `IndexedSlices`.
    TypeError: If `control_inputs` is not a list of `Operation` or `Tensor`
      objects.

  """
  return tuple(tensors=tensors, name=name, control_inputs=control_inputs)  # pylint: disable=redefined-builtin


@tf_export(v1=["tuple"])
@dispatch.add_dispatch_support
def tuple(tensors, name=None, control_inputs=None):  # pylint: disable=redefined-builtin
  """Group tensors together.

  This creates a tuple of tensors with the same values as the `tensors`
  argument, except that the value of each tensor is only returned after the
  values of all tensors have been computed.

  `control_inputs` contains additional ops that have to finish before this op
  finishes, but whose outputs are not returned.

  This can be used as a "join" mechanism for parallel computations: all the
  argument tensors can be computed in parallel, but the values of any tensor
  returned by `tuple` are only available after all the parallel computations
  are done.

  See also `tf.group` and
  `tf.control_dependencies`.

  Args:
    tensors: A list of `Tensor`s or `IndexedSlices`, some entries can be `None`.
    name: (optional) A name to use as a `name_scope` for the operation.
    control_inputs: List of additional ops to finish before returning.

  Returns:
    Same as `tensors`.

  Raises:
    ValueError: If `tensors` does not contain any `Tensor` or `IndexedSlices`.
    TypeError: If `control_inputs` is not a list of `Operation` or `Tensor`
      objects.

  """
  if context.executing_eagerly():
    return tensors
  with ops.name_scope(name, "tuple", tensors) as name:
    tensors = [
        t if (isinstance(t, ops.Operation) or tensor_util.is_tf_type(t) or
              t is None) else ops.convert_to_tensor(t) for t in tensors
    ]
    gating_ops = [
        t if isinstance(t, ops.Operation) else t.op
        for t in tensors
        if t is not None
    ]
    if control_inputs:
      for c in control_inputs:
        if isinstance(c, tensor_lib.Tensor):
          c = c.op
        elif not isinstance(c, ops.Operation):
          raise TypeError(
              "'control_inputs' must only contain Operation or Tensor. "
              f"Received: {type(c)}")
        gating_ops.append(c)
    # Note that in order to ensure ordering in the pbtxt, we must take care to
    # ensure the order here.
    gating_ops = sorted(set(gating_ops), key=lambda op: op._id)  # Uniquify ops.
    if not gating_ops:
      raise ValueError("'tensors' must have at least one Tensor. "
                       f"Received: {tensors}.")
    gate = group(*gating_ops)
    tpl = []
    for t in tensors:
      if tensor_util.is_tf_type(t):
        tpl.append(with_dependencies([gate], t))
      elif isinstance(t, ops.Operation):
        with ops.control_dependencies([gate]):
          tpl.append(group(t))
      else:
        tpl.append(None)
    return tpl


class XLAControlFlowContext(ControlFlowContext):
  """Base class for XLA and TPU control flow contexts."""

  def __init__(self):
    super(XLAControlFlowContext, self).__init__()
    self._name = "XLAControlFlowContext"

  def to_control_flow_context_def(self, context_def, export_scope=None):
    # pylint: disable=useless-super-delegation
    # NOTE(slebedev): the method is required by `ControlFlowContext`.
    super(XLAControlFlowContext,
          self).to_control_flow_context_def(context_def, export_scope)

  def IsXLAContext(self):
    return True

  def AddOp(self, _):
    pass

  def AddValue(self, x):
    return x

  def RequiresUniqueFunctionRetracing(self):
    """Returns whether the tf.function should be retraced if the context changes.
    """
    return False


@tf_export("__internal__.get_enclosing_xla_context", v1=[])
def get_enclosing_xla_context():
  """Recursively find and return the XLAControlFlowContext."""
  graph = ops.get_default_graph()
  while graph is not None:
    # pylint: disable=protected-access
    context_ = graph._get_control_flow_context()
    # pylint: enable=protected-access
    while context_ is not None:
      if isinstance(context_, XLAControlFlowContext):
        return context_
      context_ = context_.outer_context
    # This may be a FuncGraph due to defuns or v2 control flow. We need to
    # find the original graph with the XLAControlFlowContext.
    graph = getattr(graph, "outer_graph", None)
  return None


def from_control_flow_context_def(context_def, import_scope=None):
  """Deserializes `context_def` into the appropriate ControlFlowContext.

  Args:
    context_def: ControlFlowContextDef proto
    import_scope: Optional `string`. Name scope to add.

  Returns:
    A ControlFlowContext subclass
  """
  if context_def.HasField("cond_ctxt"):
    return CondContext.from_proto(
        context_def.cond_ctxt, import_scope=import_scope)
  if context_def.HasField("while_ctxt"):
    return WhileContext.from_proto(
        context_def.while_ctxt, import_scope=import_scope)
  raise NotImplementedError("Unknown ControlFlowContextDef field: %s" %
                            context_def.WhichOneof("ctxt"))


ops.register_proto_function(
    ops.GraphKeys.COND_CONTEXT,
    proto_type=control_flow_pb2.CondContextDef,
    to_proto=CondContext.to_proto,
    from_proto=CondContext.from_proto)

ops.register_proto_function(
    ops.GraphKeys.WHILE_CONTEXT,
    proto_type=control_flow_pb2.WhileContextDef,
    to_proto=WhileContext.to_proto,
    from_proto=WhileContext.from_proto)