def get_backward_walk_ops(seed_ops, inclusive=True, within_ops=None, stop_at_ts=(), control_inputs=False):
    """Do a backward graph walk and return all the visited ops.

  Args:
    seed_ops: an iterable of operations from which the backward graph
      walk starts. If a list of tensors is given instead, the seed_ops are set
      to be the generators of those tensors.
    inclusive: if True the given seed_ops are also part of the resulting set.
    within_ops: an iterable of tf.Operation whithin which the search is
      restricted. If within_ops is None, the search is performed within
      the whole graph.
    stop_at_ts: an iterable of tensors at which the graph walk stops.
    control_inputs: if True, control inputs will be used while moving backward.
  Returns:
    A Python set of all the tf.Operation behind seed_ops.
  Raises:
    TypeError: if seed_ops or within_ops cannot be converted to a list of
      tf.Operation.
  """
    if not util.is_iterable(seed_ops):
        seed_ops = [seed_ops]
    if not seed_ops:
        return []
    if isinstance(seed_ops[0], tf_ops.Tensor):
        ts = util.make_list_of_t(seed_ops, allow_graph=False)
        seed_ops = util.get_generating_ops(ts)
    else:
        seed_ops = util.make_list_of_op(seed_ops, allow_graph=False)

    stop_at_ts = frozenset(util.make_list_of_t(stop_at_ts))
    seed_ops = frozenset(util.make_list_of_op(seed_ops))
    if within_ops:
        within_ops = util.make_list_of_op(within_ops, allow_graph=False)
        within_ops = frozenset(within_ops)
        seed_ops &= within_ops

    def is_within(op):
        return within_ops is None or op in within_ops

    result = list(seed_ops)
    wave = set(seed_ops)
    while wave:
        new_wave = set()
        for op in wave:
            for new_t in op.inputs:
                if new_t in stop_at_ts:
                    continue
                if new_t.op not in result and is_within(new_t.op):
                    new_wave.add(new_t.op)
            if control_inputs:
                for new_op in op.control_inputs:
                    if new_op not in result and is_within(new_op):
                        new_wave.add(new_op)
        util.concatenate_unique(result, new_wave)
        wave = new_wave
    if not inclusive:
        result = [op for op in result if op not in seed_ops]
    return result

def get_forward_walk_ops(seed_ops, inclusive=True, within_ops=None,
                         control_outputs=True):
  """Do a forward graph walk and return all the visited ops.

  Args:
    seed_ops: an iterable of operations from which the forward graph
      walk starts. If a list of tensors is given instead, the seed_ops are set
      to be the consumers of those tensors.
    inclusive: if True the given seed_ops are also part of the resulting set.
    within_ops: an iterable of tf.Operation whithin which the search is
      restricted. If within_ops is None, the search is performed within
      the whole graph.
    control_outputs: an object convertible to a control output dictionary
      (see function util.convert_to_control_outputs for more details).
      If the dictionary can be created, it will be used while walking the graph
      forward.
  Returns:
    A Python set of all the tf.Operation ahead of seed_ops.
  Raises:
    TypeError: if seed_ops or within_ops cannot be converted to a list of
      tf.Operation.
  """
  if not util.is_iterable(seed_ops): seed_ops = [seed_ops]
  if not seed_ops: return set()
  if isinstance(seed_ops[0], tf_ops.Tensor):
    ts = util.make_list_of_t(seed_ops, allow_graph=False)
    seed_ops = get_consuming_ops(ts)
  else:
    seed_ops = util.make_list_of_op(seed_ops, allow_graph=False)

  control_outputs = util.convert_to_control_outputs(seed_ops, control_outputs)

  seed_ops = frozenset(seed_ops)
  if within_ops:
    within_ops = util.make_list_of_op(within_ops, allow_graph=False)
    within_ops = frozenset(within_ops)
    seed_ops &= within_ops
  def is_within(op):
    return within_ops is None or op in within_ops
  result = set(seed_ops)
  wave = set(seed_ops)
  while wave:
    new_wave = set()
    for op in wave:
      for new_t in op.outputs:
        for new_op in new_t.consumers():
          if new_op not in result and is_within(new_op):
            new_wave.add(new_op)
      if control_outputs is not None and op in control_outputs:
        for new_op in control_outputs[op]:
          if new_op not in result and is_within(new_op):
            new_wave.add(new_op)
    result.update(new_wave)
    wave = new_wave
  if not inclusive:
    result.difference_update(seed_ops)
  return result

def get_within_boundary_ops(ops,
                            seed_ops,
                            boundary_ops=(),
                            inclusive=True,
                            control_inputs=False,
                            control_outputs=None,
                            control_ios=None):
  """Return all the `tf.Operation` within the given boundary.

  Args:
    ops: an object convertible to a list of `tf.Operation`. those ops define the
      set in which to perform the operation (if a `tf.Graph` is given, it
      will be converted to the list of all its operations).
    seed_ops: the operations from which to start expanding.
    boundary_ops: the ops forming the boundary.
    inclusive: if `True`, the result will also include the boundary ops.
    control_inputs: A boolean indicating whether control inputs are enabled.
    control_outputs: An instance of `util.ControlOutputs` or `None`. If not
      `None`, control outputs are enabled.
    control_ios:  An instance of `util.ControlOutputs` or `None`. If not
      `None`, both control inputs and control outputs are enabled. This is
      equivalent to set control_inputs to True and control_outputs to
      the `util.ControlOutputs` instance.
  Returns:
    All the `tf.Operation` surrounding the given ops.
  Raises:
    TypeError: if `ops` or `seed_ops` cannot be converted to a list of
      `tf.Operation`.
    ValueError: if the boundary is intersecting with the seeds.
  """
  control_inputs, control_outputs = check_cios(control_inputs, control_outputs,
                                               control_ios)
  ops = util.make_list_of_op(ops)
  seed_ops = util.make_list_of_op(seed_ops, allow_graph=False)
  boundary_ops = set(util.make_list_of_op(boundary_ops))
  res = set(seed_ops)
  if boundary_ops & res:
    raise ValueError("Boundary is intersecting with the seeds.")
  wave = set(seed_ops)
  while wave:
    new_wave = set()
    ops_io = get_ops_ios(wave, control_inputs, control_outputs)
    for op in ops_io:
      if op in res:
        continue
      if op in boundary_ops:
        if inclusive:
          res.add(op)
      else:
        new_wave.add(op)
    res.update(new_wave)
    wave = new_wave
  return [op for op in ops if op in res]

def get_ops_ios(ops, control_inputs=False, control_outputs=None,
                control_ios=None):
  """Return all the tf.Operation which are connected to an op in ops.

  Args:
    ops: an object convertible to a list of tf.Operation.
    control_inputs: A boolean indicating whether control inputs are enabled.
    control_outputs: An instance of util.ControlOutputs or None. If not None,
      control outputs are enabled.
    control_ios:  An instance of util.ControlOutputs or None. If not None, both
      control inputs and control outputs are enabled. This is equivalent to set
      control_inputs to True and control_outputs to the util.ControlOutputs
      instance.
  Returns:
    All the tf.Operation surrounding the given ops.
  Raises:
    TypeError: if ops cannot be converted to a list of tf.Operation.
  """
  control_inputs, control_outputs = check_cios(control_inputs, control_outputs,
                                               control_ios)
  ops = util.make_list_of_op(ops)
  res = []
  for op in ops:
    util.concatenate_unique(res, [t.op for t in op.inputs])
    for t in op.outputs:
      util.concatenate_unique(res, t.consumers())
    if control_outputs is not None:
      util.concatenate_unique(res, control_outputs.get(op))
    if control_inputs:
      util.concatenate_unique(res, op.control_inputs)
  return res

  def __init__(self, inside_ops=(), passthrough_ts=()):
    """Create a subgraph containing the given ops and the "passthrough" tensors.

    Args:
      inside_ops: an object convertible to a list of tf.Operation. This list
        defines all the operations in the subgraph.
      passthrough_ts: an object convertible to a list of tf.Tensor. This list
        define all the "passthrough" tensors. A passthrough tensor is a tensor
        which goes directly from the input of the subgraph to it output, without
        any intermediate operations. All the non passthrough tensors are
        silently ignored.
    Raises:
      TypeError: if inside_ops cannot be converted to a list of tf.Operation or
        if passthrough_ts cannot be converted to a list of tf.Tensor.
    """
    inside_ops = util.make_list_of_op(inside_ops)
    passthrough_ts = util.make_list_of_t(passthrough_ts)
    ops_and_ts = inside_ops + passthrough_ts
    if ops_and_ts:
      self._graph = util.get_unique_graph(ops_and_ts)
    else:
      self._graph = None
    self._ops = inside_ops

    # Compute inside and outside tensor
    inputs, outputs, insides = select.compute_boundary_ts(inside_ops)

    # Compute passthrough tensors, silently ignoring the non-passthrough ones.
    all_tensors = frozenset(inputs + outputs + list(insides))
    self._passthrough_ts = [t for t in passthrough_ts if t not in all_tensors]

    # Set inputs and outputs.
    self._input_ts = inputs + self._passthrough_ts
    self._output_ts = outputs + self._passthrough_ts

def get_within_boundary_ops(ops,
                            seed_ops,
                            boundary_ops,
                            inclusive=True,
                            control_outputs=True):
  """Return all the tf.Operation within the given boundary.

  Args:
    ops: an object convertible to a list of tf.Operation. those ops define the
      set in which to perform the operation (if a tf.Graph is given, it
      will be converted to the list of all its operations).
    seed_ops: the operations from which to start expanding.
    boundary_ops: the ops forming the boundary.
    inclusive: if True, the result will also include the boundary ops.
    control_outputs: an object convertible to a control output dictionary
      (or None). If the dictionary can be created, it will be used while
      expanding.
  Returns:
    All the tf.Operation surrounding the given ops.
  Raises:
    TypeError: if ops or seed_ops cannot be converted to a list of tf.Operation.
    ValueError: if the boundary is intersecting with the seeds.
  """
  ops = util.make_list_of_op(ops)
  control_outputs = util.convert_to_control_outputs(ops, control_outputs)
  seed_ops = util.make_list_of_op(seed_ops, allow_graph=False)
  boundary_ops = set(util.make_list_of_op(boundary_ops))
  res = set(seed_ops)
  if boundary_ops & res:
    raise ValueError("Boundary is intersecting with the seeds.")
  wave = set(seed_ops)
  while wave:
    new_wave = set()
    ops_io = get_ops_ios(wave, control_outputs)
    for op in ops_io:
      if op in res:
        continue
      if op in boundary_ops:
        if inclusive:
          res.add(op)
      else:
        new_wave.add(op)
    res.update(new_wave)
    wave = new_wave
  return res

def compute_boundary_ts(ops):
  """Compute the tensors at the boundary of a set of ops.

  This function looks at all the tensors connected to the given ops (in/out)
  and classify them into three categories:
  1) input tensors: tensors whose generating operation is not in ops.
  2) output tensors: tensors whose consumer operations are not in ops
  3) inside tensors: tensors which are neither input nor output tensors.

  Note that a tensor can be both an inside tensor and an output tensor if it is
  consumed by operations both outside and inside of `ops`.

  Args:
    ops: an object convertible to a list of tf.Operation.
  Returns:
    A tuple `(outside_input_ts, outside_output_ts, inside_ts)` where:
      `outside_input_ts` is a Python list of input tensors;
      `outside_output_ts` is a python list of output tensors;
      `inside_ts` is a python list of inside tensors.
    Since a tensor can be both an inside tensor and an output tensor,
    `outside_output_ts` and `inside_ts` might intersect.
  Raises:
    TypeError: if ops cannot be converted to a list of tf.Operation.
  """
  ops = util.make_list_of_op(ops)
  input_ts = _get_input_ts(ops)
  output_ts = _get_output_ts(ops)
  output_ts_set = frozenset(output_ts)
  ops_set = frozenset(ops)

  # Compute inside tensors.
  inside_ts = []
  only_inside_ts = []
  for t in input_ts:
    # Skip if the input tensor is not also an output tensor.
    if t not in output_ts_set:
      continue
    # Mark as "inside".
    inside_ts.append(t)
    # Mark as "only inside" if the tensor is not both inside and output.
    consumers = frozenset(t.consumers())
    if consumers - ops_set:
      continue
    only_inside_ts.append(t)

  inside_ts_set = frozenset(inside_ts)
  only_inside_ts_set = frozenset(only_inside_ts)
  outside_output_ts = [t for t in output_ts if t not in only_inside_ts_set]
  outside_input_ts = [t for t in input_ts if t not in inside_ts_set]
  return outside_input_ts, outside_output_ts, inside_ts

def filter_ops(ops, positive_filter=None):
  """Get the ops passing the given filter.

  Args:
    ops: an object convertible to a list of tf.Operation.
    positive_filter: a function deciding where to keep an operation or not.
  Returns:
    A list of selected tf.Operation.
  Raises:
    TypeError: if ops cannot be converted to a list of tf.Operation.
  """
  ops = util.make_list_of_op(ops)
  if positive_filter is not None:
    ops = [op for op in ops if positive_filter(op)]
  return ops

def _get_output_ts(ops):
  """Compute the list of unique output tensors of all the op in ops.

  Args:
    ops: an object convertible to a list of tf.Operation.
  Returns:
    The list of unique output tensors of all the op in ops.
  Raises:
    TypeError: if ops cannot be converted to a list of tf.Operation.
  """
  ops = util.make_list_of_op(ops)
  ts = []
  for op in ops:
    ts += op.outputs
  return ts

def get_output_ts(ops):
  """Compute the set of output tensors of all the op in ops.

  Args:
    ops: an object convertible to a list of tf.Operation.
  Returns:
    The set of output tensors of all the op in ops.
  Raises:
    TypeError: if ops cannot be converted to a list of tf.Operation.
  """
  ops = util.make_list_of_op(ops)
  ts = set()
  for op in ops:
    ts.update(op.outputs)
  return ts

def filter_ops(ops, positive_filter):
  """Get the ops passing the given filter.

  Args:
    ops: an object convertible to a list of tf.Operation.
    positive_filter: a function deciding where to keep an operation or not.
      If True, all the operations are returned.
  Returns:
    A list of selected tf.Operation.
  Raises:
    TypeError: if ops cannot be converted to a list of tf.Operation.
  """
  ops = util.make_list_of_op(ops)
  if positive_filter is not True:  # pylint: disable=g-explicit-bool-comparison
    ops = [op for op in ops if positive_filter(op)]
  return ops

def filter_ops_from_regex(ops, regex):
  """Get all the operations that match the given regex.

  Args:
    ops: an object convertible to a list of tf.Operation.
    regex: a regular expression matching the operation's name.
      For example, "^foo(/.*)?$" will match all the operations in the "foo"
      scope.
  Returns:
    A list of tf.Operation.
  Raises:
    TypeError: if ops cannot be converted to a list of tf.Operation.
  """
  ops = util.make_list_of_op(ops)
  regex_obj = _make_regex(regex)
  return filter_ops(ops, lambda op: regex_obj.search(op.name))

def filter_ts_from_regex(ops, regex):
  r"""Get all the tensors linked to ops that match the given regex.

  Args:
    ops: an object convertible to a list of tf.Operation.
    regex: a regular expression matching the tensors' name.
      For example, "^foo(/.*)?:\d+$" will match all the tensors in the "foo"
      scope.
  Returns:
    A list of tf.Tensor.
  Raises:
    TypeError: if ops cannot be converted to a list of tf.Operation.
  """
  ops = util.make_list_of_op(ops)
  regex_obj = make_regex(regex)
  return filter_ts(ops, positive_filter=lambda op: regex_obj.search(op.name))

def compute_boundary_ts(ops, ambiguous_ts_are_outputs=True):
  """Compute the tensors at the boundary of a set of ops.

  This function looks at all the tensors connected to the given ops (in/out)
  and classify them into three categories:
  1) input tensors: tensors whose generating operation is not in ops.
  2) output tensors: tensors whose consumer operations are not in ops
  3) inside tensors: tensors which are neither input nor output tensors.

  Args:
    ops: an object convertible to a list of tf.Operation.
    ambiguous_ts_are_outputs: a tensor can have consumers both inside and
      outside ops. Such tensors are treated as outside tensor if
      ambiguous_ts_are_outputs is True, otherwise they are treated as
      inside tensor.
  Returns:
    A tuple `(outside_input_ts, outside_output_ts, inside_ts)` where:
      `outside_input_ts` is a Python list of input tensors;
      `outside_output_ts` is a python list of output tensors;
      `inside_ts` is a python list of inside tensors.
  Raises:
    TypeError: if ops cannot be converted to a list of tf.Operation.
  """
  ops = util.make_list_of_op(ops)
  input_ts = _get_input_ts(ops)
  output_ts = _get_output_ts(ops)
  output_ts_set = frozenset(output_ts)
  ops_set = frozenset(ops)

  # fill in inside
  inside_ts = []
  for t in input_ts:
    # is also output?
    if t not in output_ts_set:
      continue
    # is ambiguous_ts_are_outputs is True, don't add to inside if ambiguous
    if ambiguous_ts_are_outputs:
      consumers = frozenset(t.consumers())
      if consumers - ops_set:
        continue
    inside_ts.append(t)

  inside_ts_set = frozenset(inside_ts)
  outside_input_ts = [t for t in input_ts if t not in inside_ts_set]
  outside_output_ts = [t for t in output_ts if t not in inside_ts_set]
  return outside_input_ts, outside_output_ts, inside_ts

def compute_boundary_ts(ops, keep_order=False, ambiguous_are_outputs=True):
  """Compute the tensors at the boundary of a set of ops.

  This function looks at all the tensors connected to the given ops (in/out)
  and classify them into three categories:
  1) input tensors: tensors whose generating operation is not in ops.
  2) output tensors: tensors whose consumer operations are not in ops
  3) inside tensors: tensors which are neither input nor output tensors.

  Args:
    ops: an object convertible to a list of tf.Operation.
    keep_order: if True use ops to determine the order of the resulting input
      and output tensors.
    ambiguous_are_outputs: a tensor can have consumers both inside and outside
      ops. Such tensors are treated as outside tensor if inside_output_as_output
      is True, otherwise they are treated as inside tensor.
  Returns:
    A Python set (list if keep_order is True) of input tensors.
    A Python set (list if keep_order is True) of output tensors.
    A Python set of inside tensors.
  Raises:
    TypeError: if ops cannot be converted to a list of tf.Operation.
  """
  ops = util.make_list_of_op(ops)
  input_tensors = get_input_ts(ops)
  output_tensors = get_output_ts(ops)
  inside_tensors = input_tensors & output_tensors
  # deal with ambiguous tensors
  if ambiguous_are_outputs:
    inside_and_output_tensors = set()
    for t in inside_tensors:
      for op in t.consumers():
        if op not in ops:
          inside_and_output_tensors.add(t)
          break
    output_tensors |= inside_and_output_tensors
    inside_tensors -= inside_and_output_tensors
  outside_input_tensors = input_tensors - inside_tensors
  outside_output_tensors = output_tensors - inside_tensors
  if keep_order:
    outside_input_tensors = [t for t in input_tensors
                             if t in outside_input_tensors]
    outside_output_tensors = [t for t in output_tensors
                              if t in outside_output_tensors]
  return outside_input_tensors, outside_output_tensors, inside_tensors

def filter_ts(ops, positive_filter):
  """Get all the tensors which are input or output of an op in ops.

  Args:
    ops: an object convertible to a list of tf.Operation.
    positive_filter: a function deciding whether to keep a tensor or not.
      If True, all the tensors are returned.
  Returns:
    A list of tf.Tensor.
  Raises:
    TypeError: if ops cannot be converted to a list of tf.Operation.
  """
  ops = util.make_list_of_op(ops)
  ts = _get_input_ts(ops)
  util.concatenate_unique(ts, _get_output_ts(ops))
  if positive_filter is not True:
    ts = [t for t in ts if positive_filter(t)]
  return ts

def filter_ts(ops, positive_filter=None):
  """Get all the tensors which are input or output of an op in ops.

  Args:
    ops: an object convertible to a list of tf.Operation.
    positive_filter: a function deciding whether to keep a tensor or not.
  Returns:
    A list of tf.Tensor.
  Raises:
    TypeError: if ops cannot be converted to a list of tf.Operation.
  """
  ops = util.make_list_of_op(ops)
  tensors = set()
  tensors.update(get_input_ts(ops))
  tensors.update(get_output_ts(ops))
  if positive_filter is not None:
    tensors = [t for t in tensors if positive_filter(t)]
  return tensors

def _get_input_ts(ops):
  """Compute the list of unique input tensors of all the op in ops.

  Args:
    ops: an object convertible to a list of tf.Operation.
  Returns:
    The list of unique input tensors of all the op in ops.
  Raises:
    TypeError: if ops cannot be converted to a list of tf.Operation.
  """
  ops = util.make_list_of_op(ops)
  ts = []
  ts_set = set()
  for op in ops:
    for t in op.inputs:
      if t not in ts_set:
        ts.append(t)
        ts_set.add(t)
  return ts

def add_control_inputs(op, cops):
  """Add the control inputs cops to op.

  Warning: this function is directly manipulating the internals of the tf.Graph.

  Args:
    op: a tf.Operation to which the control inputs are added.
    cops: an object convertible to a list of `tf.Operation`.
  Raises:
    TypeError: if op is not a tf.Operation
    ValueError: if any cop in cops is already a control input of op.
  """
  if not isinstance(op, _tf_ops.Operation):
    raise TypeError("Expected a tf.Operation, got: {}", type(op))
  cops = _util.make_list_of_op(cops, allow_graph=False)
  for cop in cops:
    if cop in op.control_inputs:
      raise ValueError("{} is already a control_input of {}".format(cop.name,
                                                                    op.name))
  op._add_control_inputs(cops)  # pylint: disable=protected-access

def remove_control_inputs(op, cops):
    """Remove the control inputs cops from co.

  Warning: this function is directly manipulating the internals of the
  `tf.Graph`.

  Args:
    op: a `tf.Operation` from which to remove the control inputs.
    cops: an object convertible to a list of `tf.Operation`.
  Raises:
    TypeError: if op is not a `tf.Operation`.
    ValueError: if any cop in cops is not a control input of op.
  """
    if not isinstance(op, tf_ops.Operation):
        raise TypeError("Expected a tf.Operation, got: {}", type(op))
    cops = util.make_list_of_op(cops, allow_graph=False)
    for cop in cops:
        if cop not in op.control_inputs:
            raise ValueError("{} is not a control_input of {}".format(op.name, cop.name))
    # pylint: disable=protected-access
    op._control_inputs = [cop for cop in op._control_inputs if cop not in cops]
    op._recompute_node_def()