lookup_ops.py

# Copyright 2016 The TensorFlow Authors. All Rights Reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
# ==============================================================================
"""Lookup table operations."""

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

import collections
import functools

from tensorflow.python.framework import dtypes
from tensorflow.python.framework import ops
from tensorflow.python.framework import sparse_tensor
from tensorflow.python.framework import tensor_shape
from tensorflow.python.ops import array_ops
from tensorflow.python.ops import control_flow_ops
from tensorflow.python.ops import gen_lookup_ops
from tensorflow.python.ops import math_ops
from tensorflow.python.ops import string_ops
from tensorflow.python.training.saver import BaseSaverBuilder
from tensorflow.python.util import compat
from tensorflow.python.util.deprecation import deprecated


class LookupInterface(object):
  """Represent a lookup table that persists across different steps."""

  def __init__(self, key_dtype, value_dtype, name):
    """Construct a lookup table interface.

    Args:
      key_dtype: The table key type.
      value_dtype: The table value type.
      name: A name for the operation (optional).
    """
    self._key_dtype = dtypes.as_dtype(key_dtype)
    self._value_dtype = dtypes.as_dtype(value_dtype)
    self._name = name

  @property
  def key_dtype(self):
    """The table key dtype."""
    return self._key_dtype

  @property
  def value_dtype(self):
    """The table value dtype."""
    return self._value_dtype

  @property
  def name(self):
    """The name of the table."""
    return self._name

  @property
  def init(self):
    """The table initialization op."""
    raise NotImplementedError

  def size(self, name=None):
    """Compute the number of elements in this table."""
    raise NotImplementedError

  def lookup(self, keys, name=None):
    """Looks up `keys` in a table, outputs the corresponding values."""
    raise NotImplementedError

  def check_table_dtypes(self, key_dtype, value_dtype):
    """Check that the given key_dtype and value_dtype matches the table dtypes.

    Args:
      key_dtype: The key data type to check.
      value_dtype: The value data type to check.

    Raises:
      TypeError: when 'key_dtype' or 'value_dtype' doesn't match the table data
        types.
    """
    if key_dtype != self.key_dtype:
      raise TypeError("Invalid key dtype, expected %s but got %s." %
                      (self.key_dtype, key_dtype))
    if value_dtype != self.value_dtype:
      raise TypeError("Invalid value dtype, expected %s but got %s." %
                      (self.value_dtype, value_dtype))


class InitializableLookupTableBase(LookupInterface):
  """Initializable lookup table interface.

  An initializable lookup tables persist across different steps.
  """

  def __init__(self, table_ref, default_value, initializer):
    """Construct a table object from a table reference.

    If requires a table initializer object (subclass of `TableInitializerBase`).
    It provides the table key and value types, as well as the op to initialize
    the table. The caller is responsible to execute the initialization op.

    Args:
      table_ref: The table reference, i.e. the output of the lookup table ops.
      default_value: The value to use if a key is missing in the table.
      initializer: The table initializer to use.
    """
    super(InitializableLookupTableBase, self).__init__(
        initializer.key_dtype, initializer.value_dtype,
        table_ref.op.name.split("/")[-1])
    self._table_ref = table_ref
    self._default_value = ops.convert_to_tensor(default_value,
                                                dtype=self._value_dtype)
    self._default_value.get_shape().merge_with(tensor_shape.scalar())
    self._init = initializer.initialize(self)

  @property
  def table_ref(self):
    """Get the underlying table reference."""
    return self._table_ref

  @property
  def default_value(self):
    """The default value of the table."""
    return self._default_value

  @property
  def init(self):
    """The table initialization op."""
    return self._init

  def size(self, name=None):
    """Compute the number of elements in this table.

    Args:
      name: A name for the operation (optional).

    Returns:
      A scalar tensor containing the number of elements in this table.
    """
    with ops.name_scope(name, "%s_Size" % self._name,
                        [self._table_ref]) as scope:
      # pylint: disable=protected-access
      return gen_lookup_ops._lookup_table_size(self._table_ref, name=scope)
      # pylint: enable=protected-access

  def lookup(self, keys, name=None):
    """Looks up `keys` in a table, outputs the corresponding values.

    The `default_value` is used for keys not present in the table.

    Args:
      keys: Keys to look up. May be either a `SparseTensor` or dense `Tensor`.
      name: A name for the operation (optional).

    Returns:
      A `SparseTensor` if keys are sparse, otherwise a dense `Tensor`.

    Raises:
      TypeError: when `keys` or `default_value` doesn't match the table data
        types.
    """
    key_tensor = keys
    if isinstance(keys, sparse_tensor.SparseTensor):
      key_tensor = keys.values

    if keys.dtype != self._key_dtype:
      raise TypeError("Signature mismatch. Keys must be dtype %s, got %s." %
                      (self._key_dtype, keys.dtype))

    with ops.name_scope(
        name, "%s_Lookup" % self._name,
        (self._table_ref, key_tensor, self._default_value)) as scope:
      # pylint: disable=protected-access
      values = gen_lookup_ops._lookup_table_find(
          self._table_ref, key_tensor, self._default_value, name=scope)
      # pylint: enable=protected-access

    values.set_shape(key_tensor.get_shape())
    if isinstance(keys, sparse_tensor.SparseTensor):
      return sparse_tensor.SparseTensor(keys.indices, values, keys.dense_shape)
    else:
      return values


class HashTable(InitializableLookupTableBase):
  """A generic hash table implementation.

  Example usage:

  ```python
  table = tf.contrib.lookup.HashTable(
      tf.contrib.lookup.KeyValueTensorInitializer(keys, values), -1)
  out = table.lookup(input_tensor).
  table.init.run()
  print out.eval()
  ```
  """

  def __init__(self, initializer, default_value, shared_name=None, name=None):
    """Creates a non-initialized `HashTable` object.

    Creates a table, the type of its keys and values are specified by the
    initializer.
    Before using the table you will have to initialize it. After initialization
    the table will be immutable.

    Args:
      initializer: The table initializer to use. See `HashTable` kernel for
        supported key and value types.
      default_value: The value to use if a key is missing in the table.
      shared_name: If non-empty, this table will be shared under
        the given name across multiple sessions.
      name: A name for the operation (optional).

    Returns:
      A `HashTable` object.
    """
    with ops.name_scope(
        name, "hash_table", (initializer, default_value)) as scope:
      # pylint: disable=protected-access
      table_ref = gen_lookup_ops._hash_table(
          shared_name=shared_name,
          key_dtype=initializer.key_dtype,
          value_dtype=initializer.value_dtype,
          name=scope)
      # pylint: enable=protected-access

      super(HashTable, self).__init__(table_ref, default_value, initializer)


class TableInitializerBase(object):
  """Base class for lookup table initializers."""

  def __init__(self, key_dtype, value_dtype):
    """Construct a table initializer object.

    Args:
      key_dtype: Type of the table keys.
      value_dtype: Type of the table values.
    """
    self._key_dtype = dtypes.as_dtype(key_dtype)
    self._value_dtype = dtypes.as_dtype(value_dtype)

  @property
  def key_dtype(self):
    """The expected table key dtype."""
    return self._key_dtype

  @property
  def value_dtype(self):
    """The expected table value dtype."""
    return self._value_dtype

  def initialize(self, table):
    """Returns the table initialization op."""
    raise NotImplementedError


class KeyValueTensorInitializer(TableInitializerBase):
  """Table initializers given `keys` and `values` tensors."""

  def __init__(self, keys, values, key_dtype=None, value_dtype=None, name=None):
    """Constructs a table initializer object based on keys and values tensors.

    Args:
      keys: The tensor for the keys.
      values: The tensor for the values.
      key_dtype: The `keys` data type. Used when `keys` is a python array.
      value_dtype: The `values` data type. Used when `values` is a python array.
      name: A name for the operation (optional).
    """
    with ops.name_scope(name, "key_value_init", [keys, values]) as scope:
      self._keys = ops.convert_to_tensor(keys, dtype=key_dtype, name="keys")
      self._values = ops.convert_to_tensor(values,
                                           dtype=value_dtype,
                                           name="values")
      self._name = scope

    super(KeyValueTensorInitializer, self).__init__(self._keys.dtype,
                                                    self._values.dtype)

  def initialize(self, table):
    """Initializes the given `table` with `keys` and `values` tensors.

    Args:
      table: The table to initialize.

    Returns:
      The operation that initializes the table.

    Raises:
      TypeError: when the keys and values data types do not match the table
      key and value data types.
    """
    table.check_table_dtypes(self._keys.dtype, self._values.dtype)
    with ops.name_scope(
        self._name,
        values=(table.table_ref, self._keys, self._values)) as scope:
      # pylint: disable=protected-access
      init_op = gen_lookup_ops._initialize_table(
          table.table_ref, self._keys, self._values, name=scope)
      # pylint: enable=protected-access
    ops.add_to_collection(ops.GraphKeys.TABLE_INITIALIZERS, init_op)
    return init_op


class TextFileIndex(object):
  WHOLE_LINE = -2
  LINE_NUMBER = -1


class TextFileInitializer(TableInitializerBase):
  """Table initializers from a text file.

  This initializer assigns one entry in the table for each line in the file.

  The key and value type of the table to initialize is given by `key_dtype` and
  `value_dtype`.

  The key and value content to get from each line is specified by
  the `key_index` and `value_index`.

  * `TextFileIndex.LINE_NUMBER` means use the line number starting from zero,
    expects data type int64.
  * `TextFileIndex.WHOLE_LINE` means use the whole line content, expects data
    type string.
  * A value `>=0` means use the index (starting at zero) of the split line based
      on `delimiter`.

  For example if we have a file with the following content:

  ```
  emerson 10
  lake 20
  palmer 30
  ```

  The following snippet initializes a table with the first column as keys and
  second column as values:

  * `emerson -> 10`
  * `lake -> 20`
  * `palmer -> 30`

  ```python
  table = tf.contrib.lookup.HashTable(tf.contrib.lookup.TextFileInitializer(
      "test.txt", tf.string, 0, tf.int64, 1, delimiter=" "), -1)
  ...
  table.init.run()
  ```

  Similarly to initialize the whole line as keys and the line number as values.

  * `emerson 10 -> 0`
  * `lake 20 -> 1`
  * `palmer 30 -> 2`

  ```python
  table = tf.contrib.lookup.HashTable(tf.contrib.lookup.TextFileInitializer(
      "test.txt", tf.string, tf.contrib.lookup.TextFileIndex.WHOLE_LINE,
      tf.int64, tf.contrib.lookup.TextFileIndex.LINE_NUMBER, delimiter=" "), -1)
  ...
  table.init.run()
  ```
  """

  def __init__(self,
               filename,
               key_dtype,
               key_index,
               value_dtype,
               value_index,
               vocab_size=None,
               delimiter="\t",
               name=None):
    """Constructs a table initializer object to populate from a text file.

    It generates one key-value pair per line. The type of table key and
    value are specified by `key_dtype` and `value_dtype`, respectively.
    Similarly the content of the key and value are specified by the key_index
    and value_index.

    - TextFileIndex.LINE_NUMBER means use the line number starting from zero,
      expects data type int64.
    - TextFileIndex.WHOLE_LINE means use the whole line content, expects data
      type string.
    - A value >=0 means use the index (starting at zero) of the split line based
      on `delimiter`.

    Args:
      filename: The filename of the text file to be used for initialization.
        The path must be accessible from wherever the graph is initialized
        (eg. trainer or eval workers). The filename may be a scalar `Tensor`.
      key_dtype: The `key` data type.
      key_index: the index that represents information of a line to get the
        table 'key' values from.
      value_dtype: The `value` data type.
      value_index: the index that represents information of a line to get the
        table 'value' values from.'
      vocab_size: The number of elements in the file, if known.
      delimiter: The delimiter to separate fields in a line.
      name: A name for the operation (optional).

    Raises:
      ValueError: when the filename is empty, or when the table key and value
      data types do not match the expected data types.
    """
    if not isinstance(filename, ops.Tensor) and not filename:
      raise ValueError("Filename required for %s." % name)

    key_dtype = dtypes.as_dtype(key_dtype)
    value_dtype = dtypes.as_dtype(value_dtype)

    if key_index < -2:
      raise ValueError("Invalid key index %s." % (key_index))

    if key_index == TextFileIndex.LINE_NUMBER and key_dtype != dtypes.int64:
      raise ValueError("Signature mismatch. Keys must be dtype %s, got %s." %
                       (dtypes.int64, key_dtype))
    if ((key_index == TextFileIndex.WHOLE_LINE) and
        (not key_dtype.is_integer) and (key_dtype != dtypes.string)):
      raise ValueError(
          "Signature mismatch. Keys must be integer or string, got %s." %
          key_dtype)
    if value_index < -2:
      raise ValueError("Invalid value index %s." % (value_index))

    if value_index == TextFileIndex.LINE_NUMBER and value_dtype != dtypes.int64:
      raise ValueError("Signature mismatch. Values must be dtype %s, got %s." %
                       (dtypes.int64, value_dtype))
    if value_index == TextFileIndex.WHOLE_LINE and value_dtype != dtypes.string:
      raise ValueError("Signature mismatch. Values must be dtype %s, got %s." %
                       (dtypes.string, value_dtype))

    if (vocab_size is not None) and (vocab_size <= 0):
      raise ValueError("Invalid vocab_size %s." % vocab_size)

    self._filename = filename
    self._key_index = key_index
    self._value_index = value_index
    self._vocab_size = vocab_size
    self._delimiter = delimiter
    self._name = name

    super(TextFileInitializer, self).__init__(key_dtype, value_dtype)

  def initialize(self, table):
    """Initializes the table from a text file.

    Args:
      table: The table to be initialized.

    Returns:
      The operation that initializes the table.

    Raises:
      TypeError: when the keys and values data types do not match the table
      key and value data types.
    """
    table.check_table_dtypes(self.key_dtype, self.value_dtype)
    with ops.name_scope(
        self._name, "text_file_init", (table.table_ref,)) as scope:
      filename = ops.convert_to_tensor(self._filename,
                                       dtypes.string,
                                       name="asset_filepath")
      # pylint: disable=protected-access
      init_op = gen_lookup_ops._initialize_table_from_text_file(
          table.table_ref,
          filename,
          self._key_index,
          self._value_index,
          -1 if self._vocab_size is None else self._vocab_size,
          self._delimiter,
          name=scope)
      # pylint: enable=protected-access
    ops.add_to_collection(ops.GraphKeys.TABLE_INITIALIZERS, init_op)
    ops.add_to_collection(ops.GraphKeys.ASSET_FILEPATHS, filename)
    return init_op


class TextFileStringTableInitializer(TextFileInitializer):
  """Table initializer for `int64` IDs to string tables from a text file."""

  def __init__(self,
               filename,
               key_column_index=TextFileIndex.LINE_NUMBER,
               value_column_index=TextFileIndex.WHOLE_LINE,
               vocab_size=None,
               delimiter="\t",
               name="text_file_string_table_init"):
    """Constructs an initializer for an id-to-string table from a text file.

    It populates a table that its key and value types are int64 and string,
    respectively. It generates one key-value pair per line.
    The content of the key and value are specified by `key_column_index`
    and `value_column_index`.

    - TextFileIndex.LINE_NUMBER means use the line number starting from zero,
      expects data type int64.
    - TextFileIndex.WHOLE_LINE means use the whole line content, expects data
      type string.
    - A value >=0 means use the index (starting at zero) of the split line based
      on `delimiter`.

    Args:
      filename: The filename of the text file to be used for initialization.
        The path must be accessible from wherever the graph is initialized
        (eg. trainer or eval workers). The filename may be a scalar `Tensor`.
      key_column_index: The column index from the text file to get the keys
        from. The default is 0 that represents the whole line content.
      value_column_index: The column index from the text file to get the
        values from. The default is to use the line number, starting from zero.
      vocab_size: The number of elements in the file, if known.
      delimiter: The delimiter to separate fields in a line.
      name: Optional name for the op.

    Raises:
      TypeError: when the filename is empty, or when the table key and value
      data types do not match the expected data types.
    """
    super(TextFileStringTableInitializer, self).__init__(filename,
                                                         dtypes.int64,
                                                         key_column_index,
                                                         dtypes.string,
                                                         value_column_index,
                                                         vocab_size=vocab_size,
                                                         delimiter=delimiter,
                                                         name=name)


class TextFileIdTableInitializer(TextFileInitializer):
  """Table initializer for string to `int64` IDs tables from a text file."""

  def __init__(self,
               filename,
               key_column_index=TextFileIndex.WHOLE_LINE,
               value_column_index=TextFileIndex.LINE_NUMBER,
               vocab_size=None,
               delimiter="\t",
               name="text_file_id_table_init",
               key_dtype=dtypes.string):
    """Constructs an initializer for an string-to-id table from a text file.

    It populates a table that its key and value types are string and int64,
    respectively. It generates one key-value pair per line.
    The content of the key and value are specified by the key_index
    and value_index.

    - TextFileIndex.LINE_NUMBER means use the line number starting from zero,
      expects data type int64.
    - TextFileIndex.WHOLE_LINE means use the whole line content, expects data
      type string.
    - A value >=0 means use the index (starting at zero) of the split line based
      on `delimiter`.

    Args:
      filename: The filename of the text file to be used for initialization.
        The path must be accessible from wherever the graph is initialized
        (eg. trainer or eval workers). The filename may be a scalar `Tensor`.
      key_column_index: The column index from the text file to get the `key`
        values from. The default is to use the line number, starting from zero.
      value_column_index: The column index from the text file ro get the `value`
        values from. The default is 0 that represents the whole line content.
      vocab_size: The number of elements in the file, if known.
      delimiter: The delimiter to separate fields in a line.
      name: Optional name for the op.
      key_dtype: The `key` data type.

    Raises:
      TypeError: when the filename is empty, or when the table key and value
      data types do not match the expected data types.
    """
    super(TextFileIdTableInitializer, self).__init__(filename,
                                                     key_dtype,
                                                     key_column_index,
                                                     dtypes.int64,
                                                     value_column_index,
                                                     vocab_size=vocab_size,
                                                     delimiter=delimiter,
                                                     name=name)


class HasherSpec(collections.namedtuple("HasherSpec", ["hasher", "key"])):
  """A structure for the spec of the hashing function to use for hash buckets.

  `hasher` is the name of the hashing function to use (eg. "fasthash",
  "stronghash").
  `key` is optional and specify the key to use for the hash function if
  supported, currently only used by a strong hash.

  Fields:
    hasher: The hasher name to use.
    key: The key to be used by the hashing function, if required.
  """
  __slots__ = ()


FastHashSpec = HasherSpec("fasthash", None)  # pylint: disable=invalid-name


class StrongHashSpec(HasherSpec):
  """A structure to specify a key of the strong keyed hash spec.

  The strong hash requires a `key`, which is a list of 2 unsigned integer
  numbers. These should be non-zero; random numbers generated from random.org
  would be a fine choice.

  Fields:
    key: The key to be used by the keyed hashing function.
  """
  __slots__ = ()

  def __new__(cls, key):
    if len(key) != 2:
      raise ValueError("key must have size 2, got %s." % len(key))

    if not isinstance(key[0], compat.integral_types) or not isinstance(
        key[1], compat.integral_types):
      raise TypeError("Invalid key %s. Must be unsigned integer values." % key)

    return super(cls, StrongHashSpec).__new__(cls, "stronghash", key)


def _as_string(tensor):
  if dtypes.string == tensor.dtype.base_dtype:
    return tensor
  return string_ops.as_string(tensor)


class IdTableWithHashBuckets(LookupInterface):
  """String to Id table wrapper that assigns out-of-vocabulary keys to buckets.

  For example, if an instance of `IdTableWithHashBuckets` is initialized with a
  string-to-id table that maps:
  - emerson -> 0
  - lake -> 1
  - palmer -> 2

  The `IdTableWithHashBuckets` object will performs the following mapping:
  - emerson -> 0
  - lake -> 1
  - palmer -> 2
  - <other term> -> bucket id between 3 and 3 + num_oov_buckets, calculated by:
    hash(<term>) % num_oov_buckets + vocab_size

  If input_tensor is ["emerson", "lake", "palmer", "king", "crimson"],
  the lookup result is [0, 1, 2, 4, 7]

  If `table` is None, only out-of-vocabulary buckets are used.

  Example usage:

  ```python
  num_oov_buckets = 3
  input_tensor = tf.constant(["emerson", "lake", "palmer", "king", "crimnson"])
  table = tf.IdTableWithHashBuckets(
      tf.HashTable(tf.TextFileIdTableInitializer(filename), default_value),
      num_oov_buckets)
  out = table.lookup(input_tensor).
  table.init.run()
  print out.eval()
  ```

  The hash function used for generating out-of-vocabulary buckets ID is handled
  by `hasher_spec`.
  """

  def __init__(self,
               table,
               num_oov_buckets,
               hasher_spec=FastHashSpec,
               name=None,
               key_dtype=None):
    """Construct a `IdTableWithHashBuckets` object.

    Args:
      table: Table that maps `tf.string` or `tf.int64` keys to `tf.int64` ids.
      num_oov_buckets: Number of buckets to use for out-of-vocabulary keys.
      hasher_spec: A `HasherSpec` to specify the hash function to use for
        assignation of out-of-vocabulary buckets  (optional).
      name: A name for the operation (optional).
      key_dtype: Data type of keys passed to `lookup`. Defaults to
        `table.key_dtype` if `table` is specified, otherwise `tf.string`.
        Must be string or integer, and must be castable to `table.key_dtype`.

    Raises:
      ValueError: when `table` in None and `num_oov_buckets` is not positive.
      TypeError: when `hasher_spec` is invalid.
    """
    # If a name ends with a '/' it is a "name scope", remove all trailing '/'
    # characters to use as table name.
    if name:
      name = name.rstrip("/")
    if table:
      if key_dtype is None:
        key_dtype = table.key_dtype
      supported_table_key_dtypes = (dtypes.int64, dtypes.string)
      if table.key_dtype not in supported_table_key_dtypes:
        raise TypeError("Invalid key dtype, expected one of %s, but got %s." %
                        (supported_table_key_dtypes, key_dtype))
      if table.key_dtype.is_integer != key_dtype.is_integer:
        raise TypeError("Invalid key dtype, expected %s but got %s." %
                        ("integer" if key_dtype.is_integer else "non-integer",
                         table.key_dtype))
      if table.value_dtype != dtypes.int64:
        raise TypeError("Invalid value dtype, expected %s but got %s." %
                        (dtypes.int64, table.value_dtype))
      self._table = table
      name = name or self._table.name
    else:
      if num_oov_buckets <= 0:
        raise ValueError("oov_buckets must be > 0 if no table is supplied.")
      key_dtype = dtypes.string if key_dtype is None else key_dtype
      self._table = None
      name = name or "hash_bucket"
    if (not key_dtype.is_integer) and (dtypes.string != key_dtype):
      raise TypeError(
          "Invalid key_dtype, expected integer or string, got %s." % key_dtype)
    self._num_oov_buckets = num_oov_buckets

    if not isinstance(hasher_spec, HasherSpec):
      raise TypeError("hasher_spec must be of type HasherSpec, got %s" %
                      hasher_spec)
    self._hasher_spec = hasher_spec
    super(IdTableWithHashBuckets, self).__init__(key_dtype, dtypes.int64,
                                                 name.split("/")[-1])

  @property
  def init(self):
    """The table initialization op."""
    if self._table:
      return self._table.init
    with ops.name_scope(None, "init"):
      return control_flow_ops.no_op()

  def size(self, name=None):
    """Compute the number of elements in this table."""
    with ops.name_scope(name, "%s_Size" % self.name) as scope:
      if self._table:
        tsize = self._table.size(scope)
      else:
        tsize = ops.convert_to_tensor(0, dtype=dtypes.int64)
      return tsize + self._num_oov_buckets

  def _get_string_to_hash_bucket_fn(self, hasher_spec):
    """Returns the string_to_hash_bucket op to use based on `hasher_spec`."""
    if not isinstance(hasher_spec, HasherSpec):
      raise TypeError("hasher_spec must be of type HasherSpec %s" % hasher_spec)
    if hasher_spec.hasher == "fasthash":
      return string_ops.string_to_hash_bucket_fast
    if hasher_spec.hasher == "legacy":
      return string_ops.string_to_hash_bucket
    if hasher_spec.hasher == "stronghash":
      return functools.partial(
          string_ops.string_to_hash_bucket_strong, key=hasher_spec.key)
    raise ValueError("Unknown hasher %s" % hasher_spec.hasher)

  def lookup(self, keys, name=None):
    """Looks up `keys` in the table, outputs the corresponding values.

    It assigns out-of-vocabulary keys to buckets based in their hashes.

    Args:
      keys: Keys to look up. May be either a `SparseTensor` or dense `Tensor`.
      name: Optional name for the op.

    Returns:
      A `SparseTensor` if keys are sparse, otherwise a dense `Tensor`.

    Raises:
      TypeError: when `keys` doesn't match the table key data type.
    """
    if keys.dtype != self._key_dtype:
      raise TypeError("Signature mismatch. Keys must be dtype %s, got %s." %
                      (self._key_dtype, keys.dtype))
    values = keys
    if isinstance(keys, sparse_tensor.SparseTensor):
      values = keys.values
    if self._table and (self._table.key_dtype.base_dtype == dtypes.int64):
      values = math_ops.to_int64(values)

    if self._num_oov_buckets == 0:
      ids = self._table.lookup(values, name=name)
    else:
      # TODO(yleon): Consider moving this functionality to its own kernel.
      with ops.name_scope(name, "%s_Lookup" % self.name) as scope:
        str_to_hash_bucket = self._get_string_to_hash_bucket_fn(
            self._hasher_spec)
        buckets = str_to_hash_bucket(
            _as_string(values),
            num_buckets=self._num_oov_buckets,
            name="hash_bucket")
        if self._table:
          ids = self._table.lookup(values)
          buckets = math_ops.add(buckets, self._table.size())
          is_id_non_default = math_ops.not_equal(ids, self._table.default_value)
          ids = array_ops.where(is_id_non_default, ids, buckets, name=scope)
        else:
          ids = buckets
    if isinstance(keys, sparse_tensor.SparseTensor):
      return sparse_tensor.SparseTensor(keys.indices, ids, keys.dense_shape)
    return ids


@deprecated("2017-04-10", "Use `index_table_from_file`.")
def string_to_index_table_from_file(vocabulary_file=None,
                                    num_oov_buckets=0,
                                    vocab_size=None,
                                    default_value=-1,
                                    hasher_spec=FastHashSpec,
                                    name=None):
  return index_table_from_file(
      vocabulary_file, num_oov_buckets, vocab_size, default_value, hasher_spec,
      key_dtype=dtypes.string, name=name)


def index_table_from_file(vocabulary_file=None,
                          num_oov_buckets=0,
                          vocab_size=None,
                          default_value=-1,
                          hasher_spec=FastHashSpec,
                          key_dtype=dtypes.string,
                          name=None):
  """Returns a lookup table that converts a string tensor into int64 IDs.

  This operation constructs a lookup table to convert tensor of strings into
  int64 IDs. The mapping can be initialized from a vocabulary file specified in
  `vocabulary_file`, where the whole line is the key and the zero-based line
  number is the ID.

  Any lookup of an out-of-vocabulary token will return a bucket ID based on its
  hash if `num_oov_buckets` is greater than zero. Otherwise it is assigned the
  `default_value`.
  The bucket ID range is `[vocabulary size, vocabulary size + num_oov_buckets]`.

  The underlying table must be initialized by calling
  `tf.tables_initializer.run()` or `table.init.run()` once.

  Sample Usages:

  If we have a vocabulary file "test.txt" with the following content:

  ```
  emerson
  lake
  palmer
  ```

  ```python
  features = tf.constant(["emerson", "lake", "and", "palmer"])
  table = tf.contrib.lookup.index_table_from_file(
      vocabulary_file="test.txt", num_oov_buckets=1)
  ids = table.lookup(features)
  ...
  tf.tables_initializer().run()

  ids.eval()  ==> [0, 1, 3, 2]  # where 3 is the out-of-vocabulary bucket
  ```

  Args:
    vocabulary_file: The vocabulary filename.
    num_oov_buckets: The number of out-of-vocabulary buckets.
    vocab_size: Number of the elements in the vocabulary, if known.
    default_value: The value to use for out-of-vocabulary feature values.
      Defaults to -1.
    hasher_spec: A `HasherSpec` to specify the hash function to use for
      assignation of out-of-vocabulary buckets.
    key_dtype: The `key` data type.
    name: A name for this op (optional).

  Returns:
    The lookup table to map a `key_dtype` `Tensor` to index `int64` `Tensor`.

  Raises:
    ValueError: If `vocabulary_file` is not set.
    ValueError: If `num_oov_buckets` is negative or `vocab_size` is not greater
      than zero.
  """
  if not vocabulary_file:
    raise ValueError("vocabulary_file must be specified.")
  if num_oov_buckets < 0:
    raise ValueError("num_oov_buckets must be greater or equal than 0, got %d."
                     % num_oov_buckets)
  if vocab_size is not None and vocab_size < 1:
    raise ValueError("vocab_size must be greater than 0, got %d." % vocab_size)
  if (not key_dtype.is_integer) and (dtypes.string != key_dtype.base_dtype):
    raise TypeError("Only integer and string keys are supported.")

  with ops.name_scope(name, "string_to_index") as feat_to_id_scope:
    table = None
    shared_name = ""
    with ops.name_scope(None, "hash_table") as hash_table_scope:
      if vocab_size:
        # Keep the shared_name:
        # <table_type>_<filename>_<vocab_size>_<key_index>_<value_index>
        shared_name = "hash_table_%s_%d_%s_%s" % (vocabulary_file, vocab_size,
                                                  TextFileIndex.WHOLE_LINE,
                                                  TextFileIndex.LINE_NUMBER)
      else:
        # Keep the shared_name
        # <table_type>_<filename>_<key_index>_<value_index>
        shared_name = "hash_table_%s_%s_%s" % (vocabulary_file,
                                               TextFileIndex.WHOLE_LINE,
                                               TextFileIndex.LINE_NUMBER)
      init = TextFileIdTableInitializer(
          vocabulary_file, vocab_size=vocab_size,
          key_dtype=dtypes.int64 if key_dtype.is_integer else key_dtype,
          name="table_init")

      table = HashTable(
          init, default_value, shared_name=shared_name, name=hash_table_scope)
    if num_oov_buckets:
      table = IdTableWithHashBuckets(
          table,
          num_oov_buckets=num_oov_buckets,
          hasher_spec=hasher_spec,
          name=feat_to_id_scope,
          key_dtype=key_dtype)

    return table


@deprecated("2017-04-10", "Use `index_table_from_tensor`.")
def string_to_index_table_from_tensor(mapping,
                                      num_oov_buckets=0,
                                      default_value=-1,
                                      hasher_spec=FastHashSpec,
                                      name=None):
  with ops.name_scope(name, "string_to_index") as scope:
    mapping = ops.convert_to_tensor(mapping)
  if dtypes.string != mapping.dtype.base_dtype:
    raise ValueError("string_to_index_table_from_tensor requires string.")
  return index_table_from_tensor(
      mapping, num_oov_buckets, default_value, hasher_spec, name=scope)


def index_table_from_tensor(mapping,
                            num_oov_buckets=0,
                            default_value=-1,
                            hasher_spec=FastHashSpec,
                            dtype=dtypes.string,
                            name=None):
  """Returns a lookup table that converts a string tensor into int64 IDs.

  This operation constructs a lookup table to convert tensor of strings into
  int64 IDs. The mapping can be initialized from a string `mapping` 1-D tensor
  where each element is a key and corresponding index within the tensor is the
  value.

  Any lookup of an out-of-vocabulary token will return a bucket ID based on its
  hash if `num_oov_buckets` is greater than zero. Otherwise it is assigned the
  `default_value`.
  The bucket ID range is `[mapping size, mapping size + num_oov_buckets]`.

  The underlying table must be initialized by calling
  `tf.tables_initializer.run()` or `table.init.run()` once.

  Elements in `mapping` cannot have duplicates, otherwise when executing the
  table initializer op, it will throw a `FailedPreconditionError`.

  Sample Usages:

  ```python
  mapping_strings = t.constant(["emerson", "lake", "palmer"])
  table = tf.contrib.lookup.index_table_from_tensor(
      mapping=mapping_strings, num_oov_buckets=1, default_value=-1)
  features = tf.constant(["emerson", "lake", "and", "palmer"])
  ids = table.lookup(features)
  ...
  tf.tables_initializer().run()

  ids.eval()  ==> [0, 1, 4, 2]
  ```

  Args:
    mapping: A 1-D `Tensor` that specifies the mapping of keys to indices. The
      type of this object must be castable to `dtype`.
    num_oov_buckets: The number of out-of-vocabulary buckets.
    default_value: The value to use for out-of-vocabulary feature values.
      Defaults to -1.
    hasher_spec: A `HasherSpec` to specify the hash function to use for
      assignment of out-of-vocabulary buckets.
    dtype: The type of values passed to `lookup`. Only string and integers are
      supported.
    name: A name for this op (optional).

  Returns:
    The lookup table to map an input `Tensor` to index `int64` `Tensor`.

  Raises:
    ValueError: If `mapping` is invalid.
    ValueError: If `num_oov_buckets` is negative.
  """
  if mapping is None:
    raise ValueError("mapping must be specified.")

  if num_oov_buckets < 0:
    raise ValueError("num_oov_buckets must be greater or equal than 0, got %d."
                     % num_oov_buckets)

  if (not dtype.is_integer) and (dtypes.string != dtype.base_dtype):
    raise TypeError("Only integer and string keys are supported.")

  with ops.name_scope(name, "string_to_index") as feat_to_id_scope:
    keys = ops.convert_to_tensor(mapping)
    if keys.dtype.is_integer != dtype.is_integer:
      raise ValueError("Expected %s, got %s." % (
          "integer" if dtype.is_integer else "non-integer", keys.dtype))
    if (not dtype.is_integer) and (keys.dtype.base_dtype != dtype):
      raise ValueError("Expected %s, got %s." % (dtype, keys.dtype))
    num_elements = array_ops.size(keys)
    values = math_ops.to_int64(math_ops.range(num_elements))

    shared_name = ""
    with ops.name_scope(None, "hash_table") as hash_table_scope:
      table_keys = math_ops.to_int64(keys) if keys.dtype.is_integer else keys
      init = KeyValueTensorInitializer(
          table_keys, values, table_keys.dtype.base_dtype, dtypes.int64,
          name="table_init")
      table = HashTable(
          init, default_value, shared_name=shared_name, name=hash_table_scope)
    if num_oov_buckets:
      table = IdTableWithHashBuckets(
          table,
          num_oov_buckets=num_oov_buckets,
          hasher_spec=hasher_spec,
          name=feat_to_id_scope,
          key_dtype=dtype)

    return table


@deprecated(
    "2017-01-07", "This op will be removed after the deprecation date. "
    "Please switch to index_table_from_tensor and call the lookup "
    "method of the returned table.")
def string_to_index(tensor, mapping, default_value=-1, name=None):
  """Maps `tensor` of strings into `int64` indices based on `mapping`.

  This operation converts `tensor` of strings into `int64` indices.
  The mapping is initialized from a string `mapping` tensor where each element
  is a key and corresponding index within the tensor is the value.

  Any entry in the input which does not have a corresponding entry in 'mapping'
  (an out-of-vocabulary entry) is assigned the `default_value`

  Elements in `mapping` cannot be duplicated, otherwise the initialization
  will throw a FailedPreconditionError.

  The underlying table must be initialized by calling
  `tf.tables_initializer.run()` once.

  For example:

  ```python
  mapping_strings = tf.constant(["emerson", "lake", "palmer"])
  feats = tf.constant(["emerson", "lake", "and", "palmer"])
  ids = tf.contrib.lookup.string_to_index(
      feats, mapping=mapping_strings, default_value=-1)
  ...
  tf.tables_initializer().run()

  ids.eval()  ==> [0, 1, -1, 2]
  ```

  Args:
    tensor: A 1-D input `Tensor` with the strings to map to indices.
    mapping: A 1-D string `Tensor` that specifies the mapping of strings to
      indices.
    default_value: The `int64` value to use for out-of-vocabulary strings.
      Defaults to -1.
    name: A name for this op (optional).

  Returns:
    The mapped indices. It has the same shape and tensor type (dense or sparse)
    as `tensor`.
  """
  table = index_table_from_tensor(
      mapping=mapping, default_value=default_value, name=name)
  return table.lookup(tensor)


def index_to_string_table_from_file(vocabulary_file,
                                    vocab_size=None,
                                    default_value="UNK",
                                    name=None):
  """Returns a lookup table that maps a `Tensor` of indices into strings.

  This operation constructs a lookup table to map int64 indices into string
  values. The table is initialized from a vocabulary file specified in
  `vocabulary_file`, where the whole line is the value and the
  zero-based line number is the index.

  Any input which does not have a corresponding index in the vocabulary file
  (an out-of-vocabulary entry) is assigned the `default_value`

  The underlying table must be initialized by calling
  `tf.tables_initializer.run()` or `table.init.run()` once.

  Sample Usages:

  If we have a vocabulary file "test.txt" with the following content:

  ```
  emerson
  lake
  palmer
  ```

  ```python
  indices = tf.constant([1, 5], tf.int64)
  table = tf.contrib.lookup.index_to_string_table_from_file(
      vocabulary_file="test.txt", default_value="UNKNOWN")
  values = table.lookup(indices)
  ...
  tf.tables_initializer().run()

  values.eval() ==> ["lake", "UNKNOWN"]
  ```

  Args:
    vocabulary_file: The vocabulary filename.
    vocab_size: Number of the elements in the vocabulary, if known.
    default_value: The value to use for out-of-vocabulary indices.
    name: A name for this op (optional).

  Returns:
    The lookup table to map a string values associated to a given index `int64`
    `Tensors`.

  Raises:
    ValueError: when `vocabulary_file` is empty.
    ValueError: when `vocab_size` is invalid.
  """
  if not vocabulary_file:
    raise ValueError("vocabulary_file must be specified.")
  if vocab_size is not None and vocab_size < 1:
    raise ValueError("vocab_size must be greater than 0, got %d." % vocab_size)

  with ops.name_scope(name, "index_to_string") as scope:
    shared_name = ""
    if vocab_size:
      # Keep a shared_name
      # <table_type>_<filename>_<vocab_size>_<key_index>_<value_index>
      shared_name = "hash_table_%s_%d_%s_%s" % (vocabulary_file, vocab_size,
                                                TextFileIndex.LINE_NUMBER,
                                                TextFileIndex.WHOLE_LINE)
    else:
      # Keep a shared_name <table_type>_<filename>_<key_index>_<value_index>
      shared_name = "hash_table_%s_%s_%s" % (vocabulary_file,
                                             TextFileIndex.LINE_NUMBER,
                                             TextFileIndex.WHOLE_LINE)
    init = TextFileStringTableInitializer(
        vocabulary_file, vocab_size=vocab_size, name="table_init")

    # TODO(yleon): Use a more effienct structure.
    return HashTable(init, default_value, shared_name=shared_name, name=scope)


def index_to_string_table_from_tensor(mapping, default_value="UNK", name=None):
  """Returns a lookup table that maps a `Tensor` of indices into strings.

  This operation constructs a lookup table to map int64 indices into string
  values. The mapping is initialized from a string `mapping` 1-D `Tensor` where
  each element is a value and the corresponding index within the tensor is the
  key.

  Any input which does not have a corresponding index in 'mapping'
  (an out-of-vocabulary entry) is assigned the `default_value`

  The underlying table must be initialized by calling
  `tf.tables_initializer.run()` or `table.init.run()` once.

  Elements in `mapping` cannot have duplicates, otherwise when executing the
  table initializer op, it will throw a `FailedPreconditionError`.

  Sample Usages:

  ```python
  mapping_string = t.constant(["emerson", "lake", "palmer")
  indices = tf.constant([1, 5], tf.int64)
  table = tf.contrib.lookup.index_to_string_table_from_tensor(
      mapping_string, default_value="UNKNOWN")
  values = table.lookup(indices)
  ...
  tf.tables_initializer().run()

  values.eval() ==> ["lake", "UNKNOWN"]
  ```

  Args:
    mapping: A 1-D string `Tensor` that specifies the strings to map from
      indices.
    default_value: The value to use for out-of-vocabulary indices.
    name: A name for this op (optional).

  Returns:
    The lookup table to map a string values associated to a given index `int64`
    `Tensors`.

  Raises:
    ValueError: when `mapping` is not set.
  """

  if mapping is None:
    raise ValueError("mapping must be specified.")

  with ops.name_scope(name, "index_to_string") as scope:
    values = ops.convert_to_tensor(mapping, dtypes.string)
    num_elements = array_ops.size(values)
    keys = math_ops.to_int64(math_ops.range(num_elements))

    shared_name = ""
    init = KeyValueTensorInitializer(
        keys, values, dtypes.int64, dtypes.string, name="table_init")
    # TODO(yleon): Use a more effienct structure.
    return HashTable(init, default_value, shared_name=shared_name, name=scope)


@deprecated(
    "2017-01-07", "This op will be removed after the deprecation date. "
    "Please switch to index_to_string_table_from_tensor and call the lookup "
    "method of the returned table.")
def index_to_string(tensor, mapping, default_value="UNK", name=None):
  """Maps `tensor` of indices into string values based on `mapping`.

  This operation converts `int64` indices into string values. The mapping is
  initialized from a string `mapping` tensor where each element is a value and
  the corresponding index within the tensor is the key.

  Any input which does not have a corresponding index in 'mapping'
  (an out-of-vocabulary entry) is assigned the `default_value`

  The underlying table must be initialized by calling
  `tf.tables_initializer.run()` once.

  For example:

  ```python
  mapping_string = t.constant(["emerson", "lake", "palmer")
  indices = tf.constant([1, 5], tf.int64)
  values = tf.contrib.lookup.index_to_string(
      indices, mapping=mapping_string, default_value="UNKNOWN")
  ...
  tf.tables_initializer().run()

  values.eval() ==> ["lake", "UNKNOWN"]
  ```

  Args:
    tensor: A `int64` `Tensor` with the indices to map to strings.
    mapping: A 1-D string `Tensor` that specifies the strings to map from
      indices.
    default_value: The string value to use for out-of-vocabulary indices.
    name: A name for this op (optional).

  Returns:
    The strings values associated to the indices. The resultant dense
    feature value tensor has the same shape as the corresponding `indices`.
  """
  table = index_to_string_table_from_tensor(
      mapping=mapping, default_value=default_value, name=name)
  return table.lookup(tensor)


class MutableHashTable(LookupInterface):
  """A generic mutable hash table implementation.

  Data can be inserted by calling the insert method. It does not support
  initialization via the init method.

  Example usage:

  ```python
  table = tf.contrib.lookup.MutableHashTable(key_dtype=tf.string,
                                             value_dtype=tf.int64,
                                             default_value=-1)
  table.insert(keys, values)
  out = table.lookup(query_keys)
  print out.eval()
  ```
  """

  def __init__(self,
               key_dtype,
               value_dtype,
               default_value,
               shared_name=None,
               name="MutableHashTable",
               checkpoint=True):
    """Creates an empty `MutableHashTable` object.

    Creates a table, the type of its keys and values are specified by key_dtype
    and value_dtype, respectively.

    Args:
      key_dtype: the type of the key tensors.
      value_dtype: the type of the value tensors.
      default_value: The value to use if a key is missing in the table.
      shared_name: If non-empty, this table will be shared under
        the given name across multiple sessions.
      name: A name for the operation (optional).
      checkpoint: if True, the contents of the table are saved to and restored
        from checkpoints. If `shared_name` is empty for a checkpointed table, it
        is shared using the table node name.

    Returns:
      A `MutableHashTable` object.

    Raises:
      ValueError: If checkpoint is True and no name was specified.
    """
    self._default_value = ops.convert_to_tensor(default_value,
                                                dtype=value_dtype)
    self._value_shape = self._default_value.get_shape()

    # The table must be shared if checkpointing is requested for multi-worker
    # training to work correctly. Use the node name if no shared_name has been
    # explicitly specified.
    use_node_name_sharing = checkpoint and shared_name is None
    # pylint: disable=protected-access
    if self._default_value.get_shape().ndims == 0:
      self._table_ref = gen_lookup_ops._mutable_hash_table(
          shared_name=shared_name,
          use_node_name_sharing=use_node_name_sharing,
          key_dtype=key_dtype,
          value_dtype=value_dtype,
          name=name)
    else:
      self._table_ref = gen_lookup_ops._mutable_hash_table_of_tensors(
          shared_name=shared_name,
          use_node_name_sharing=use_node_name_sharing,
          key_dtype=key_dtype,
          value_dtype=value_dtype,
          value_shape=self._default_value.get_shape(),
          name=name)
    # pylint: enable=protected-access
    super(MutableHashTable, self).__init__(key_dtype, value_dtype,
                                           self._table_ref.op.name.split(
                                               "/")[-1])

    if checkpoint:
      saveable = MutableHashTable._Saveable(self, name)
      ops.add_to_collection(ops.GraphKeys.SAVEABLE_OBJECTS, saveable)

  def size(self, name=None):
    """Compute the number of elements in this table.

    Args:
      name: A name for the operation (optional).

    Returns:
      A scalar tensor containing the number of elements in this table.
    """
    with ops.name_scope(name, "%s_Size" % self._name,
                        [self._table_ref]) as name:
      # pylint: disable=protected-access
      return gen_lookup_ops._lookup_table_size(self._table_ref, name=name)

  def lookup(self, keys, name=None):
    """Looks up `keys` in a table, outputs the corresponding values.

    The `default_value` is used for keys not present in the table.

    Args:
      keys: Keys to look up. Can be a tensor of any shape. Must match the
        table's key_dtype.
      name: A name for the operation (optional).

    Returns:
      A tensor containing the values in the same shape as `keys` using the
        table's value type.

    Raises:
      TypeError: when `keys` do not match the table data types.
    """
    if keys.dtype != self._key_dtype:
      raise TypeError("Signature mismatch. Keys must be dtype %s, got %s." %
                      (self._key_dtype, keys.dtype))

    with ops.name_scope(name, "%s_lookup_table_find" % self._name,
                        (self._table_ref, keys, self._default_value)) as name:
      # pylint: disable=protected-access
      values = gen_lookup_ops._lookup_table_find(
          self._table_ref, keys, self._default_value, name=name)

    values.set_shape(keys.get_shape().concatenate(self._value_shape))
    return values

  def insert(self, keys, values, name=None):
    """Associates `keys` with `values`.

    Args:
      keys: Keys to insert. Can be a tensor of any shape. Must match the
        table's key type.
      values: Values to be associated with keys. Must be a tensor of the same
        shape as `keys` and match the table's value type.
      name: A name for the operation (optional).

    Returns:
      The created Operation.

    Raises:
      TypeError: when `keys` or `values` doesn't match the table data
        types.
    """
    self.check_table_dtypes(keys.dtype, values.dtype)
    with ops.name_scope(name, "%s_lookup_table_insert" % self._name,
                        [self._table_ref, keys, values]) as name:
      # pylint: disable=protected-access
      op = gen_lookup_ops._lookup_table_insert(
          self._table_ref, keys, values, name=name)
      return op

  def export(self, name=None):
    """Returns tensors of all keys and values in the table.

    Args:
      name: A name for the operation (optional).

    Returns:
      A pair of tensors with the first tensor containing all keys and the
        second tensors containing all values in the table.
    """
    with ops.name_scope(name, "%s_lookup_table_export_values" % self._name,
                        [self._table_ref]) as name:
      # pylint: disable=protected-access
      exported_keys, exported_values = gen_lookup_ops._lookup_table_export(
          self._table_ref, self._key_dtype, self._value_dtype, name=name)

    exported_values.set_shape(exported_keys.get_shape().concatenate(
        self._value_shape))
    return exported_keys, exported_values

  class _Saveable(BaseSaverBuilder.SaveableObject):
    """SaveableObject implementation for MutableHashTable."""

    def __init__(self, table, name):
      tensors = table.export()
      specs = [
          BaseSaverBuilder.SaveSpec(tensors[0], "", name + "-keys"),
          BaseSaverBuilder.SaveSpec(tensors[1], "", name + "-values")
      ]
      # pylint: disable=protected-access
      super(MutableHashTable._Saveable, self).__init__(table, specs, name)

    def restore(self, restored_tensors, unused_restored_shapes):
      # pylint: disable=protected-access
      return gen_lookup_ops._lookup_table_import(
          self.op._table_ref, restored_tensors[0], restored_tensors[1])


class MutableDenseHashTable(LookupInterface):
  """A generic mutable hash table implementation using tensors as backing store.

  Data can be inserted by calling the insert method. It does not support
  initialization via the init method.

  It uses "open addressing" with quadratic reprobing to resolve collisions.
  Compared to `MutableHashTable` the insert and lookup operations in a
  `MutableDenseHashTable` are typically faster, but memory usage can be higher.
  However, `MutableDenseHashTable` does not require additional memory for
  temporary tensors created during checkpointing and restore operations.

  Example usage:

  ```python
  table = tf.contrib.lookup.MutableDenseHashTable(key_dtype=tf.int64,
                                                  value_dtype=tf.int64,
                                                  default_value=-1,
                                                  empty_key=0)
  table.insert(keys, values)
  out = table.lookup(query_keys)
  print out.eval()
  ```
  """

  # TODO(andreasst): consider extracting common code with MutableHashTable into
  # a common superclass.
  def __init__(self,
               key_dtype,
               value_dtype,
               default_value,
               empty_key,
               initial_num_buckets=None,
               shared_name=None,
               name="MutableDenseHashTable",
               checkpoint=True):
    """Creates an empty `MutableDenseHashTable` object.

    Creates a table, the type of its keys and values are specified by key_dtype
    and value_dtype, respectively.

    Args:
      key_dtype: the type of the key tensors.
      value_dtype: the type of the value tensors.
      default_value: The value to use if a key is missing in the table.
      empty_key: the key to use to represent empty buckets internally. Must not
        be used in insert or lookup operations.
      initial_num_buckets: the initial number of buckets.
      shared_name: If non-empty, this table will be shared under
        the given name across multiple sessions.
      name: A name for the operation (optional).
      checkpoint: if True, the contents of the table are saved to and restored
        from checkpoints. If `shared_name` is empty for a checkpointed table, it
        is shared using the table node name.

    Returns:
      A `MutableHashTable` object.

    Raises:
      ValueError: If checkpoint is True and no name was specified.
    """
    self._default_value = ops.convert_to_tensor(
        default_value, dtype=value_dtype)
    self._value_shape = self._default_value.get_shape()

    # The table must be shared if checkpointing is requested for multi-worker
    # training to work correctly. Use the node name if no shared_name has been
    # explicitly specified.
    use_node_name_sharing = checkpoint and shared_name is None
    empty_key = ops.convert_to_tensor(empty_key, dtype=key_dtype)
    # pylint: disable=protected-access
    self._table_ref = gen_lookup_ops._mutable_dense_hash_table(
        empty_key=empty_key,
        shared_name=shared_name,
        use_node_name_sharing=use_node_name_sharing,
        value_dtype=value_dtype,
        value_shape=self._value_shape,
        initial_num_buckets=initial_num_buckets,
        name=name)
    # pylint: enable=protected-access
    super(MutableDenseHashTable, self).__init__(
        key_dtype, value_dtype, self._table_ref.op.name.split("/")[-1])

    if checkpoint:
      saveable = MutableDenseHashTable._Saveable(self, name)
      ops.add_to_collection(ops.GraphKeys.SAVEABLE_OBJECTS, saveable)

  def size(self, name=None):
    """Compute the number of elements in this table.

    Args:
      name: A name for the operation (optional).

    Returns:
      A scalar tensor containing the number of elements in this table.
    """
    with ops.name_scope(name, "%s_Size" % self._name,
                        [self._table_ref]) as name:
      # pylint: disable=protected-access
      return gen_lookup_ops._lookup_table_size(self._table_ref, name=name)

  def lookup(self, keys, name=None):
    """Looks up `keys` in a table, outputs the corresponding values.

    The `default_value` is used for keys not present in the table.

    Args:
      keys: Keys to look up. Can be a tensor of any shape. Must match the
        table's key_dtype.
      name: A name for the operation (optional).

    Returns:
      A tensor containing the values in the same shape as `keys` using the
        table's value type.

    Raises:
      TypeError: when `keys` do not match the table data types.
    """
    if keys.dtype != self._key_dtype:
      raise TypeError("Signature mismatch. Keys must be dtype %s, got %s." %
                      (self._key_dtype, keys.dtype))

    with ops.name_scope(name, "%s_lookup_table_find" % self._name,
                        [self._table_ref, keys]) as name:
      # pylint: disable=protected-access
      values = gen_lookup_ops._lookup_table_find(
          self._table_ref, keys, self._default_value, name=name)

    if keys.get_shape().ndims is not None and keys.get_shape().ndims > 0:
      values.set_shape(
          tensor_shape.TensorShape([keys.get_shape().dims[0]]).concatenate(
              self._value_shape))
    return values

  def insert(self, keys, values, name=None):
    """Associates `keys` with `values`.

    Args:
      keys: Keys to insert. Can be a tensor of any shape. Must match the
        table's key type.
      values: Values to be associated with keys. Must be a tensor of the same
        shape as `keys` and match the table's value type.
      name: A name for the operation (optional).

    Returns:
      The created Operation.

    Raises:
      TypeError: when `keys` or `values` doesn't match the table data
        types.
    """
    self.check_table_dtypes(keys.dtype, values.dtype)
    with ops.name_scope(name, "%s_lookup_table_insert" % self._name,
                        [self._table_ref, keys, values]) as name:
      # pylint: disable=protected-access
      op = gen_lookup_ops._lookup_table_insert(
          self._table_ref, keys, values, name=name)
      return op

  def export(self, name=None):
    """Returns tensors of all keys and values in the table.

    Args:
      name: A name for the operation (optional).

    Returns:
      A pair of tensors with the first tensor containing all keys and the
        second tensors containing all values in the table.
    """
    with ops.name_scope(name, "%s_lookup_table_export_values" % self._name,
                        [self._table_ref]) as name:
      # pylint: disable=protected-access
      exported_keys, exported_values = gen_lookup_ops._lookup_table_export(
          self._table_ref, self._key_dtype, self._value_dtype, name=name)

    exported_values.set_shape(exported_keys.get_shape().concatenate(
        self._value_shape))
    return exported_keys, exported_values

  class _Saveable(BaseSaverBuilder.SaveableObject):
    """SaveableObject implementation for MutableDenseHashTable."""

    def __init__(self, table, name):
      tensors = table.export()
      specs = [
          BaseSaverBuilder.SaveSpec(tensors[0], "", name + "-keys"),
          BaseSaverBuilder.SaveSpec(tensors[1], "", name + "-values")
      ]
      # pylint: disable=protected-access
      super(MutableDenseHashTable._Saveable, self).__init__(table, specs, name)

    def restore(self, restored_tensors, unused_restored_shapes):
      # pylint: disable=protected-access
      return gen_lookup_ops._lookup_table_import(
          self.op._table_ref, restored_tensors[0], restored_tensors[1])