相信很多同志，在做深度学习模型的过程中，往往会需要将特征的原始值映射为数值类型的ID，然后再通过tf.nn.embedding_lookup转化为dense向量。最终，在上线的时候，映射关系一般保存为哈希表（dict），但如果特征很多，那么管理起来就很麻烦。

今天这篇博客会讲述《如何将这个过程在tensorflow实现》！

MutableHashTable

首先，先附上官方的API文档

tf.contrib.lookup.MutableHashTable(
    key_dtype, value_dtype, default_value, name='MutableHashTable', checkpoint=True
)

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.
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.

Raises
ValueError	If checkpoint is True and no name was specified.

Attributes
key_dtype	The table key dtype.
name	The name of the table.
resource_handle	Returns the resource handle associated with this Resource.
value_dtype	The table value dtype.

并且，它提供了哈希表的基本操作：

insert：插入键值对
export：导出hashtable
lookup：key查询
remove：删除key
size：hashtable的容量

demo代码

import tensorflow as tfÅ
import time


def demo():
    """
    insert：插入键值对
    export：导出hashtable
    lookup：key查询
    remove：删除key
    size：hashtable的容量
    :return:
    """
    keys = tf.placeholder(dtype=tf.string, shape=[None])
    values = tf.placeholder(dtype=tf.int64, shape=[None])
    # 如果有多个表，则需要name命名，否则保存加载时，会因为都是默认命名而导致被覆盖
    table1 = tf.contrib.lookup.MutableHashTable(key_dtype=tf.string, value_dtype=tf.int64, default_value=-1,
                                                name="HashTable_1")
    table2 = tf.contrib.lookup.MutableHashTable(tf.string, tf.int64, -1)
    insert_table1 = table1.insert(keys, values)
    insert_table2 = table2.insert(keys, values)
    saver = tf.train.Saver()
    with tf.Session() as sess:
        sess.run(insert_table1, feed_dict={keys: ["a"], values: [1]})
        sess.run(insert_table2, feed_dict={keys: ["b"], values: [2]})
        print("table1:", sess.run(table1.export()))
        print("table2:", sess.run(table2.export()))
        saver.saverve(sess, "checkpoint/test")


def run():
    """
    测试50W容量的hashtable，保存的大小和查询速度
    :return:
    """
    size = 500000
    keys = tf.placeholder(dtype=tf.string, shape=[None])
    values = tf.placeholder(dtype=tf.int64, shape=[None])
    # 如果有多个表，则需要name命名，否则保存加载时，会因为都是默认命名而导致被覆盖
    table1 = tf.contrib.lookup.MutableHashTable(key_dtype=tf.string, value_dtype=tf.int64, default_value=-1,
                                                name="HashTable_1")
    insert_table1 = table1.insert(keys, values)
    lookup = table1.lookup(keys)
    saver = tf.train.Saver()
    with tf.Session() as sess:
        sess.run(insert_table1, feed_dict={keys: ["id_" + str(i) for i in range(size)], values: list(range(size))})
        # print("table1:", sess.run(table1.export()))

        # 查询时间：0.007218122482299805
        # 模型大小：8.9M
        s1 = time.time()
        print(sess.run(lookup, feed_dict={keys: ["id_1", "id_100"]}))
        print(time.time() - s1)
        saver.save(sess, "checkpoint/test")


if __name__ == '__main__':
    run()