Bert+Bilstm+Crf 命名实体识别(NER) Keras实战

0.引言

最近做了一个命名实体识别(NER)的任务，开始用的是keras中的Embedding层+bilstm+crf，但是模型训练精度太低了，没有实用意义。后来考虑用bert来embedding，因为bert毕竟是Google花了大精力训练的模型，还是很强大的。其实网上相关tensorflow，pytorch的代码很多，但是用Keras的感觉没有一个是简单明了的，所以我在写这个网络的时候遇到不少麻烦，幸运的是最后还是实现了这个网络，正确率有99%+。作为总结，把自己的经验分享给大家，尽量做到简单明了可实现，代码粗陋，适合新手学习，也欢迎大家的宝贵意见。

1.前期准备

keras中要用bert和crf，我用到了两个封装好的函数包：keras_bert和keras_contrib，安装命令：

pip install git+https://www.github.com/keras-team/keras-contrib.git
pip install keras-bert

此外还要用到bert训练好的模型chinese_L-12_H-768_A-12，下载网址：

https://storage.googleapis.com/bert_models/2018_11_03/chinese_L-12_H-768_A-12.zip

2.任务与整体思路

NER的任务是根据自己的需要，识别机构名或者人名地名等，本次的任务就用识别公司名作为简单的例子，整体的思路就是首先使用下载的bert模型把语料样本embedding，然后输入bilstm+crf的网络训练模型。我们知道进入bilstm网络的张量应该是一个3D张量，所以任务的难点就是使用bert来embedding语料的过程，而且要值得注意的是语料样本embedding以后的squence要和标签的squence一致。这个会在后面用例子详细介绍。为了便于初学者理解语料的embedding和网络的输入，请看下图：

3.训练样本向量化

把语料向量化是这个任务的关键，在这里直观的用例子来解释向量化的过程，后面读者再结合代码，就能有比较好的理解。首先，我是把语料的样本和标签放到了不同的文档里，sentence放的是语料，label是语料的标签也就是标注的公司名。(没有公司名的就空行)

对语料的向量化：使用extract_embedding函数直接把sentence中的语料embedding成一个3D张量。【extract_embedding()处理样本太多的时候会很慢，bert_service的方法见我的另一篇博客：bert_serving 安装使用教程】
对标签的向量化：根据语料tokenize的结果进行标注。例如语料tokenizer以后的向量是:
‘[CLS]’, ‘我’, ‘爱’, ‘中’, ‘国’, ‘[SEP]’
‘[CLS]’, ‘阿’, ‘里’, ‘巴’, ‘巴’, ‘捐’, ‘款’, ‘武’, ‘汉’, ‘10’, ‘亿’, ‘[SEP]’
那对应的标注向量就是
[ [ [0],[0],[0],[0],[0],[0] ] , [ [0],[1],[2],[2],[2],[0],[0],[0],[0],[0],[0],[0] ] ]
1:代表公司名的首个字
2:代表公司名的非首个字
注：
1.bert的tokenize以后句子的首尾会多出’[CLS]‘和’[SEP]’
2.语料和标签向量化的时候要padding，否则squence长度不统一，无法统一成一个张量输入网络

4.代码

import

import numpy as np
import os
from keras_bert import extract_embeddings
from keras_bert import load_trained_model_from_checkpoint
import codecs
from keras_bert import Tokenizer
from keras.models import Sequential
from keras.layers import Embedding, Bidirectional, LSTM, TimeDistributed,Dense
from keras_contrib.layers import CRF
from keras.callbacks import Callback
from sklearn.metrics import confusion_matrix, f1_score, precision_score, recall_score

声明文件路径

pretrained_path = './chinese_L-12_H-768_A-12'
config_path = os.path.join(pretrained_path, 'bert_config.json')
checkpoint_path = os.path.join(pretrained_path, 'bert_model.ckpt')
vocab_path = os.path.join(pretrained_path, 'vocab.txt')
sentence_path=r'./sentence'
label_path=r'./label'
test_path=r'./test'

载入bert模型字典

#模型字典
token_dict = {}
with codecs.open(vocab_path, 'r', 'utf8') as reader:
    for line in reader:
        token = line.strip()
        token_dict[token] = len(token_dict)
tokenizer = Tokenizer(token_dict)

样本向量化

#语料样本向量化
#pad_num：自定义语料squence长度
def pre_y(sentence,label,pad_num):
    y=np.zeros((len(sentence),pad_num))
    for i in range(len(sentence)):
        sen=sentence[i]
        tokens = tokenizer.tokenize(sen)
        lab=label[i]
        lab_y = np.zeros(len(tokens))
        if lab!='':
            top = None
            #label只有一项
            if ',' not in lab:
                tokens_lab = tokenizer.tokenize(lab)
                num_lab=len(tokens_lab)-2
                for j in range(len(tokens)):
                    if j+num_lab-1<len(tokens):
                        if (tokens[j] == tokens_lab[1]) & (tokens[j + num_lab - 1] == tokens_lab[1 + num_lab - 1]):
                            top = j
                if top==None:
                    print('error:' + sen)
                    print(tokens,tokens_lab)
                    break
                lab_y[top] = 1  # B-COM
                for g in range(top + 1, top + num_lab):
                    lab_y[g] = 2  # I-COM
            #label有多项
            else:
                t=[]
                n=[]
                for u in lab.split(','):
                    tokens_lab = tokenizer.tokenize(u)
                    num_lab = len(tokens_lab) - 2
                    for j in range(len(tokens)):
                        if j + num_lab - 1 < len(tokens):
                            if (tokens[j] == tokens_lab[1]) & (tokens[j + num_lab - 1] == tokens_lab[1 + num_lab - 1]):
                                t.append(j)
                                n.append(num_lab)
                if len(t)==0:
                    print('error:' + sen)
                    break
                for t_num in range(len(t)):
                    lab_y[t[t_num]] = 1  # B-COM
                    for g in range(t[t_num]+1,t[t_num]+n[t_num]):
                        lab_y[g] = 2  # I-COM
        y[i]=np.lib.pad(lab_y, (0,pad_num-len(tokens)), 'constant', constant_values=(0,0))
    return y.reshape((len(sentence),pad_num,1))

#语料标签向量化
#pad_num：自定义语料squence长度
def pre_x(sentence,pad_num):
    x = extract_embeddings(pretrained_path, sentence)
    #padding
    x_train = np.zeros((len(sentence), pad_num, x[0].shape[1]))
    for i in range(len(sentence)):
        for j in range(len(x[i])):
            if len(x[i]) > pad_num:
                print('error:超出范围！'+str(len(x[i]))+str(sentence[i]))
                break
            x_train[i][j] = x[i][j]
    return x_train

class Metrics(Callback):
  def on_train_begin(self, logs={}):
    self.val_f1s = []
    self.val_recalls = []
    self.val_precisions = []

  def on_epoch_end(self, epoch, logs={}):
    val_predict=(np.asarray(self.model.predict(self.validation_data[0]))).round()
    val_targ = self.validation_data[1]
    # _val_f1 = f1_score(val_targ, val_predict,average='micro')
    _val_recall = recall_score(val_targ, val_predict,average=None)
    # _val_precision = precision_score(val_targ, val_predict,average=None)
    # self.val_f1s.append(_val_f1)
    self.val_recalls.append(_val_recall)
    # self.val_precisions.append(_val_precision)
    # print('-val_f1: %.4f --val_precision: %.4f --val_recall: %.4f'%(_val_f1, _val_precision, _val_recall))
    print("— val_recall: %f " % _val_recall)
    return

模型训练

EMBED_DIM = 200
BiRNN_UNITS = 200
chunk_tags=3#O,B-COM,I-COM
with open(sentence_path, encoding="utf-8", errors='ignore') as sentence_file_object:
    sentence = sentence_file_object.read()
with open(label_path, encoding="utf-8", errors='ignore') as label_path_file_object:
    label = label_path_file_object.read()
with open(test_path, encoding="utf-8", errors='ignore') as test_path_file_object:
    test = test_path_file_object.read()
s = sentence.split('\n')
l = label.split('\n')
test_s=test.split('\n')
y=pre_y(s,l,128)
x=pre_x(s,128)
test_x=pre_x(test_s,128)

model = Sequential()
model.add(Bidirectional(LSTM(BiRNN_UNITS // 2, return_sequences=True)))
crf = CRF(chunk_tags, sparse_target=True)
model.add(crf)
model.compile('adam', loss=crf.loss_function, metrics=[crf.accuracy])
model.fit(x[:1000], y[:1000],batch_size=6,epochs=6, validation_data=(x[1000:], y[1000:]))#前1000个样本作为训练集
pre = model.predict(test_x)

最近很多人找我要源码，最后把整个项目分享给大家，至于数据涉及到商业因素，不方便公开，我用了10条样本供参考。记得收藏点赞！感谢大家支持！
获取项目代码戳这里
提取码：aylx