TensorFlow word2vec_basic 代码分析
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将字母向量化(即 word embedding)是NLP的基础,在TensorFlow中有一个简单的实现即word2vec(这篇文章主要关注他的基本实现,demo在这个目录下:tensorflow/examples/tutorials/word2vec/word2vec_basic.py)。关于word2vec的介绍非常多理论也一堆一堆的,下面是本文参考的几篇主要的理论文章:
http://www.jeyzhang.com/tensorflow-learning-notes-3.html
https://liusida.github.io/2016/11/14/study-embeddings/
https://github.com/tensorflow/tensorflow/blob/master/tensorflow/examples/tutorials/word2vec/word2vec_basic.py
本文希望抛开理论,三言两语的把word2vec用人话说清楚。
一些基本概念不在复述,这里主要总结几个结论:
1.word2vec的学习过程目标是把每个单词映射成一个向量。
2.我们希望这些向量能满足一些基本规律,比如如果两个单词的含义是相近的,那么他们对应的向量的余弦相似度尽可能小。
3.在一个上下文环境中如果单词a出现过多次,单词b也出现过多次,那么a和b极有可能是同个类型的或者意思相近的词。
4.在这里上下文主要指一个单词周围的其他单词。
5.word2vec使用的是skip-gram模型,skip-gram模型是从目标词来预测上下文信息。
所以这里word2vec的模型其实很简单,取其中一个词,作为目标词,当输入目标词的时候,那么理论上就可以输出他的上下文信息。但是它的上下文信息很多,一个目标词周围有很多的词,目标词对应其中任何一个词都是合理的输出。所以将目标词与周围的N个词组成N组训练样本,分别作为输入输出进行训练。
举个例子理解起来就很简单了,一个句子: Y X A B C D E X Y,假设将单词左右两边的两个单词以内的词作为当前词的上下文,当将C作为输入(这时A,B,D,E都是他的上下文),那么他的输出是什么呢, 输出可以是A ,B , D,E中的任何一个,所以(C,E),(C,B)(C,D)(C,A),都可以作为训练样本,一般随机选其中的几种进行训练,比如我们只选(C,E),这是C是输入,E是输出(也就是label,机器学习,当给数据打了标签就好办了)。
到现在这个模型应该已经非常清晰了,他的输入是一个单词(应该是一个单词word embedding后对应的数字化后的向量),他的输出也是一个单词,不过输出这里的单词表示方式跟输入不太一样,采用的是One-hot Representation,为什么输入使用word embedding的方式而输出使用One-hot Representation的方式,我的理解是word embedding的方式表示一个单词开始时是有参数的,不是真实的结果,没法用来作为输出去优化模型。
所以对于一个总共有5000个单词的词库,进行word embedding,假设词向量是128维的,那么这个模型的输入是一个1x128的向量,输出是一个1x5000的向量。
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最后我们把代码贴出来看下:
# Copyright 2015 The TensorFlow Authors. All Rights Reserved.## Licensed under the Apache License, Version 2.0 (the "License");# you may not use this file except in compliance with the License.# You may obtain a copy of the License at## http://www.apache.org/licenses/LICENSE-2.0## Unless required by applicable law or agreed to in writing, software# distributed under the License is distributed on an "AS IS" BASIS,# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.# See the License for the specific language governing permissions and# limitations under the License.# =============================================================================="""Basic word2vec example."""from __future__ import absolute_importfrom __future__ import divisionfrom __future__ import print_functionimport collectionsimport mathimport osimport randomimport zipfileimport numpy as npfrom six.moves import urllibfrom six.moves import xrange # pylint: disable=redefined-builtinimport tensorflow as tf# Step 1: Download the data.url = 'http://mattmahoney.net/dc/'def maybe_download(filename, expected_bytes): """Download a file if not present, and make sure it's the right size.""" if not os.path.exists(filename): filename, _ = urllib.request.urlretrieve(url + filename, filename) statinfo = os.stat(filename) if statinfo.st_size == expected_bytes: print('Found and verified', filename) else: print(statinfo.st_size) raise Exception( 'Failed to verify ' + filename + '. Can you get to it with a browser?') return filenamefilename = maybe_download('text8.zip', 31344016)# Read the data into a list of strings.def read_data(filename): """Extract the first file enclosed in a zip file as a list of words.""" with zipfile.ZipFile(filename) as f: data = tf.compat.as_str(f.read(f.namelist()[0])).split() return datavocabulary = read_data(filename)print('Data size', len(vocabulary))# Step 2: Build the dictionary and replace rare words with UNK token.vocabulary_size = 50000def build_dataset(words, n_words): """Process raw inputs into a dataset.""" count = [['UNK', -1]] count.extend(collections.Counter(words).most_common(n_words - 1)) dictionary = dict() for word, _ in count: dictionary[word] = len(dictionary) data = list() unk_count = 0 for word in words: if word in dictionary: index = dictionary[word] else: index = 0 # dictionary['UNK'] unk_count += 1 data.append(index) count[0][1] = unk_count reversed_dictionary = dict(zip(dictionary.values(), dictionary.keys())) return data, count, dictionary, reversed_dictionarydata, count, dictionary, reverse_dictionary = build_dataset(vocabulary, vocabulary_size)del vocabulary # Hint to reduce memory.print('Most common words (+UNK)', count[:5])print('Sample data', data[:10], [reverse_dictionary[i] for i in data[:10]])data_index = 0# Step 3: Function to generate a training batch for the skip-gram model.def generate_batch(batch_size, num_skips, skip_window): global data_index assert batch_size % num_skips == 0 assert num_skips <= 2 * skip_window batch = np.ndarray(shape=(batch_size), dtype=np.int32) labels = np.ndarray(shape=(batch_size, 1), dtype=np.int32) span = 2 * skip_window + 1 # [ skip_window target skip_window ] buffer = collections.deque(maxlen=span) if data_index + span > len(data): data_index = 0 buffer.extend(data[data_index:data_index + span]) data_index += span for i in range(batch_size // num_skips): target = skip_window # target label at the center of the buffer targets_to_avoid = [skip_window] for j in range(num_skips): while target in targets_to_avoid: target = random.randint(0, span - 1) targets_to_avoid.append(target) batch[i * num_skips + j] = buffer[skip_window] labels[i * num_skips + j, 0] = buffer[target] if data_index == len(data): buffer[:] = data[:span] data_index = span else: buffer.append(data[data_index]) data_index += 1 # Backtrack a little bit to avoid skipping words in the end of a batch data_index = (data_index + len(data) - span) % len(data) return batch, labelsbatch, labels = generate_batch(batch_size=8, num_skips=2, skip_window=1)for i in range(8): print(batch[i], reverse_dictionary[batch[i]], '->', labels[i, 0], reverse_dictionary[labels[i, 0]])# Step 4: Build and train a skip-gram model.batch_size = 128embedding_size = 128 # Dimension of the embedding vector.skip_window = 1 # How many words to consider left and right.num_skips = 2 # How many times to reuse an input to generate a label.# We pick a random validation set to sample nearest neighbors. Here we limit the# validation samples to the words that have a low numeric ID, which by# construction are also the most frequent.valid_size = 16 # Random set of words to evaluate similarity on.valid_window = 100 # Only pick dev samples in the head of the distribution.valid_examples = np.random.choice(valid_window, valid_size, replace=False)num_sampled = 64 # Number of negative examples to sample.graph = tf.Graph()with graph.as_default(): # Input data. train_inputs = tf.placeholder(tf.int32, shape=[batch_size]) train_labels = tf.placeholder(tf.int32, shape=[batch_size, 1]) valid_dataset = tf.constant(valid_examples, dtype=tf.int32) # Ops and variables pinned to the CPU because of missing GPU implementation with tf.device('/cpu:0'): # Look up embeddings for inputs. embeddings = tf.Variable( tf.random_uniform([vocabulary_size, embedding_size], -1.0, 1.0)) embed = tf.nn.embedding_lookup(embeddings, train_inputs) # Construct the variables for the NCE loss nce_weights = tf.Variable( tf.truncated_normal([vocabulary_size, embedding_size], stddev=1.0 / math.sqrt(embedding_size))) nce_biases = tf.Variable(tf.zeros([vocabulary_size])) # Compute the average NCE loss for the batch. # tf.nce_loss automatically draws a new sample of the negative labels each # time we evaluate the loss. loss = tf.reduce_mean( tf.nn.nce_loss(weights=nce_weights, biases=nce_biases, labels=train_labels, inputs=embed, num_sampled=num_sampled, num_classes=vocabulary_size)) # Construct the SGD optimizer using a learning rate of 1.0. optimizer = tf.train.GradientDescentOptimizer(1.0).minimize(loss) # Compute the cosine similarity between minibatch examples and all embeddings. norm = tf.sqrt(tf.reduce_sum(tf.square(embeddings), 1, keep_dims=True)) normalized_embeddings = embeddings / norm valid_embeddings = tf.nn.embedding_lookup( normalized_embeddings, valid_dataset) similarity = tf.matmul( valid_embeddings, normalized_embeddings, transpose_b=True) # Add variable initializer. init = tf.global_variables_initializer()# Step 5: Begin training.num_steps = 100001with tf.Session(graph=graph) as session: # We must initialize all variables before we use them. init.run() print('Initialized') average_loss = 0 for step in xrange(num_steps): batch_inputs, batch_labels = generate_batch( batch_size, num_skips, skip_window) feed_dict = {train_inputs: batch_inputs, train_labels: batch_labels} # We perform one update step by evaluating the optimizer op (including it # in the list of returned values for session.run() _, loss_val = session.run([optimizer, loss], feed_dict=feed_dict) average_loss += loss_val if step % 2000 == 0: if step > 0: average_loss /= 2000 # The average loss is an estimate of the loss over the last 2000 batches. print('Average loss at step ', step, ': ', average_loss) average_loss = 0 # Note that this is expensive (~20% slowdown if computed every 500 steps) if step % 10000 == 0: sim = similarity.eval() for i in xrange(valid_size): valid_word = reverse_dictionary[valid_examples[i]] top_k = 8 # number of nearest neighbors nearest = (-sim[i, :]).argsort()[1:top_k + 1] log_str = 'Nearest to %s:' % valid_word for k in xrange(top_k): close_word = reverse_dictionary[nearest[k]] log_str = '%s %s,' % (log_str, close_word) print(log_str) final_embeddings = normalized_embeddings.eval()# Step 6: Visualize the embeddings.def plot_with_labels(low_dim_embs, labels, filename='tsne.png'): assert low_dim_embs.shape[0] >= len(labels), 'More labels than embeddings' plt.figure(figsize=(18, 18)) # in inches for i, label in enumerate(labels): x, y = low_dim_embs[i, :] plt.scatter(x, y) plt.annotate(label, xy=(x, y), xytext=(5, 2), textcoords='offset points', ha='right', va='bottom') plt.savefig(filename)try: # pylint: disable=g-import-not-at-top from sklearn.manifold import TSNE import matplotlib.pyplot as plt tsne = TSNE(perplexity=30, n_components=2, init='pca', n_iter=5000) plot_only = 500 low_dim_embs = tsne.fit_transform(final_embeddings[:plot_only, :]) labels = [reverse_dictionary[i] for i in xrange(plot_only)] plot_with_labels(low_dim_embs, labels)except ImportError: print('Please install sklearn, matplotlib, and scipy to show embeddings.')
其他的就不多说,我想重点分析一下generate_batch这个函数。
# Step 3: Function to generate a training batch for the skip-gram model.#从文本总体的第二个单词开始,每个单词依次作为输入,它的输出可以是上下文范围内的单词中的任何一个单词。一般不是取全部而是随机取其中的几组,以增加随机性。#batch_size 就是每次训练用多少数据,skip_window是确定取一个词周边多远的词来训练,num_skips对于一个输入数据,产生多少个标签数据。所以skip_window决定上下文的长度,就是当前词的周围多少个词内的词被视为它的上下文的范围内,然后从这上下文范围内的词中随机取num_skips个与输入组合成num_skips组训练数据def generate_batch(batch_size, num_skips, skip_window): global data_index assert batch_size % num_skips == 0 assert num_skips <= 2 * skip_window #保证num_skips不会超过当前输入的的上下文的总个数 batch = np.ndarray(shape=(batch_size), dtype=np.int32) labels = np.ndarray(shape=(batch_size, 1), dtype=np.int32) span = 2 * skip_window + 1 # [ skip_window target skip_window ] #这个很重要,最大长度是span,后面如果数据超过这个长度,前面的会被挤掉,这样使得buffer里面永远是data_index周围的span歌数据, #而buffer的第skip_window个数据永远是当前处理循环里的输入数据 buffer = collections.deque(maxlen=span) if data_index + span > len(data): data_index = 0 buffer.extend(data[data_index:data_index + span]) data_index += span for i in range(batch_size // num_skips): target = skip_window # target label at the center of the buffer targets_to_avoid = [skip_window] #自己肯定要排除掉,不能自己作为自己的上下文 for j in range(num_skips): while target in targets_to_avoid: target = random.randint(0, span - 1) #随机取一个,增强随机性,减少训练时进入局部最优解 targets_to_avoid.append(target) batch[i * num_skips + j] = buffer[skip_window] #这里保存的是训练的输入序列 labels[i * num_skips + j, 0] = buffer[target] #这里保存的是训练时的输出序列,也就是标签 if data_index == len(data): #超长时回到开始 buffer[:] = data[:span] data_index = span else: buffer.append(data[data_index]) #append时会把queue的开始的几个挤掉 data_index += 1 # Backtrack a little bit to avoid skipping words in the end of a batch data_index = (data_index + len(data) - span) % len(data) return batch, labels
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