PyTorch翻译官网教程-TEXT CLASSIFICATION WITH THE TORCHTEXT LIBRARY

官网链接

Text classification with the torchtext library — PyTorch Tutorials 2.0.1+cu117 documentation

使用torchtext库进行文本分类

在本教程中，我们将展示如何使用torchtext库来构建用于文本分类分析的数据集。

• 使用迭代器访问原始数据
• 构建数据处理管道，将原始文本字符串转换为torch.Tensor ,可以用来训练模型
• 使用torch.utils.data.DataLoader对数据进行清洗和迭代

先决条件

在运行本教程之前，需要安装portalocker包的最新2.x版本。例如，在Colab环境中，可以通过在脚本顶部添加以下行来完成:

!pip install -U portalocker>=2.0.0`

访问原始数据集迭代器

torchtext库提供了一些原始数据集迭代器，用于生成原始文本字符串。例如，AG_NEWS数据集迭代器以标签和文本元组的形式生成原始数据。

要访问torchtext数据集，请按照https://github.com/pytorch/data上的说明安装torchdata。

import torch
from torchtext.datasets import AG_NEWS

train_iter = iter(AG_NEWS(split="train"))

next(train_iter)
>>> (3, "Fears for T N pension after talks Unions representing workers at Turner
Newall say they are 'disappointed' after talks with stricken parent firm Federal
Mogul.")

next(train_iter)
>>> (4, "The Race is On: Second Private Team Sets Launch Date for Human
Spaceflight (SPACE.com) SPACE.com - TORONTO, Canada -- A second\\team of
rocketeers competing for the  #36;10 million Ansari X Prize, a contest
for\\privately funded suborbital space flight, has officially announced
the first\\launch date for its manned rocket.")

next(train_iter)
>>> (4, 'Ky. Company Wins Grant to Study Peptides (AP) AP - A company founded
by a chemistry researcher at the University of Louisville won a grant to develop
a method of producing better peptides, which are short chains of amino acids, the
building blocks of proteins.')

准备数据处理管道

我们回顾了torchtext库的基本组件，包括词汇表、词向量和分词器。这些是原始文本字符串的基本数据处理模块。

下面是一个使用分词器和词汇表进行典型NLP数据处理的示例。第一步是使用原始训练数据集构建一个词汇表。这里我们使用内置的工厂函数build_vocab_from_iterator，它接受生成列表的迭代器或者单词列表的迭代器。用户还可以传递任何要添加到词汇表中的特殊符号。

from torchtext.data.utils import get_tokenizer
from torchtext.vocab import build_vocab_from_iterator

tokenizer = get_tokenizer("basic_english")
train_iter = AG_NEWS(split="train")


def yield_tokens(data_iter):
    for _, text in data_iter:
        yield tokenizer(text)


vocab = build_vocab_from_iterator(yield_tokens(train_iter), specials=[""])
vocab.set_default_index(vocab[""])

词汇表块将单词列表转换为整数。

vocab(['here', 'is', 'an', 'example'])
>>> [475, 21, 30, 5297]

使用分词器和词汇表准备文本处理管道。文本和标签管道将用于处理来自数据集迭代器的原始数据字符串。

text_pipeline = lambda x: vocab(tokenizer(x))
label_pipeline = lambda x: int(x) - 1

文本管道根据词汇表中定义的查找表将文本字符串转换为整数列表。标签管道将标签转换为整数。例如,

text_pipeline('here is the an example')
>>> [475, 21, 2, 30, 5297]
label_pipeline('10')
>>> 9

生成数据批处理和迭代器

建议PyTorch用户使用torch.utils.data.DataLoader(这里有一个教程here).它使用map风格的数据集，实现了getitem()和len()协议，表示从索引/键到数据样本的映射。它也适用于shuffle参数为False的可迭代数据集。

在发送到模型之前，collate_fn函数处理从DataLoader生成的一批样本。collate_fn的输入是DataLoader中批量大小相同的一批数据，collate_fn根据之前声明的数据处理管道来处理它们。注意这里，确保collate_fn被声明为顶级定义。这确保了该函数在每个worker中都可用。

在这个例子中，原始数据批量输入中的文本条目被打包到一个列表中，并连接为一个张量，用于nn.EmbeddingBag 的输入。偏移量是一个分隔符张量，表示文本张量中单个序列的起始索引。Label是一个张量，保存每个文本条目的标签。

from torch.utils.data import DataLoader

device = torch.device("cuda" if torch.cuda.is_available() else "cpu")


def collate_batch(batch):
    label_list, text_list, offsets = [], [], [0]
    for _label, _text in batch:
        label_list.append(label_pipeline(_label))
        processed_text = torch.tensor(text_pipeline(_text), dtype=torch.int64)
        text_list.append(processed_text)
        offsets.append(processed_text.size(0))
    label_list = torch.tensor(label_list, dtype=torch.int64)
    offsets = torch.tensor(offsets[:-1]).cumsum(dim=0)
    text_list = torch.cat(text_list)
    return label_list.to(device), text_list.to(device), offsets.to(device)


train_iter = AG_NEWS(split="train")
dataloader = DataLoader(
    train_iter, batch_size=8, shuffle=False, collate_fn=collate_batch
)

定义模型

该模型由 nn.EmbeddingBag 层和用于分类的线性层组成。使用模式模式”mean“ 的nn.EmbeddingBag 计算”bag“ Embedding的平均值。尽管这里的文本条目具有不同的长度，但nn.EmbeddingBag模块在这里不需要填充，因为文本长度保存在偏移量中。

此外，由于nn.EmbeddingBag在运行中累计了所有Embedding的平均值，因此nn.EmbeddingBag可以提高处理一系列张量的性能和内存效率。

from torch import nn


class TextClassificationModel(nn.Module):
    def __init__(self, vocab_size, embed_dim, num_class):
        super(TextClassificationModel, self).__init__()
        self.embedding = nn.EmbeddingBag(vocab_size, embed_dim, sparse=False)
        self.fc = nn.Linear(embed_dim, num_class)
        self.init_weights()

    def init_weights(self):
        initrange = 0.5
        self.embedding.weight.data.uniform_(-initrange, initrange)
        self.fc.weight.data.uniform_(-initrange, initrange)
        self.fc.bias.data.zero_()

    def forward(self, text, offsets):
        embedded = self.embedding(text, offsets)
        return self.fc(embedded)

初始化实例

AG_NEWS数据集有4个标签，因此类别数为4

1 : World
2 : Sports
3 : Business
4 : Sci/Tec

我们构建了一个embedding维度为64的模型。词汇表的大小等于词汇表实例的长度。类别的数量等于标签的数量，

train_iter = AG_NEWS(split="train")
num_class = len(set([label for (label, text) in train_iter]))
vocab_size = len(vocab)
emsize = 64
model = TextClassificationModel(vocab_size, emsize, num_class).to(device)

定义函数来训练模型和评估结果

import time


def train(dataloader):
    model.train()
    total_acc, total_count = 0, 0
    log_interval = 500
    start_time = time.time()

    for idx, (label, text, offsets) in enumerate(dataloader):
        optimizer.zero_grad()
        predicted_label = model(text, offsets)
        loss = criterion(predicted_label, label)
        loss.backward()
        torch.nn.utils.clip_grad_norm_(model.parameters(), 0.1)
        optimizer.step()
        total_acc += (predicted_label.argmax(1) == label).sum().item()
        total_count += label.size(0)
        if idx % log_interval == 0 and idx > 0:
            elapsed = time.time() - start_time
            print(
                "| epoch {:3d} | {:5d}/{:5d} batches "
                "| accuracy {:8.3f}".format(
                    epoch, idx, len(dataloader), total_acc / total_count
                )
            )
            total_acc, total_count = 0, 0
            start_time = time.time()


def evaluate(dataloader):
    model.eval()
    total_acc, total_count = 0, 0

    with torch.no_grad():
        for idx, (label, text, offsets) in enumerate(dataloader):
            predicted_label = model(text, offsets)
            loss = criterion(predicted_label, label)
            total_acc += (predicted_label.argmax(1) == label).sum().item()
            total_count += label.size(0)
    return total_acc / total_count

分割数据集并运行模型

由于原始AG_NEWS没有验证数据集，我们将训练数据集划分为训练集/验证集，划分比为0.95(训练集)和0.05(验证集)。这里我们使用torch.utils.data.dataset.random_split 。PyTorch核心库中的random_split函数。

CrossEntropyLoss(交叉熵损失)准则将nn.LogSoftmax()和nn.NLLLoss()组合在一个类中。当训练包含C类的分类问题时，它很有用。SGD采用随机梯度下降法作为优化器。初始学习率设置为5.0。这里使用StepLR 通过epoch调整学习率。

from torch.utils.data.dataset import random_split
from torchtext.data.functional import to_map_style_dataset

# Hyperparameters
EPOCHS = 10  # epoch
LR = 5  # learning rate
BATCH_SIZE = 64  # batch size for training

criterion = torch.nn.CrossEntropyLoss()
optimizer = torch.optim.SGD(model.parameters(), lr=LR)
scheduler = torch.optim.lr_scheduler.StepLR(optimizer, 1.0, gamma=0.1)
total_accu = None
train_iter, test_iter = AG_NEWS()
train_dataset = to_map_style_dataset(train_iter)
test_dataset = to_map_style_dataset(test_iter)
num_train = int(len(train_dataset) * 0.95)
split_train_, split_valid_ = random_split(
    train_dataset, [num_train, len(train_dataset) - num_train]
)

train_dataloader = DataLoader(
    split_train_, batch_size=BATCH_SIZE, shuffle=True, collate_fn=collate_batch
)
valid_dataloader = DataLoader(
    split_valid_, batch_size=BATCH_SIZE, shuffle=True, collate_fn=collate_batch
)
test_dataloader = DataLoader(
    test_dataset, batch_size=BATCH_SIZE, shuffle=True, collate_fn=collate_batch
)

for epoch in range(1, EPOCHS + 1):
    epoch_start_time = time.time()
    train(train_dataloader)
    accu_val = evaluate(valid_dataloader)
    if total_accu is not None and total_accu > accu_val:
        scheduler.step()
    else:
        total_accu = accu_val
    print("-" * 59)
    print(
        "| end of epoch {:3d} | time: {:5.2f}s | "
        "valid accuracy {:8.3f} ".format(
            epoch, time.time() - epoch_start_time, accu_val
        )
    )
    print("-" * 59)

输出

| epoch   1 |   500/ 1782 batches | accuracy    0.694
| epoch   1 |  1000/ 1782 batches | accuracy    0.856
| epoch   1 |  1500/ 1782 batches | accuracy    0.877
-----------------------------------------------------------
| end of epoch   1 | time: 12.46s | valid accuracy    0.886
-----------------------------------------------------------
| epoch   2 |   500/ 1782 batches | accuracy    0.898
| epoch   2 |  1000/ 1782 batches | accuracy    0.899
| epoch   2 |  1500/ 1782 batches | accuracy    0.906
-----------------------------------------------------------
| end of epoch   2 | time: 11.99s | valid accuracy    0.895
-----------------------------------------------------------
| epoch   3 |   500/ 1782 batches | accuracy    0.916
| epoch   3 |  1000/ 1782 batches | accuracy    0.913
| epoch   3 |  1500/ 1782 batches | accuracy    0.915
-----------------------------------------------------------
| end of epoch   3 | time: 12.07s | valid accuracy    0.894
-----------------------------------------------------------
| epoch   4 |   500/ 1782 batches | accuracy    0.930
| epoch   4 |  1000/ 1782 batches | accuracy    0.932
| epoch   4 |  1500/ 1782 batches | accuracy    0.929
-----------------------------------------------------------
| end of epoch   4 | time: 12.06s | valid accuracy    0.902
-----------------------------------------------------------
| epoch   5 |   500/ 1782 batches | accuracy    0.932
| epoch   5 |  1000/ 1782 batches | accuracy    0.933
| epoch   5 |  1500/ 1782 batches | accuracy    0.931
-----------------------------------------------------------
| end of epoch   5 | time: 12.04s | valid accuracy    0.902
-----------------------------------------------------------
| epoch   6 |   500/ 1782 batches | accuracy    0.933
| epoch   6 |  1000/ 1782 batches | accuracy    0.932
| epoch   6 |  1500/ 1782 batches | accuracy    0.935
-----------------------------------------------------------
| end of epoch   6 | time: 12.02s | valid accuracy    0.903
-----------------------------------------------------------
| epoch   7 |   500/ 1782 batches | accuracy    0.934
| epoch   7 |  1000/ 1782 batches | accuracy    0.933
| epoch   7 |  1500/ 1782 batches | accuracy    0.935
-----------------------------------------------------------
| end of epoch   7 | time: 12.09s | valid accuracy    0.903
-----------------------------------------------------------
| epoch   8 |   500/ 1782 batches | accuracy    0.935
| epoch   8 |  1000/ 1782 batches | accuracy    0.933
| epoch   8 |  1500/ 1782 batches | accuracy    0.935
-----------------------------------------------------------
| end of epoch   8 | time: 12.10s | valid accuracy    0.904
-----------------------------------------------------------
| epoch   9 |   500/ 1782 batches | accuracy    0.934
| epoch   9 |  1000/ 1782 batches | accuracy    0.934
| epoch   9 |  1500/ 1782 batches | accuracy    0.934
-----------------------------------------------------------
| end of epoch   9 | time: 12.05s | valid accuracy    0.904
-----------------------------------------------------------
| epoch  10 |   500/ 1782 batches | accuracy    0.934
| epoch  10 |  1000/ 1782 batches | accuracy    0.936
| epoch  10 |  1500/ 1782 batches | accuracy    0.933
-----------------------------------------------------------
| end of epoch  10 | time: 12.00s | valid accuracy    0.905
-----------------------------------------------------------

使用测试数据集评估模型

检查测试数据集的结果……

print("Checking the results of test dataset.")
accu_test = evaluate(test_dataloader)
print("test accuracy {:8.3f}".format(accu_test))

输出

Checking the results of test dataset.
test accuracy    0.907

测试一个随机的新闻

使用迄今为止最好的模型，测试一个高尔夫新闻。

ag_news_label = {1: "World", 2: "Sports", 3: "Business", 4: "Sci/Tec"}


def predict(text, text_pipeline):
    with torch.no_grad():
        text = torch.tensor(text_pipeline(text))
        output = model(text, torch.tensor([0]))
        return output.argmax(1).item() + 1


ex_text_str = "MEMPHIS, Tenn. – Four days ago, Jon Rahm was \
    enduring the season’s worst weather conditions on Sunday at The \
    Open on his way to a closing 75 at Royal Portrush, which \
    considering the wind and the rain was a respectable showing. \
    Thursday’s first round at the WGC-FedEx St. Jude Invitational \
    was another story. With temperatures in the mid-80s and hardly any \
    wind, the Spaniard was 13 strokes better in a flawless round. \
    Thanks to his best putting performance on the PGA Tour, Rahm \
    finished with an 8-under 62 for a three-stroke lead, which \
    was even more impressive considering he’d never played the \
    front nine at TPC Southwind."

model = model.to("cpu")

print("This is a %s news" % ag_news_label[predict(ex_text_str, text_pipeline)])

输出

This is a Sports news