【深度学习入门篇 ③】PyTorch的数据加载

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掌握PyTorch数据通常的处理方法，是构建高效、可扩展模型的关键一步。今天，我们就利用PyTorch高效地处理数据，为模型训练打下坚实基础。

在前面的线性回归模型中，我们使用的数据很少，所以直接把全部数据放到模型中去使用。

但是在深度学习中，数据量通常是都非常多，非常大的，如此大量的数据，不可能一次性的在模型中进行向前的计算和反向传播，经常我们会对整个数据进行随机的打乱顺序，把数据处理成一个个的batch，同时还会对数据进行预处理。

所以，接下来我们来学习pytorch中的数据加载的方法~

Dataset基类介绍

dataset定义了这个数据集的总长度，以及会返回哪些参数，模板：

from torch.utils.data import Dataset
 
class MyDataset(Dataset):
    def __init__(self, ):
        # 定义数据集包含的数据和标签
 
    def __len__(self):
        return len(...)
    def __getitem__(self, index):
        # 当数据集被读取时，返回一个包含数据和标签的元组

数据加载案例

数据来源：http://archive.ics.uci.edu/ml/datasets/SMS+Spam+Collection

该数据集包含了5574条 短信，其中正常短信（标记为"ham"）4831条 ，骚扰短信（标记为"spam"）743条。

from torch.utils.data import Dataset,DataLoader
import pandas as pd

data_path = r"data/SMSSpamCollection"    # 路径

class SMSDataset(Dataset):
    def __init__(self):
        lines = open(data_path,"r",encoding="utf-8")
        # 前4个为label,后面的为短信内容
        lines = [[i[:4].strip(),i[4:].strip()] for i in lines]
        # 转为dataFrame类型
        self.df = pd.DataFrame(lines,columns=["label","sms"])

    def __getitem__(self, index):
        single_item = self.df.iloc[index,:]
        return single_item.values[0],single_item.values[1]

    def __len__(self):
        return self.df.shape[0]

我们现在已经成功地构建了一个数据集类 SMSDataset，这个类能够加载SMS 垃圾短信数据集，并将每条短信及其对应的标签（ham 或 spam）封装为可迭代的形式，以便于后续的数据加载和模型训练。

d = SMSDataset()
for i in range(len(d)):
    print(i,d[i])

输出：

...
5566 ('ham', "Why don't you wait 'til at least wednesday to see if you get your .")
5567 ('ham', 'Huh y lei...')
5568 ('spam', 'REMINDER FROM O2: To get 2.50 pounds free call credit and details of great offers pls reply 2 this text with your valid name, house no and postcode')
5569 ('spam', 'This is the 2nd time we have tried 2 contact u. U have won the £750 Pound prize. 2 claim is easy, call 087187272008 NOW1! Only 10p per minute. BT-national-rate.')
5570 ('ham', 'Will ü b going to esplanade fr home?')
5571 ('ham', 'Pity, * was in mood for that. So...any other suggestions?')
5572 ('ham', "The guy did some bitching but I acted like i'd be interested in buying something else next week and he gave it to us for free")
5573 ('ham', 'Rofl. Its true to its name')

DataLoader格式说明

my_dataset = DataLoader(mydataset, batch_size=2, shuffle=True,num_workers=4)
 # num_workers:多进程读取数据

DataLoader的使用方法示例：

from torch.utils.data import DataLoader

dataset = MyDataset()
data_loader = DataLoader(dataset=dataset,batch_size=10,shuffle=True,num_workers=2)

#遍历，获取其中的每个batch的结果
for index, (label, context) in enumerate(data_loader):
    print(index,label,context)
    print("*"*100)

1. dataset：提前定义的dataset的实例

2. batch_size:传入数据的batch的大小，常用128,256等等

3. shuffle：bool类型，表示是否在每次获取数据的时候提前打乱数据

4. num_workers:加载数据的线程数

导入两个列表到Dataset

class MyDataset(Dataset):
    def __init__(self, ):
        # 定义数据集包含的数据和标签
        self.x_data = [i for i in range(10)]
        self.y_data = [2*i for i in range(10)]
 
    def __len__(self):
        return len(self.x_data)
    def __getitem__(self, index):
        # 当数据集被读取时，返回一个包含数据和标签的元组
        return self.x_data[index], self.y_data[index]
 
mydataset = MyDataset()
my_dataset = DataLoader(mydataset)
 
for x_i ,y_i in my_dataset:
    print(x_i,y_i)

💬输出：

tensor([0]) tensor([0])
tensor([1]) tensor([2])
tensor([2]) tensor([4])
tensor([3]) tensor([6])
tensor([4]) tensor([8])
tensor([5]) tensor([10])
tensor([6]) tensor([12])
tensor([7]) tensor([14])
tensor([8]) tensor([16])
tensor([9]) tensor([18])

💬如果修改batch_size为2，则输出：

tensor([0, 1]) tensor([0, 2])
tensor([2, 3]) tensor([4, 6])
tensor([4, 5]) tensor([ 8, 10])
tensor([6, 7]) tensor([12, 14])
tensor([8, 9]) tensor([16, 18])

• 我们可以看出，这是管理每次输出的批次的
• 还可以控制用多少个线程来加速读取数据（Num Workers），这参数和电脑cpu核心数有关系，尽量不超过电脑的核心数

我们看到可以不使用DataLoader，但这样就不能批次处理，只能for i in range(len(d))这样得到数据，也不能自动实现打乱逻辑，也不能串行加载。

data_loader = DataLoader(dataset=Dataset,batch_size=10,shuffle=True,num_workers=2)
# 获取其中的每个batch的结果
for index, (label, context) in enumerate(data_loader):
    print(index,label,context)
    print("*"*100)

输出：

555 ('ham', 'ham', 'ham', 'ham', 'ham', 'ham', 'ham', 'ham', 'ham', 'spam') ("I forgot 2 ask ü all smth.. There's a card on da present lei... How? Ü all want 2 write smth or sign on it?", 'Am i that much dirty fellow?', 'have got * few things to do. may be in * pub later.', 'Ok lor. Anyway i thk we cant get tickets now cos like quite late already. U wan 2 go look 4 ur frens a not? Darren is wif them now...', 'When you came to hostel.', 'Well i know Z will take care of me. So no worries.', 'I REALLY NEED 2 KISS U I MISS U MY BABY FROM UR BABY 4EVA', 'Booked ticket for pongal?', 'Awww dat is sweet! We can think of something to do he he! Have a nice time tonight ill probably txt u later cos im lonely :( xxx.', 'We tried to call you re your reply to our sms for a video mobile 750 mins UNLIMITED TEXT + free camcorder Reply of call 08000930705 Now')
****************************************************************************************************
556 ('ham', 'ham', 'ham', 'ham', 'ham', 'ham', 'ham', 'ham', 'ham', 'spam') (':-( sad puppy noise', 'G.W.R', 'Otherwise had part time job na-tuition..', 'They finally came to fix the ceiling.', 'The word "Checkmate" in chess comes from the Persian phrase "Shah Maat" which means; "the king is dead.." Goodmorning.. Have a good day..:)', 'Yup', 'I am real, baby! I want to bring out your inner tigress...', 'THANX4 TODAY CER IT WAS NICE 2 CATCH UP BUT WE AVE 2 FIND MORE TIME MORE OFTEN OH WELL TAKE CARE C U SOON.C', "She said,'' do u mind if I go into the bedroom for a minute ? '' ''OK'', I sed in a sexy mood. She came out 5 minuts latr wid a cake...n My Wife,", 'Ur cash-balance is currently 500 pounds - to maximize ur cash-in now send COLLECT to 83600 only 150p/msg. CC: 08718720201 PO BOX 114/14 TCR/W1')
****************************************************************************************************
557 ('ham', 'ham', 'ham', 'ham') ('It shall be fine. I have avalarr now. Will hollalater', "Nah it's straight, if you can just bring bud or drinks or something that's actually a little more useful than straight cash", 'U sleeping now.. Or you going to take? Haha.. I got spys wat.. Me online checking n replying mails lor..', 'In other news after hassling me to get him weed for a week andres has no money. HAUGHAIGHGTUJHYGUJ')
****************************************************************************************************

导入Excel数据到Dataset中

💥dataset只是一个类，因此数据可以从外部导入，我们也可以在dataset中规定数据在返回时进行更多的操作，数据在返回时也不一定是有两个。

pip install pandas
pip install openpyxl

class myDataset(Dataset):
    def __init__(self, data_loc):
        data = pd.read_ecl(data_loc)
        self.x1,self.x2,self.x3,self.x4,self.y = data['x1'],data['x2'],data['x3'] ,data['x4'],data['y']
 
    def __len__(self):
        return len(self.x1)
 
    def __getitem__(self, idx):
        return self.x1[idx],self.x2[idx],self.x3[idx],self.x4[idx],self.y[idx]
 
mydataset = myDataset(data_loc='e:\pythonProject Pytorch1\data.xls')
my_dataset = DataLoader(mydataset,batch_size=2)
for x1_i ,x2_i,x3_i,x4_i,y_i in my_dataset:
    print(x1_i,x2_i,x3_i,x4_i,y_i)

💯加载官方数据集

有一些数据集是PyTorch自带的，它被保存在TorchVision和torchtext 中

1. torchvision提供了对图片数据处理相关的api和数据

   • 数据位置：torchvision.datasets，例如：torchvision.datasets.MNIST(手写数字图片数据)

2. torchtext提供了对文本数据处理相关的API和数据

   • 数据位置：torchtext.datasets,例如：torchtext.datasets.IMDB（电影评论文本数据）

我们以Mnist手写数字为例 ，看看pytorch如何加载其中自带的数据集

torchvision.datasets.MNIST(root='/files/', train=True, download=True, transform=)`

1. root参数表示数据存放的位置

2. train：bool类型，表示是使用训练集的数据还是测试集的数据

3. download：bool类型，表示是否需要下载数据到root目录

4. transform:实现的对图片的处理函数