收集、读取、预处理数据，模型搭建、训练。

Pokemon Dataset

数据集加载

自定义数据集

__len__()函数返回数据集的数量，限制数据集迭代次数；
__getitem__索引样本；

import torch
from  torch.utils.data import Dataset 

class NumberDataset(Dataset):
    def __init__(self, training = True) -> None:
        super().__init__()
        self.training = training
        if training:
            self.samples = list(range(1, 1001))
        else:
            self.samples = list(range(1001, 1501))
    def __len__(self):
        return len(self.samples)
    def __getitem__(self, idx):
        return self.samples[idx]

if __name__ == '__main__':
    data = NumberDataset(True)
    print(len(data))
    print(data[10])

输出：
1000
11

数据预处理

resize
数据增强
增加数据集规模，辅助性提升一部分性能；
归一化
将数据分布缩放为一个指定均值和方差的正态分布；
转换为Tensor
将其它数据类型转换为pytorch的Tensor

图像数据存储结构

推荐采用一个label文件夹存储该label的图像；pytorch易于管理，它提供了一个API可以直接读取出这种存储结构的数据，而不用我们人为去写一个读取这些数据的代码；

代码

import torch
import os, csv
import random, glob
from  torch.utils.data import Dataset
import visdom,time
from torchvision import transforms
from PIL import Image

class Pokemon(Dataset):
    def __init__(self, root, resize, mode):
        super(Pokemon, self).__init__()
        self.root = root
        self.resize = resize
        self.name2label = dict()  # 将string转换为label
        for name in sorted(os.listdir(root)):
            if not os.path.isdir(os.path.join(root, name)):
                continue
            self.name2label[name] = len(self.name2label.keys())
        
        # image, label  将图像数据和label一一对应
        self.images, self.labels = self.load_csv('img_label.csv')

        if mode == 'train': # 60%
            self.images = self.images[:int(0.6*len(self.images))]  #取数据集的前60%作为训练集
            self.labels = self.labels[:int(0.6*len(self.labels))]
        elif mode == 'val': # 20% : 60%->80%
            self.images = self.images[int(0.6*len(self.images)): int(0.8*len(self.images))]  #取数据集的前60%-80%作为验证集
            self.labels = self.labels[int(0.6*len(self.labels)): int(0.8*len(self.labels))]
        else:  #20% : 80%->100%
            self.images = self.images[int(0.8*len(self.images)):]  #取数据集的最后20%作为测试集
            self.labels = self.labels[int(0.8*len(self.labels)):]

    def load_csv(self, filename):
        if not os.path.exists(os.path.join(self.root, filename)):
            images = []
            #将每一类图像的path提取出来存入image
            for name in self.name2label.keys():
                images += glob.glob(os.path.join(self.root, name, '*.png'))  #Return a list of paths matching a pathname pattern.
                images += glob.glob(os.path.join(self.root, name, '*.jpg'))
                images += glob.glob(os.path.join(self.root, name, '*.jpeg'))
            print(len(images))  

            #保存image path和label的对应关系，这里保存到csv文件中，节约内存
            with open(os.path.join(self.root, filename), mode = 'w', newline='') as f:
                writer = csv.writer(f)
                for img in images:
                    name = img.split(os.sep)[-2]
                    label = self.name2label[name]
                    writer.writerow([img, label])
            print('write to csv file:', filename)

        images = []
        labels = []
        #将image path和label的对应关系再重新读取出来
        with open(os.path.join(self.root, filename), mode='r') as f:
            reader = csv.reader(f)
            for row in reader:
                img, label = row
                label = int(label)
                images.append(img)
                labels.append(label)

        assert len(images) == len(labels)

        return images,labels

    def denormalize(self, x):  #c,h,w
        mean = [0.485, 0.456, 0.406] # c
        std = [0.229, 0.224, 0.225]  # c
        x = x*(torch.tensor(std).unsqueeze(1).unsqueeze(1)) + \
            torch.tensor(mean).unsqueeze(1).unsqueeze(1)
        return x

    def __len__(self):
        return len(self.images)

    def __getitem__(self, index):
        # indx: [0~len(slef.images)]
        img_path = self.images[index]
        label = self.labels[index]
        tf = transforms.Compose([
            lambda x: Image.open(img_path).convert('RGB'), # string path => image
            transforms.Resize((int(self.resize*1.5), int(self.resize*1.5))),
            transforms.RandomRotation(15), #如果旋转角度太大，可能会导致网络不收敛
            transforms.CenterCrop(self.resize), #裁剪成一个指定大小的形状
            transforms.ToTensor(),
            transforms.Normalize(mean = [0.485, 0.456, 0.406],
                                 std = [0.229, 0.224, 0.225])
        ])
        img = tf(img_path)
        label = torch.tensor(label)
        return img, label

def main():
    vis = visdom.Visdom()
    data = Pokemon('G:\BaiduNetdiskDownload\pokemon\pokeman', 224, 'train')
    x,y = next(iter(data))
    print(x.shape)
    print(y.shape)
    vis.image(data.denormalize(x), win = 'sample_x', opts=dict(title='sample_x'))
if __name__ == '__main__':
    main()

输出:
Setting up a new session...
torch.Size([3, 224, 224])
torch.Size([])

visdom：http://localhost:8097/

上述代码，可以通过torchvision.datasets.ImageFolder实现数据的读取和封装；
这种方式不是适合所有情况，只适合数据非常规整的存储了，并且如果对数据有一些额外的操作，还是要自己定义数据类。

import torchvision
    tf = transforms.Compose([
            transforms.Resize((64, 64)),
            transforms.ToTensor(),
        ])
    db = torchvision.datasets.ImageFolder(root='G:\BaiduNetdiskDownload\pokemon\pokeman', transform=tf)
    print(len(db))
    x,y = next(iter(db))
    print(x.shape)
    print(y)
    print(db.class_to_idx)
    vis.image(x, win = 'sample_x', opts=dict(title='sample_x'))
输出:
1167
torch.Size([3, 64, 64])
0
{'bulbasaur': 0, 'charmander': 1, 'mewtwo': 2, 'pikachu': 3, 'squirtle': 4}

visdom：http://localhost:8097/

构建模型

ResNet18

import torch
import torch.nn as nn
import torch.nn.functional as F

class ResBlk(nn.Module):
    def __init__(self, in_channels, out_channels, stride):
        super().__init__()
        self.conv1 = nn.Conv2d(in_channels, out_channels, kernel_size=3, stride= stride, padding=1)
        self.bn1 = nn.BatchNorm2d(out_channels)
        self.conv2 = nn.Conv2d(out_channels, out_channels, kernel_size=3, stride=1, padding=1)
        self.bn2 = nn.BatchNorm2d(out_channels)

        self.extra = nn.Sequential()
        if in_channels != out_channels:
            self.extra = nn.Sequential(nn.Conv2d(in_channels, out_channels, kernel_size=1, stride=stride, padding=0),
            nn.BatchNorm2d(out_channels)
            )
    def forward(self, x):
        out = F.relu(self.bn1(self.conv1(x)))
        out = self.bn2(self.conv2(out))
        out = F.relu(out + self.extra(x))
        return out

class ResNet(nn.Module):
    def __init__(self):
        super().__init__()
        self.conv1 = nn.Sequential(
            nn.Conv2d(3, 16, kernel_size=3, stride=1, padding=0),
            nn.BatchNorm2d(16)
        )
        # fllow 4 blocks
        self.block1 = ResBlk(16, 32 ,2)
        self.block2 = ResBlk(32, 64, 2)
        self.block3 = ResBlk(64, 128, 2)
        self.block4 = ResBlk(128, 256, 2)
        self.pool = nn.AdaptiveAvgPool2d((1,1))
        self.outlayer = nn.Linear(256, 5)
    def forward(self, x):
        x = self.conv1(x)
        x = self.block1(x)
        x = self.block2(x)
        x = self.block3(x)
        x = self.block4(x)
        x = self.pool(x).flatten(1)
        logits = self.outlayer(x)
        return logits
if __name__ == '__main__':
    x = torch.rand(3,3, 224, 224)
    model = ResNet()
    out = model(x)
    p = sum(map(lambda p:p.numel(), model.parameters())) # torch.numel()函数,查看一个张量有多少元素
    print(out.shape)
    print('parameters size:', p)
    
输出：
torch.Size([3, 5])
parameters size: 1224645

训练模型

import torch
import torch.nn as nn
import torchvision
import visdom
from torch.utils.data import DataLoader
from pokemon import Pokemon
from resnet import ResNet

batch_size = 32
lr = 1e-3
epoches = 10
device = torch.device('cpu')
torch.manual_seed(1234)
root = 'G:\BaiduNetdiskDownload\pokemon\pokeman'

def evaluate(model, loader):
    correct = 0.
    for x,y in loader:
        x, y  =  x.to(device), y.to(device)
        with torch.no_grad():
            logits = model(x)
            preds = logits.argmax(dim = 1)
        correct += preds.eq(y).sum().float().item() 
    print('total correct:', correct)
    acc = correct / len(loader.dataset)
    return acc

def train():
    vis = visdom.Visdom()
    train_db = Pokemon(root, 224, 'train')
    val_db = Pokemon(root, 224, 'val')
    test_db = Pokemon(root, 224, 'test')
    print('train:', len(train_db))
    print('val:', len(val_db))
    print('test:', len(test_db))

    train_loader = DataLoader(train_db, batch_size=batch_size, shuffle = True, num_workers=4)
    val_loader = DataLoader(val_db, batch_size=batch_size, num_workers=2)
    test_loader = DataLoader(test_db, batch_size=batch_size, num_workers=2)

    model = ResNet().to(device)
    criteon = nn.CrossEntropyLoss()
    optimizer = torch.optim.Adam(model.parameters(), lr = lr)
    best_acc = 0.
    best_epoch = 0
    global_step = 0
    for epoch in range(epoches):
        model.train()
        for step, (x,y) in enumerate(train_loader):
            x,y = x.to(device), y.to(device)
            # print(y)
            logits = model(x)
            loss = criteon(logits, y)
            optimizer.zero_grad()
            loss.backward()
            optimizer.step()
            global_step += 1
            if step %2 == 0:
                print('epoch:[{}/{}]\tloss:{}'.format(step, epoch, loss.item()))
                vis.line([loss.item()], [global_step], win='loss', update='append')
        # evaluate
        model.eval()
        val_acc = evaluate(model, val_loader)
        print('epoch:[{}]\t accuracy:{}'.format(epoch, val_acc))
        vis.line([val_acc], [global_step], win='val_acc', update='append')
        if val_acc > best_acc:
            torch.save(model.state_dict(), 'best_model.pth')
            best_acc = val_acc
            best_epoch = epoch
            print('save model....')

    # test
    best_model = ResNet()
    best_model.load_state_dict(torch.load('best_model.pth'))
    best_acc = evaluate(best_model, test_loader)
    print('best acc:', best_acc, 'best epoch:', best_epoch)

if __name__ == '__main__':
    train()

输出：
...
epoch:[10/1]    loss:0.32095620036125183
epoch:[12/1]    loss:0.8680893182754517
epoch:[14/1]    loss:0.5944045782089233
epoch:[16/1]    loss:0.8467034101486206
epoch:[18/1]    loss:0.442536860704422
epoch:[20/1]    loss:0.5616939663887024
total correct: 192.0
epoch:[1]        accuracy:0.8240343347639485
save model....
epoch:[0/2]     loss:0.5097338557243347
...

训练过程的visdom：
loss曲线

acc曲线；

迁移学习

图像数据集与源域数据集存在比较多的重合的话（或者分布相似）比如ImageNet，那么可以使用源域数据集训练好的模型来辅助现在的特定任务，即将在A任务上训练好一个分类器，然后transfer到B任务上去；在B任务上叫微调，finetuning;

from torchvision.models import resnet18
class Flatten(nn.Module):
    def __init__(self) -> None:
        super().__init__()
    def forward(self, x):
        return x.flatten(1)
        
trained_model = resnet18(pretrained=True).to(device)
model = nn.Sequential(
    *list(trained_model.children())[:-1],  #取resnet18前17层, 该层输出为[b,512,1,1]
    Flatten(),
    nn.Linear(512,5)
)

输出：
...
epoch:[0/0]     loss:1.823586344718933
epoch:[2/0]     loss:0.30410656332969666
epoch:[4/0]     loss:0.7876781821250916
epoch:[6/0]     loss:0.8662126660346985
epoch:[8/0]     loss:0.5194013714790344
epoch:[10/0]    loss:0.390007346868515
...

部分visdom可视化：
loss曲线

acc曲线

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pytorch基础（九）- 自定义数据集训练模型 和 迁移学习