翻译过来就是： torchvision包由流行的数据集、模型体系结构和通用的计算机视觉图像转换组成。简单地说就是常用数据集+常见模型+常见图像增强方法
这个torchvision中主要有包组成：
torchvision.datasets torchvision.models torchvision.transforms 1 torchvision.datssets 包含贼多的数据集，包含下面的：
官方说明了：All the datasets have almost similar API. They all have two common arguments: transform and target_transform to transform the input and target respectively.
翻译过来就是：每一个数据集的API都是基本相同的。他们都有两个相同的参数：transform和target_transform(后面细讲)
我们就用最经典最简单的MNIST手写数字数据集作为例子，先看这个的API：
包含5个参数：
root：就是你想要保存MNIST数据集的位置，如果download是Flase的话，则会从目标位置读取数据集； download：True的话就会自动从网上下载这个数据集，到root的位置； train：True的话，数据集下载的是训练数据集；False的话则下载测试数据集（真方便，都不用自己划分了） transform：这个是对图像进行处理的transform，比方说旋转平移缩放，输入的是PIL格式的图像（不是tensor矩阵）； target_transform：这个是对图像标签进行处理的函数（这个我没用过不太确定，也许是做标签平滑那种的处理？） 【下面用代码进一步理解】
import torchvision mydataset = torchvision.datasets.MNIST(root=’./’, train=True, transform=None, target_transform=None, download=True) 运行结果如下，表示下载完毕（我不太确定这个下载数据集是否需要翻墙，我会把这次需要用的代码和数据集放到公众号，后台回复【torchvision】获取，下载出现问题请务必私戳我）
之后我们需要用到上一节课讲到的dataloader的内容：
from torch.utils.data import Dataset,DataLoader myloader = DataLoader(dataset=mydataset, batch_size=16) for i,(data,label) in enumerate(myloader): print(data.shape) print(label.shape) break 这时候会抛出一个错误：
大致看一看，就是pytorch的这个dataloader不是可以把数据集分成batch嘛，这个dataloder只能把tensor或者numpy这样的组合成batch，而现在的数据集的格式是PIL格式。这里验证了之前说到的，transform这个输入是PIL格式的图片，解决方法是：transform不能是None，我们需要将PIL转化成tensor才可以
所以我们把上面的transform稍作修改：
mydataset = torchvision.datasets.MNIST(root=’./’, train=True, transform=torchvision.transforms.ToTensor(), target_transform=None, download=True) 重新运行的时候可以得到结果：
结果中，16表示一个batch有16个样本，1表示这是单通道的灰度图片，28表示MNIST数据集图片是28×28的大小，然后每一个图片有一个label。
想要获取其他的数据集也是一样的，不过这里就用MNIST作为举例，其他的相同。
2 torchvision.models 预训练模型中torchvision提供了很多种，大体分成下面四类：
分别是分类模型，语义模型，目标检测模型和视频分类模型。这里呢因为分类模型比较常见也比较基础，就主要介绍这个好啦。
在torch1.6.0版本中(应该是比较近的版本)，主要包含下面的预训练模型：
构建模型可以通过下面的代码：
import torchvision.models as models resnet18 = models.resnet18() alexnet = models.alexnet() vgg16 = models.vgg16() squeezenet = models.squeezenet1_0() densenet = models.densenet161() inception = models.inception_v3() googlenet = models.googlenet() shufflenet = models.shufflenet_v2_x1_0() mobilenet = models.mobilenet_v2() resnext50_32x4d = models.resnext50_32x4d() wide_resnet50_2 = models.wide_resnet50_2() mnasnet = models.mnasnet1_0() 这样构建的模型的权重值是随机的，只有结构是保存的。想要获取预训练的模型，则需要设置参数pretrained: http://erecord.irm.cn/index.php?doc-view-639787 http://erecord.irm.cn/index.php?doc-view-639798 http://erecord.irm.cn/index.php?doc-view-639815 http://erecord.irm.cn/index.php?doc-view-639831 http://erecord.irm.cn/index.php?doc-view-639837 http://erecord.irm.cn/index.php?doc-view-639844 http://erecord.irm.cn/index.php?doc-view-639849 http://erecord.irm.cn/index.php?doc-view-639857 http://erecord.irm.cn/index.php?doc-view-639866 http://erecord.irm.cn/index.php?doc-view-639872 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http://erecord.irm.cn/index.php?doc-view-634866 http://erecord.irm.cn/index.php?doc-view-632809 http://erecord.irm.cn/index.php?doc-view-633600 http://erecord.irm.cn/index.php?doc-view-634468 http://erecord.irm.cn/index.php?doc-view-635228 http://erecord.irm.cn/index.php?doc-view-633287 http://erecord.irm.cn/index.php?doc-view-634152 http://erecord.irm.cn/index.php?doc-view-635055 http://erecord.irm.cn/index.php?doc-view-632988 http://erecord.irm.cn/index.php?doc-view-633802 http://erecord.irm.cn/index.php?doc-view-634704 http://erecord.irm.cn/index.php?doc-view-633518 http://erecord.irm.cn/index.php?doc-view-634351 http://erecord.irm.cn/index.php?doc-view-633176 http://erecord.irm.cn/index.php?doc-view-635600 http://erecord.irm.cn/index.php?doc-view-634033 http://erecord.irm.cn/index.php?doc-view-632887 http://erecord.irm.cn/index.php?doc-view-635410 http://erecord.irm.cn/index.php?doc-view-633685 http://erecord.irm.cn/index.php?doc-view-634557 http://erecord.irm.cn/index.php?doc-view-635275 http://erecord.irm.cn/index.php?doc-view-633373 http://erecord.irm.cn/index.php?doc-view-634214 http://erecord.irm.cn/index.php?doc-view-633036 http://erecord.irm.cn/index.php?doc-view-635479 http://erecord.irm.cn/index.php?doc-view-633880 http://erecord.irm.cn/index.php?doc-view-634800 http://erecord.irm.cn/index.php?doc-view-634409 http://erecord.irm.cn/index.php?doc-view-635181 http://erecord.irm.cn/index.php?doc-view-635019 http://erecord.irm.cn/index.php?doc-view-634644 http://erecord.irm.cn/index.php?doc-view-635315 http://erecord.irm.cn/index.php?doc-view-633454 http://erecord.irm.cn/index.php?doc-view-635874 http://erecord.irm.cn/index.php?doc-view-634291 http://erecord.irm.cn/index.php?doc-view-635114 http://erecord.irm.cn/index.php?doc-view-633121 http://erecord.irm.cn/index.php?doc-view-633967 http://erecord.irm.cn/index.php?doc-view-634890 http://erecord.irm.cn/index.php?doc-view-632829 http://erecord.irm.cn/index.php?doc-view-633624 http://erecord.irm.cn/index.php?doc-view-634493 http://erecord.irm.cn/index.php?doc-view-635247 http://erecord.irm.cn/index.php?doc-view-633310 http://erecord.irm.cn/index.php?doc-view-635709 http://erecord.irm.cn/index.php?doc-view-634173 import torchvision.models as models resnet18 = models.resnet18(pretrained=True) alexnet = models.alexnet(pretrained=True) squeezenet = models.squeezenet1_0(pretrained=True) vgg16 = models.vgg16(pretrained=True) densenet = models.densenet161(pretrained=True) inception = models.inception_v3(pretrained=True) googlenet = models.googlenet(pretrained=True) shufflenet = models.shufflenet_v2_x1_0(pretrained=True) mobilenet = models.mobilenet_v2(pretrained=True) resnext50_32x4d = models.resnext50_32x4d(pretrained=True) wide_resnet50_2 = models.wide_resnet50_2(pretrained=True) mnasnet = models.mnasnet1_0(pretrained=True) 我看官网的英文讲解，提到了一点：似乎这些模型的预训练数据集都是ImageNet的那个数据集，输入图片都是3通道的，并且要求输入图片的宽高不小于224像素，并且要求输入图片像素值的范围在0到1之间，然后做一个normalization标准化。
不知道各位在看一些案例的时候，有没有看到这个标准化：mean = [0.485, 0.456, 0.406] 和 std = [0.229, 0.224, 0.225]，这个应该是ImageNet的图片的标准化的参数。
这些预训练的模型参数不确定能不能直接下载，我也就把这些模型存起来一并放在了公众号的后台，依然是回复【torchvision】获取。
得到了.pth文件之后使用torch.load来加载即可。
model = torch.load(‘model.pth’) 模型比较 最后呢，torchvision官方提供了一个不同模型在Imagenet 1-crop 的一个错误率的比较。可以一起来看看到底哪个模型比较好使。这里我放了一些常见的模型。。像是Wide ResNet这种变种我就不放了。
网络 Top-1 error Top-5 error AlexNet 43.45 20.91 VGG-11 30.98 11.37 VGG-13 30.07 10.75 VGG-16 28.41 9.62 VGG-19 27.62 9.12 VGG-13 with BN 28.45 9.63 VGG-19 with BN 25.76 8.15 Resnet-18 30.24 10.92 Resnet-34 26.70 8.58 Resnet-50 23.85 7.13 Resnet-101 22.63 6.44 Resnet-152 21.69 5.94 SqueezeNet 1.1 41.81 19.38 Densenet-161 22.35 6.2 整体来看，还是Resnet残差网络效果好。不过EfficientNet效果更好，不过Torchvision中没有预训练，在之后会讲解EfficientNet的预训练模型的代码方便使用（先挖坑）。
