import torch import torch.nn as nn import torch.nn.functional as F import torch.optim as optim from torchvision import datasets, transforms torch.__version__
'1.0.0'
3.2 MNIST数据集手写数字识别¶
3.2.1 数据集介绍¶
MNIST 包括6万张28x28的训练样本，1万张测试样本，很多教程都会对它”下手”几乎成为一个 “典范”，可以说它就是计算机视觉里面的Hello World。所以我们这里也会使用MNIST来进行实战。
前面在介绍卷积神经网络的时候说到过LeNet-5，LeNet-5之所以强大就是因为在当时的环境下将MNIST数据的识别率提高到了99%，这里我们也自己从头搭建一个卷积神经网络，也达到99%的准确率
3.2.2 手写数字识别¶
首先，我们定义一些超参数：
BATCH_SIZE=512 #大概需要2G的显存 EPOCHS=20 # 总共训练批次 DEVICE = torch.device("cuda" if torch.cuda.is_available() else "cpu") # 让torch判断是否使用GPU，建议使用GPU环境，因为会快很多
因为Pytorch里面包含了MNIST的数据集，所以我们这里直接使用即可。 如果第一次执行会生成data文件夹，并且需要一些时间下载，如果以前下载过就不会再次下载了。
由于官方已经实现了dataset，所以这里可以直接使用DataLoader来对数据进行读取：
train_loader = torch.utils.data.DataLoader( datasets.MNIST('data', train=True, download=True, transform=transforms.Compose([ transforms.ToTensor(), transforms.Normalize((0.1307,), (0.3081,)) ])), batch_size=BATCH_SIZE, shuffle=True)
Downloading http://yann.lecun.com/exdb/mnist/train-images-idx3-ubyte.gz Downloading http://yann.lecun.com/exdb/mnist/train-labels-idx1-ubyte.gz Downloading http://yann.lecun.com/exdb/mnist/t10k-images-idx3-ubyte.gz Downloading http://yann.lecun.com/exdb/mnist/t10k-labels-idx1-ubyte.gz Processing... Done!
测试集：
test_loader = torch.utils.data.DataLoader( datasets.MNIST('data', train=False, transform=transforms.Compose([ transforms.ToTensor(), transforms.Normalize((0.1307,), (0.3081,)) ])), batch_size=BATCH_SIZE, shuffle=True)
下面我们定义一个网络，网络包含两个卷积层，conv1和conv2，然后紧接着两个线性层作为输出，最后输出10个维度，这10个维度我们作为0-9的标识来确定识别出的是那个数字。
在这里建议大家将每一层的输入和输出维度都作为注释标注出来，这样后面阅读代码的会方便很多。
class ConvNet(nn.Module): def __init__(self): super().__init__() # 1,28x28 self.conv1=nn.Conv2d(1,10,5) # 10, 24x24 self.conv2=nn.Conv2d(10,20,3) # 128, 10x10 self.fc1 = nn.Linear(20*10*10,500) self.fc2 = nn.Linear(500,10) def forward(self,x): in_size = x.size(0) out = self.conv1(x) #24 out = F.relu(out) out = F.max_pool2d(out, 2, 2) #12 out = self.conv2(out) #10 out = F.relu(out) out = out.view(in_size,-1) out = self.fc1(out) out = F.relu(out) out = self.fc2(out) out = F.log_softmax(out,dim=1) return out
我们实例化一个网络，实例化后使用.to方法将网络移动到GPU
优化器我们也直接选择简单暴力的Adam
model = ConvNet().to(DEVICE) optimizer = optim.Adam(model.parameters())
下面定义一下训练的函数，我们将训练的所有操作都封装到这个函数中
def train(model, device, train_loader, optimizer, epoch): model.train() for batch_idx, (data, target) in enumerate(train_loader): data, target = data.to(device), target.to(device) optimizer.zero_grad() output = model(data) loss = F.nll_loss(output, target) loss.backward() optimizer.step() if(batch_idx+1)%30 == 0: print('Train Epoch: {} [{}/{} ({:.0f}%)]\tLoss: {:.6f}'.format( epoch, batch_idx * len(data), len(train_loader.dataset), 100. * batch_idx / len(train_loader), loss.item()))
测试的操作也一样封装成一个函数：
def test(model, device, test_loader): model.eval() test_loss = 0 correct = 0 with torch.no_grad(): for data, target in test_loader: data, target = data.to(device), target.to(device) output = model(data) test_loss += F.nll_loss(output, target, reduction='sum').item() # 将一批的损失相加 pred = output.max(1, keepdim=True)[1] # 找到概率最大的下标 correct += pred.eq(target.view_as(pred)).sum().item() test_loss /= len(test_loader.dataset) print('\nTest set: Average loss: {:.4f}, Accuracy: {}/{} ({:.0f}%)\n'.format( test_loss, correct, len(test_loader.dataset), 100. * correct / len(test_loader.dataset)))
下面开始训练，这里就体现出封装起来的好处了，只要写两行就可以了：
for epoch in range(1, EPOCHS + 1): train(model, DEVICE, train_loader, optimizer, epoch) test(model, DEVICE, test_loader)
Train Epoch: 1 [14848/60000 (25%)] Loss: 0.272529 Train Epoch: 1 [30208/60000 (50%)] Loss: 0.235455 Train Epoch: 1 [45568/60000 (75%)] Loss: 0.101858 Test set: Average loss: 0.1018, Accuracy: 9695/10000 (97%) Train Epoch: 2 [14848/60000 (25%)] Loss: 0.057989 Train Epoch: 2 [30208/60000 (50%)] Loss: 0.083935 Train Epoch: 2 [45568/60000 (75%)] Loss: 0.051921 Test set: Average loss: 0.0523, Accuracy: 9825/10000 (98%) Train Epoch: 3 [14848/60000 (25%)] Loss: 0.045383 Train Epoch: 3 [30208/60000 (50%)] Loss: 0.049402 Train Epoch: 3 [45568/60000 (75%)] Loss: 0.061366 Test set: Average loss: 0.0408, Accuracy: 9866/10000 (99%) Train Epoch: 4 [14848/60000 (25%)] Loss: 0.035253 Train Epoch: 4 [30208/60000 (50%)] Loss: 0.038444 Train Epoch: 4 [45568/60000 (75%)] Loss: 0.036877 Test set: Average loss: 0.0433, Accuracy: 9859/10000 (99%) Train Epoch: 5 [14848/60000 (25%)] Loss: 0.038996 Train Epoch: 5 [30208/60000 (50%)] Loss: 0.020670 Train Epoch: 5 [45568/60000 (75%)] Loss: 0.034658 Test set: Average loss: 0.0339, Accuracy: 9885/10000 (99%) Train Epoch: 6 [14848/60000 (25%)] Loss: 0.067320 Train Epoch: 6 [30208/60000 (50%)] Loss: 0.016328 Train Epoch: 6 [45568/60000 (75%)] Loss: 0.017037 Test set: Average loss: 0.0348, Accuracy: 9881/10000 (99%) Train Epoch: 7 [14848/60000 (25%)] Loss: 0.022150 Train Epoch: 7 [30208/60000 (50%)] Loss: 0.009608 Train Epoch: 7 [45568/60000 (75%)] Loss: 0.012742 Test set: Average loss: 0.0346, Accuracy: 9895/10000 (99%) Train Epoch: 8 [14848/60000 (25%)] Loss: 0.010173 Train Epoch: 8 [30208/60000 (50%)] Loss: 0.019482 Train Epoch: 8 [45568/60000 (75%)] Loss: 0.012159 Test set: Average loss: 0.0323, Accuracy: 9886/10000 (99%) Train Epoch: 9 [14848/60000 (25%)] Loss: 0.007792 Train Epoch: 9 [30208/60000 (50%)] Loss: 0.006970 Train Epoch: 9 [45568/60000 (75%)] Loss: 0.004989 Test set: Average loss: 0.0294, Accuracy: 9909/10000 (99%) Train Epoch: 10 [14848/60000 (25%)] Loss: 0.003764 Train Epoch: 10 [30208/60000 (50%)] Loss: 0.005944 Train Epoch: 10 [45568/60000 (75%)] Loss: 0.001866 Test set: Average loss: 0.0361, Accuracy: 9902/10000 (99%) Train Epoch: 11 [14848/60000 (25%)] Loss: 0.002737 Train Epoch: 11 [30208/60000 (50%)] Loss: 0.014134 Train Epoch: 11 [45568/60000 (75%)] Loss: 0.001365 Test set: Average loss: 0.0309, Accuracy: 9905/10000 (99%) Train Epoch: 12 [14848/60000 (25%)] Loss: 0.003344 Train Epoch: 12 [30208/60000 (50%)] Loss: 0.003090 Train Epoch: 12 [45568/60000 (75%)] Loss: 0.004847 Test set: Average loss: 0.0318, Accuracy: 9902/10000 (99%) Train Epoch: 13 [14848/60000 (25%)] Loss: 0.001278 Train Epoch: 13 [30208/60000 (50%)] Loss: 0.003016 Train Epoch: 13 [45568/60000 (75%)] Loss: 0.001328 Test set: Average loss: 0.0358, Accuracy: 9906/10000 (99%) Train Epoch: 14 [14848/60000 (25%)] Loss: 0.002219 Train Epoch: 14 [30208/60000 (50%)] Loss: 0.003487 Train Epoch: 14 [45568/60000 (75%)] Loss: 0.014429 Test set: Average loss: 0.0376, Accuracy: 9896/10000 (99%) Train Epoch: 15 [14848/60000 (25%)] Loss: 0.003042 Train Epoch: 15 [30208/60000 (50%)] Loss: 0.002974 Train Epoch: 15 [45568/60000 (75%)] Loss: 0.000871 Test set: Average loss: 0.0346, Accuracy: 9909/10000 (99%) Train Epoch: 16 [14848/60000 (25%)] Loss: 0.000618 Train Epoch: 16 [30208/60000 (50%)] Loss: 0.003164 Train Epoch: 16 [45568/60000 (75%)] Loss: 0.007245 Test set: Average loss: 0.0357, Accuracy: 9905/10000 (99%) Train Epoch: 17 [14848/60000 (25%)] Loss: 0.001874 Train Epoch: 17 [30208/60000 (50%)] Loss: 0.013951 Train Epoch: 17 [45568/60000 (75%)] Loss: 0.000729 Test set: Average loss: 0.0322, Accuracy: 9922/10000 (99%) Train Epoch: 18 [14848/60000 (25%)] Loss: 0.002581 Train Epoch: 18 [30208/60000 (50%)] Loss: 0.001396 Train Epoch: 18 [45568/60000 (75%)] Loss: 0.015521 Test set: Average loss: 0.0389, Accuracy: 9914/10000 (99%) Train Epoch: 19 [14848/60000 (25%)] Loss: 0.000283 Train Epoch: 19 [30208/60000 (50%)] Loss: 0.001385 Train Epoch: 19 [45568/60000 (75%)] Loss: 0.011184 Test set: Average loss: 0.0383, Accuracy: 9901/10000 (99%) Train Epoch: 20 [14848/60000 (25%)] Loss: 0.000472 Train Epoch: 20 [30208/60000 (50%)] Loss: 0.003306 Train Epoch: 20 [45568/60000 (75%)] Loss: 0.018017 Test set: Average loss: 0.0393, Accuracy: 9899/10000 (99%)
我们看一下结果，准确率99%，没问题。
如果你的模型连MNIST都搞不定，那么你的模型没有任何的价值。
即使你的模型搞定了MNIST，你的模型也可能没有任何的价值。
MNIST是一个很简单的数据集，由于它的局限性只能作为研究用途，对实际应用带来的价值非常有限。但是通过这个例子，我们可以完全了解一个实际项目的工作流程。
我们找到数据集，对数据做预处理，定义我们的模型，调整超参数，测试训练，再通过训练结果对超参数进行调整或者对模型进行调整。
并且通过这个实战我们已经有了一个很好的模板，以后的项目都可以以这个模板为样例。