pytorch实现mnist分类的示例讲解

torchvision包 包含了目前流行的数据集，模型结构和常用的图片转换工具。

torchvision.datasets中包含了以下数据集

MNIST
COCO（用于图像标注和目标检测）(Captioning and Detection)
LSUN Classification
ImageFolder
Imagenet-12
CIFAR10 and CIFAR100
STL10

torchvision.models

torchvision.models模块的 子模块中包含以下模型结构。
AlexNet
VGG
ResNet
SqueezeNet
DenseNet You can construct a model with random weights by calling its constructor:

pytorch torchvision transform

对PIL.Image进行变换

from __future__ import print_function
import argparse #Python 命令行解析工具
import torch
import torch.nn as nn
import torch.nn.functional as F
import torch.optim as optim
from torchvision import datasets,transforms

class Net(nn.Module):
  def __init__(self):
    super(Net,self).__init__()
    self.conv1 = nn.Conv2d(1,10,kernel_size=5)
    self.conv2 = nn.Conv2d(10,20,kernel_size=5)
    self.conv2_drop = nn.Dropout2d()
    self.fc1 = nn.Linear(320,50)
    self.fc2 = nn.Linear(50,10)

  def forward(self,x):
    x = F.relu(F.max_pool2d(self.conv1(x),2))
    x = F.relu(F.max_pool2d(self.conv2_drop(self.conv2(x)),2))
    x = x.view(-1,320)
    x = F.relu(self.fc1(x))
    x = F.dropout(x,training=self.training)
    x = self.fc2(x)
    return F.log_softmax(x,dim=1)

def train(args,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 % args.log_interval == 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()))

def test(args,test_loader):
  model.eval()
  test_loss = 0
  correct = 0
  with torch.no_grad():
    for data,target in test_loader:
      data,target.to(device)
      output = model(data)
      test_loss += F.nll_loss(output,target,size_average=False).item() # sum up batch loss
      pred = output.max(1,keepdim=True)[1] # get the index of the max log-probability
      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)))

def main():
  # Training settings
  parser = argparse.ArgumentParser(description='PyTorch MNIST Example')
  parser.add_argument('--batch-size',type=int,default=64,metavar='N',help='input batch size for training (default: 64)')
  parser.add_argument('--test-batch-size',default=1000,help='input batch size for testing (default: 1000)')
  parser.add_argument('--epochs',default=10,help='number of epochs to train (default: 10)')
  parser.add_argument('--lr',type=float,default=0.01,metavar='LR',help='learning rate (default: 0.01)')
  parser.add_argument('--momentum',default=0.5,metavar='M',help='SGD momentum (default: 0.5)')
  parser.add_argument('--no-cuda',action='store_true',default=False,help='disables CUDA training')
  parser.add_argument('--seed',default=1,metavar='S',help='random seed (default: 1)')
  parser.add_argument('--log-interval',help='how many batches to wait before logging training status')
  args = parser.parse_args()
  use_cuda = not args.no_cuda and torch.cuda.is_available()

  torch.manual_seed(args.seed)

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

  kwargs = {'num_workers': 1,'pin_memory': True} if use_cuda else {}
  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=args.batch_size,shuffle=True,**kwargs)
  test_loader = torch.utils.data.DataLoader(
    datasets.MNIST('../data',train=False,batch_size=args.test_batch_size,**kwargs)

  model = Net().to(device)
  optimizer = optim.SGD(model.parameters(),lr=args.lr,momentum=args.momentum)

  for epoch in range(1,args.epochs + 1):
    train(args,epoch)
    test(args,test_loader)

if __name__ == '__main__':
  main()

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（编辑：李大同）

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