本文代碼主要參考:
https://www.kaggle.com/blankitdl/using-pytorch-resnet
https://www.kaggle.com/blankitdl/use-pretrained-pytorch-models

Pytorch下的遷移學習

一、基本介紹

比賽題目
Dog Breed Identification
官網對于比賽的介紹

In this playground competition, you are provided a strictly canine subset of ImageNet in order to practice fine-grained image categorization. How well you can tell your Norfolk Terriers from your Norwich Terriers? With 120 breeds of dogs and a limited number training images per class, you might find the problem more, err, ruff than you anticipated.

在這個比賽中，您將獲得ImageNet的犬類子集，以便練習細粒度的圖像分類。你能把Norwich Terriers和Norfolk Terriers 區別開來嗎？有120個品種的狗和數量有限的訓練圖像，你可能會發現問題比你預期的更多。
數據
查看數據labels.csv是帶有標簽的訓練數據,10.2kx2，sample_submission.csv是測試的數據，10.4k x 121。訓練集和測試集幾乎是一樣的。

二、代碼解析

讀取數據信息
查看labels信息。id是圖片名稱，id.jpg保存在train文件夾下。breed是狗的類別

df_train = pd.read_csv('labels.csv') submission = pd.read_csv('sample_submission.csv') df_train.head()

類別名轉成數字標簽
df_train.breed.unique()獲取df_train這個數據表的breed列中唯一值得個數。
將120種狗的類別對應成數字標簽0~119.

class_to_idx = {x:i for i,x in enumerate(df_train.breed.unique())}#類別名轉成數字標簽 idx_to_class = {i:x for i,x in enumerate(df_train.breed.unique())}#數字標簽轉成類別名，便于測試時知道輸出類別 df_train['target'] = [class_to_idx[x] for x in df_train.breed]#在原始的表格中添加數字標簽列

劃分訓練集
需要調用sklearn.model_selection的train_test_split.
將練集的一部份劃分出來作為驗證集的目的是挑選模型，防止模型過擬合。

train,val =train_test_split(df_train,test_size=0.4, random_state=0)#將訓練集的一部分劃分為測試集

構建數據集

class DogsDataset(Dataset):'''df: df_train,有id,breed和新增的target信息root_dir:圖片存放的目錄transform: 圖像處理方法'''def __init__(self, df, root_dir, transform=None):self.df = dfself.root_dir = root_dirself.transform = transformdef __len__(self):return len(self.df)#數據量def __getitem__(self, idx):img_name = '{}.jpg'.format(self.df.iloc[idx, 0])#圖片名fullname = os.path.join(self.root_dir, img_name)#圖片路徑image = Image.open(fullname)#PIL的Image方法cls = self.df.iloc[idx,2]#2是target信息if self.transform:image = self.transform(image)return [image, cls]#返回PIL對象和數字標簽

定義圖像處理方法
用到torchvision.transforms庫

normalize = transforms.Normalize(mean=[0.485, 0.456, 0.406],std=[0.229, 0.224, 0.225] ) ds_trans = transforms.Compose([transforms.Resize(224),transforms.CenterCrop(224),transforms.ToTensor(),normalize])

定義dataloader
用到from torch.utils.data 的DataLoader

BATCH_SIZE = 128 data_dir = '/train/'#注意地址 train_ds = DogsDataset(train, data_dir+'train/', transform=ds_trans)#形成Dataset val_ds = DogsDataset(val, data_dir+'train/', transform=ds_trans) train_dl = DataLoader(train_ds, batch_size=BATCH_SIZE, shuffle=True, num_workers=1)#構建Dataloader val_dl = DataLoader(val_ds, batch_size=4, shuffle=True, num_workers=1) dataloaders= {'train':train_dl,'val':val_dl}

• 查看Dataloader是否構建成功

for data in train_dl:x,y = dataprint(x.shape,y.shape)print(y)break

定義模型
數據集是從Imagnet的一個子集，可以使用在這個數據集上預訓練的模型，這里選用的是resnet18,再次基礎上微調。
NUM_CLASS狗的種類數，即最后預測結果的維度.
model.fc.in_features是 resnet18最后一層輸入神經元的個數
用in_fc_nums和NUM_CLASS作為輸入和輸出神經元的個數，替換resnet18的全連接層

model = models.resnet18(pretrained=True) NUM_CLASS = 120#狗的種類數，即最后預測結果的維度 in_fc_nums = model.fc.in_features#resnet18最后一層輸入神經元的個數 fc = nn.Linear(in_fc_nums,NUM_CLASS) model.fc = fc model = model.cuda()

定義優化器及學習率的調節方法
使用的庫分別是torch.optim,torch.nn,torch.optim.lr_scheduler

optimizer = optim.Adam(model.parameters(), lr=0.001, betas=(0.9, 0.999)) # 選用AdamOptimizer #optimizer = optim.Adam(model.fc.parameters(), lr=0.001, betas=(0.9, 0.999)) # 只優化全連接層 criterion = nn.CrossEntropyLoss() # 定義損失函數，交叉熵 scheduler = lr_scheduler.StepLR(optimizer, step_size=7, gamma=0.1)

模型訓練
模型訓練函數有以下幾個功能：

• 訓練模型
• 訓練一個epoch后驗證，記錄訓練的 訓練和驗證集的誤差及精度
• 保存在驗證集上精度最高的模型
• 記錄每個epoch和整個訓練過程的時間

def train_model(model, criterion, optimizer, scheduler, num_epochs=25):since = time.time()best_model_wts = model.cuda().state_dict()best_acc = 0.0for epoch in range(num_epochs):print('Epoch {}/{}'.format(epoch+1, num_epochs))print('-' * 20)# Each epoch has a training and validation phasefor phase in ['train', 'val']:since_epoch = time.time()if phase == 'train': #訓練階段更新學習率scheduler.step()model.train(True) # Set model to training modeelse:model.train(False) # Set model to evaluate moderunning_loss = 0.0running_corrects = 0# Iterate over data.for data in dataloaders[phase]:# get the inputsinputs, labels = datainputs = inputs.float().cuda()labels = labels.cuda()# zero the parameter gradientsoptimizer.zero_grad()# forwardoutputs = model(inputs)_, preds = torch.max(outputs.data, 1) # print(outputs.data.shape) # print(preds.shape)loss = criterion(outputs, labels)# backward + optimize only if in training phaseif phase == 'train':loss.backward()optimizer.step()# statisticsrunning_loss += loss.item() # item(),將torch數據轉成python數據（數據只有一個元素）running_corrects += torch.sum(preds == labels.data)if phase == 'train':train_epoch_loss = running_loss / len(dataloaders[phase])train_epoch_acc = running_corrects / len(dataloaders[phase])if phase == 'val':val_epoch_loss = running_loss / len(dataloaders[phase])val_epoch_acc = running_corrects / len(dataloaders[phase])time_elapsed_epoch = time.time() - since_epoch# deep copy the modelif phase == 'val' and val_epoch_acc > best_acc:best_acc = val_epoch_accbest_model_wts = model.state_dict()print('{} Train Loss: {:.4f} Train Acc: {:.4f} Valdation Loss: {:.4f} Valdation Acc: {:.4f} in {:.0f}m {:.0f}s'.format(phase, train_epoch_loss, train_epoch_acc, val_epoch_loss, val_epoch_acc, time_elapsed_epoch // 60,time_elapsed_epoch % 60))print()time_elapsed = time.time() - sinceprint('Training complete in {:.0f}m {:.0f}s'.format(time_elapsed // 60, time_elapsed % 60))print('Best val Acc: {:4f}'.format(best_acc))# load best model weightsmodel.load_state_dict(best_model_wts)return model

調用訓練模塊，得到最優模型

model = train_model(model, criterion, optimizer, scheduler, num_epochs=25)

驗證單張圖像

image_path = '/train/train/cb7fb54008ef21a8b55da46d5145acb3.jpg' img = Image.open(image_path) img = ds_trans(img)#處理圖像 #顯示圖像 inp = img.numpy().transpose((1, 2, 0)) mean = np.array([0.485, 0.456, 0.406]) std = np.array([0.229, 0.224, 0.225]) inp = std * inp + mean plt.imshow(inp)model = model.cpu() out = model(img)#獲得輸出 idx = torch.argmax(out).item() cls = idx_to_class[idx]#獲取測試圖像類別 print('The breed of testing dog is: ',cls)

總結

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