# Function train transfer learning

hello everyone, i’m in a big troble. does someone can explain to me this function line by line pleaaaase, it really blocks me i still a beginner

def train_model(model, criterion, optimizer, scheduler, num_epochs=25):
since = time.time()

``````best_model_wts = copy.deepcopy(model.state_dict())
best_acc = 0.0

for epoch in range(num_epochs):
print('Epoch {}/{}'.format(epoch, num_epochs - 1))
print('-' * 10)

# Each epoch has a training and validation phase
for phase in ['train', 'val']:
if phase == 'train':
model.train()  # Set model to training mode
else:
model.eval()   # Set model to evaluate mode

running_loss = 0.0
running_corrects = 0

# Iterate over data.
inputs = inputs.to(device)
labels = labels.to(device)

# forward
# track history if only in train
outputs = model(inputs)
_, preds = torch.max(outputs, 1)
loss = criterion(outputs, labels)

# backward + optimize only if in training phase
if phase == 'train':
loss.backward()
optimizer.step()

# statistics
running_loss += loss.item() * inputs.size(0)
running_corrects += torch.sum(preds == labels.data)
if phase == 'train':
scheduler.step()

epoch_loss = running_loss / dataset_sizes[phase]
epoch_acc = running_corrects.double() / dataset_sizes[phase]

print('{} Loss: {:.4f} Acc: {:.4f}'.format(
phase, epoch_loss, epoch_acc))

# deep copy the model
if phase == 'val' and epoch_acc > best_acc:
best_acc = epoch_acc
best_model_wts = copy.deepcopy(model.state_dict())

print()

time_elapsed = time.time() - since
print('Training complete in {:.0f}m {:.0f}s'.format(
time_elapsed // 60, time_elapsed % 60))
print('Best val Acc: {:4f}'.format(best_acc))

return model
``````

It’s unclear how much details you would need to understand each line of code. Instead of directly describing the code, could you let us know which lines are unclear?
I would expect that e.g.:

``````running_loss = 0.0 # initializes the running_loss variable with 0.0
``````

would be clear, but:

``````with torch.set_grad_enabled(phase == 'train'): # enabled gradient computation if phase=='train' is used
``````

might be more complicated to understand.

i wan to know waht this function do and what’s the output