For loops in a dictionary, pytorch

Question

Hi guys I have a question, for the variable "image_datasets" there is a for loop for x in ['train', 'val']. I have never seen the implementation of a for loop in a dict before.

data_transforms = {
'train': transforms.Compose([
    transforms.RandomResizedCrop(224),
    transforms.RandomHorizontalFlip(),
    transforms.ToTensor(),
    transforms.Normalize(mean, std)
]),
'val': transforms.Compose([
    transforms.Resize(256),
    transforms.CenterCrop(224),
    transforms.ToTensor(),
    transforms.Normalize(mean, std)
]),
}

data_dir = 'data/hymenoptera_data'
image_datasets = {x: datasets.ImageFolder(os.path.join(data_dir, x),
                                          data_transforms[x])
                  for x in ['train', 'val']}
dataloaders = {x: torch.utils.data.DataLoader(image_datasets[x], batch_size=4,
                                             shuffle=True, num_workers=0)
              for x in ['train', 'val']}

dataset_sizes = {x: len(image_datasets[x]) for x in ['train', 'val']}
class_names = image_datasets['train'].classes

Answer 1

This is called dictionary comprehension and it is iterating over a list.

The code

dataloaders = {
    x: torch.utils.data.DataLoader(
        image_datasets[x],
        batch_size=4,
        shuffle=True,
        num_workers=0
    )
    for x in ['train', 'val']
}

is equivalent to

dataloaders = {
    'train': torch.utils.data.DataLoader(
        image_datasets['train'],
        batch_size=4,
        shuffle=True,
        num_workers=0
    ),
    'val': torch.utils.data.DataLoader(
        image_datasets['val'],
        batch_size=4,
        shuffle=True,
        num_workers=0
    )
}

Answer 2

It's called dict comprehension and there's also list comprehensions: https://book.pythontips.com/en/latest/comprehensions.html

They basically function as you'd expect it:

Do X with x for x in List 

and the result is then used as the input for a dict or list and so on.