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fastai fundamentals


DataLoaders is a thin class around DataLoader, and makes them available as train and valid.

Same thing applies to Datasets and Dataset.

In pytorch, Dataset is fed into a DataLoader.


Use this to create DataLoaders

bears = DataBlock(
blocks=(ImageBlock, CategoryBlock),
splitter=RandomSplitter(valid_pct=0.2, seed=42),

DataBlocks are a template for creating DataLoaders, and need to be instantiated somehow - for example given a path where to find the data:

dls = bears.dataloaders(path)

You can modify the settings of a DataBlock with new:

bears =, min_scale=0.3)) #book has more examples
dls = bears.dataloaders(path)

You can sanity check / see transformed data with show_batch:

>>> dls.train.show_batch(max_n=8, nrows=2, unique=True)
... images

You also use DataBlocks for data augmentation, with batch_tfms:

bears =
dls = bears.dataloaders(path)
dls.train.show_batch(max_n=8, nrows=2, unique=True)


Most things use learn.fine_tune(), when you cannot fine-tune like tabular data, you often use learn.fit_one_cycle

You can also do learn.show_results(...)

from import *
path = untar_data(URLs.PETS)/'images'

def is_cat(x):
return x[0].isupper()
dls = ImageDataLoaders.from_name_func(

learn = cnn_learner(dls, resnet34, metrics=error_rate)

More info on what this is in later sections.


interp = ClassificationInterpretation.from_learner(learn)

Also see top losses:

interp.plot_top_losses(5, nrows=1)


You can get a ImageClassifierCleaner which allows you to choose (1) a category and (2) data partition (train/val) and shows you the highest loss items so you can decide whether to Keep, Delete, Change etc.

cleaner = ImageClassifierCleaner(learn)

The thing doesn't actually delete/change anything but gives you the idxs that allow you to do things with them

for idx in cleaner.delete(): cleaner.fns[idx].unlink()
for idx,cat in cleaner.change(): shutil.move(str(cleaner.fns[idx]), path/cat)

Loading / Saving

Saving a model can be done with learn.export, when you do this, fastai will save a file called "export.pkl"


load_learner can be used to load a model

learn_inf = load_learner(path/'export.pkl')


When you call predict, you will get three things: (1) class, (2) the index of the predicted category (3) Probabilities of each category

>>> learn_inf.predict('images/grizzly.jpg')
('grizzly', tensor(1), tensor([9.0767e-06, 9.9999e-01, 1.5748e-07]))

You can see all the classes with dls.vocab:

>>> learn_inf.dls.vocab
(#3) ['black','grizzly','teddy']

Zach: learn.dls.vocab or learn.dls.categorize.vocab is another way to get the class names.

Computer Vision

You can open an image with Pilow (PIL)

im3 =

#convert to numpy
# convert to pytorch tensor

Pixel Similarity Baseline

  1. Compute avg pixel value for 3's and 7's
  2. At inference time, see which one its similar too, using RMSE (L2 Norm) and MAE (L1 Norm)

Kind of like KNN

Taking an inference tensor, a_3 and calculate distance to mean 3 and 7:

# MAE & RMSE for 3  vs avg3
dist_3_abs = (a_3 - mean3).abs().mean()
dist_3_sqr = ((a_3 - mean3)**2).mean().sqrt()

# MAE & RMSE for 3 vs avg7
dist_7_abs = (a_3 - mean7).abs().mean()
dist_7_sqr = ((a_3 - mean7)**2).mean().sqrt()

# Use Pytorch Losses to do the same thing for 3 vs avg 7
F.l1_loss(a_3.float(),mean7), F.mse_loss(a_3,mean7).sqrt()


Take the mean over an axis:

def mnist_distance(a,b): 
#(-2,1) means take the average of the last 2 axis
return (a-b).abs().mean((-2,-1))

SGD from scratch

Minimal Example

# the loss function
def mse(y, yhat):
return (y - yhat).square().mean().sqrt()

# the function that produces the data
def quadratic(x, params=[.75, -25.5, 15]):
a,b,c = params
noise = (torch.randn(len(x)) * 3)
return a*(x**2) + b*x +c + noise

# generate training data
x = torch.arange(1, 40, 1)
y = quadratic(x)

# define the training loop
def apply_step(params, pr=True):
lr = 1.05e-4
preds = quadratic(x, params)
loss = mse(preds, y)
loss.backward() -= * lr
if pr: print(f'loss: {loss}')
params.grad = None

# initialize random params
params = torch.rand(3)
assert params.requires_grad

# train the model
for _ in range(1000):


A Dataset in pytorch is required to return a tuple of (x,y) when indexed. You can do this in python as follows:

# Turn mnist data into vectors 3dim -> 2dim
train_x =[stacked_threes, stacked_sevens]).view(-1, 28*28)
# Generate label tensor
train_y = tensor([1]*len(threes) + [0]*len(sevens)).unsqueeze(1)
# Create dataset
dset = list(zip(train_x,train_y))

# See shapes from first datum in the dataset
>>> x,y = dset[0]
>>> x.shape, y.shape
(torch.Size([784]), torch.Size([1]))

# Do the same thing for the validation set

Mini Batch SGD

# `@` and dot product is the same:
a, b = torch.rand(10), torch.rand(10)
assert == a@b

# define model
def init_params(size, std=1.0):
return (torch.randn(size)*std).requires_grad_()
weights = init_params((28*28,1))
bias = init_params(1)

def linear1(xb): return xb@weights + bias

#naive loss (for illustration)
corrects = (preds>0.0).float() == train_y

# define loss
def mnist_loss(preds, targets):
preds = preds.sigmoid() #squash b/w 0 and 1
return torch.where(targets==1, 1-preds, preds).mean() # average distance loss
Create a dataloader

You want to load your data in batches, so you will want to create a dataloader. Recall that in pytorch, a Dataset is required to return a tuple of (x,y) when indexed, which is quite easy to do:

# define a data loader using `dset`
dset = list(zip(train_x,train_y))

Pytorch offers a utility to then create a Dataloader from a dataset, but Jeremy basically rolled his own (w/same api):

dl = DataLoader(dset, batch_size=256)
valid_dl = DataLoader(valid_dset, batch_size=256)

The Training Loop

def calc_grad(xb, yb, model):
preds = model(xb)
loss = mnist_loss(preds, yb)

def train_epoch(model, lr, params):
for xb,yb in dl:
calc_grad(xb, yb, model)
for p in params: -= p.grad*lr
p.grad.zero_() #updates in place

### Calculate metrics
def batch_accuracy(xb, yb):
preds = xb.sigmoid()
correct = (preds>0.5) == yb
return correct.float().mean()

def validate_epoch(model):
accs = [batch_accuracy(model(xb), yb) for xb,yb in valid_dl]
return round(torch.stack(accs).mean().item(), 4)

# Train model
lr = 1.
params = weights,bias
train_epoch(linear1, lr, params)

# Train model w/epochs
for i in range(20):
train_epoch(linear1, lr, params)
print(validate_epoch(linear1), end=' ')

Using Pytorch


  1. Define a dataset and then a dataloader
  2. Create a model, which will have parameters
  3. Create an optimizer, that:
    • Updates the params: -= * lr
    • Zeros out the gradients: setting params.grad = None or zeroing out the gradients with params.grad.zero_()
  4. Generate the predictions
  5. Calculate the loss
  6. Calculate the gradients loss.backward()
  7. Using the optimizer, update the weights step and zero out the gradients zero_grad
  8. Put 4-7 in a loop.

Create an optimizer and use nn.Linear

linear_model = nn.Linear(28*28,1)
w,b = linear_model.parameters()

# Define an optimizer
class BasicOptim:
def __init__(self,params,lr): self.params, = list(params),lr

def step(self, *args, **kwargs):
for p in self.params: -= *

def zero_grad(self, *args, **kwargs):
for p in self.params: p.grad = None

opt = BasicOptim(linear_model.parameters(), lr)
# alternative, fastai provides SGD
opt = SGD(linear_model.parameters(), lr)

# Define Metrics
def batch_accuracy(xb, yb):
preds = xb.sigmoid()
correct = (preds>0.5) == yb
return correct.float().mean()

# Helper to calculate metrics on validation set
def validate_epoch(model):
accs = [batch_accuracy(model(xb), yb) for xb,yb in valid_dl]
return round(torch.stack(accs).mean().item(), 4)

def train_epoch(model):
for xb,yb in dl:
calc_grad(xb, yb, model)

def train_model(model, epochs):
for i in range(epochs):
print(validate_epoch(model), end=' ')

train_model(linear_model, 20)

Using fastai

We can substitute the above with from fastai We just have to supply the following:

  1. Dataloaders
  2. Model
  3. Optimization function
  4. Loss function
  5. Metrics
dls = DataLoaders(dl, valid_dl)
learn = Learner(dls, nn.Linear(28*28,1), opt_func=SGD,
metrics=batch_accuracy), lr=lr)

What if you used the full power of fastai? It would look like this:

dls = ImageDataLoaders.from_folder(path)
# Lots of things have defaults like optimization func
learn = cnn_learner(dls, resnet18, pretrained=False,
learn.fit_one_cycle(1, 0.1)

Simple Neural Nets

The next step is to introduce a non-linearity

simple_net = nn.Sequential(
nn.Linear(28*28, 30),
nn.Linear(30, 1)

# Construct the learner as before
learn = learner(dls, simple_net, opt_func=SGD,
loss_func=mnist_loss, metrics=batch_accuracy), 0.1)

Inspecting Training History

The training history is saved in learn.recorder. You can plot your training progress with: