Learner class and training loop

Basic training functionality

basic_train wraps together the data (in a DataBunch object) with a pytorch model to define a Learner object. This is where the basic training loop is defined for the fit function. The Learner object is the entry point of most of the Callback functions that will customize this training loop in different ways (and made available through the train module), notably:

  • Learner.lr_find will launch an LR range test that will help you select a good learning rate
  • Learner.fit_one_cycle will launch a training using the 1cycle policy, to help you train your model fast.
  • Learner.to_fp16 will convert your model in half precision and help you launch a training in mixed precision.

class Learner[source]

Learner(data:DataBunch, model:Module, opt_func:Callable='Adam', loss_func:Callable=None, metrics:Collection[Callable]=None, true_wd:bool=True, bn_wd:bool=True, wd:Floats=0.01, train_bn:bool=True, path:str=None, model_dir:str='models', callback_fns:Collection[Callable]=None, callbacks:Collection[Callback]=<factory>, layer_groups:ModuleList=None)

Trainer for model using data to minimize loss_func with optimizer opt_func.

The main purpose of Learner is to train model using Learner.fit. After every epoch, all metrics will be printed, and will also be available to callbacks.

The default weight decay will be wd, which will be handled using the method from Fixing Weight Decay Regularization in Adam if true_wd is set (otherwise it's L2 regularization). If bn_wd is False then weight decay will be removed from batchnorm layers, as recommended in Accurate, Large Minibatch SGD: Training ImageNet in 1 Hour. You can ensure that batchnorm layer learnable params are trained even for frozen layer groups, by enabling train_bn.

To use discriminative layer training pass an nn.Module for each layer group to be optimized with different settings.

Any model files created will be saved in path/model_dir.

You can pass a list of callbacks that you have already created, or (more commonly) simply pass a list of callback functions to callback_fns and each function will be called (passing self) on object initialization, with the results stored as callback objects. For a walk-through, see the training overview page. You may also want to use an application to fit your model, e.g. using the create_cnn method:

path = untar_data(URLs.MNIST_SAMPLE)
data = ImageDataBunch.from_folder(path)
learn = create_cnn(data, models.resnet18, metrics=accuracy)
Total time: 00:09

epoch train_loss valid_loss accuracy
1 0.140986 0.080924 0.973013

Model fitting methods


fit(epochs:int, lr:Union[float, Collection[float], slice]=slice(None, 0.003, None), wd:Floats=None, callbacks:Collection[Callback]=None)

Fit the model on this learner with lr learning rate, wd weight decay for epochs with callbacks.

Uses discriminative layer training if multiple learning rates or weight decay values are passed. To control training behaviour, use the callback system or one or more of the pre-defined callbacks.


fit_one_cycle(learn:Learner, cyc_len:int, max_lr:Union[float, Collection[float], slice]=slice(None, 0.003, None), moms:Point=(0.95, 0.85), div_factor:float=25.0, pct_start:float=0.3, wd:float=None, callbacks:Optional[Collection[Callback]]=None, kwargs)

Fit a model following the 1cycle policy.

Uses the OneCycleScheduler callback.


lr_find(learn:Learner, start_lr:Floats=1e-07, end_lr:Floats=10, num_it:int=100, stop_div:bool=True, kwargs:Any)

Explore lr from start_lr to end_lr over num_it iterations in learn. If stop_div, stops when loss diverges.

Runs the learning rate finder defined in LRFinder, as discussed in Cyclical Learning Rates for Training Neural Networks.

See results


get_preds(ds_type:DatasetType=<DatasetType.Valid: 2>, with_loss:bool=False, n_batch:Optional[int]=None, pbar:Union[MasterBar, ProgressBar, NoneType]=None) → List[Tensor]

Return predictions and targets on ds_type dataset.


validate(dl=None, callbacks=None, metrics=None)

Validate on dl with potential callbacks and metrics.


show_results(ds_type=<DatasetType.Valid: 2>, rows:int=5, kwargs)

Show rows result of predictions on ds_type dataset.


predict(item:ItemBase, kwargs)

Return predicted class, label and probabilities for item.


pred_batch(ds_type:DatasetType=<DatasetType.Valid: 2>, batch:Tuple=None, reconstruct:bool=False) → List[Tensor]

Return output of the model on one batch from ds_type dataset.


interpret(learn:Learner, ds_type:DatasetType=<DatasetType.Valid: 2>, tta=False)

Create a ClassificationInterpretation object from learner on ds_type with tta.

Model summary


model_summary(m:ModuleList, n:int=70)

Print a summary of m using a output text width of n chars

Test time augmentation


TTA(learn:Learner, beta:float=0.4, scale:float=1.35, ds_type:DatasetType=<DatasetType.Valid: 2>, with_loss:bool=False) → Tensors

Applies TTA to predict on ds_type dataset.

Applies Test Time Augmentation to learn on the dataset ds_type. We take the average of our regular predictions (with a weight beta) with the average of predictions obtained thourh augmented versions of the training set (with a weight 1-beta). The transforms decided for the training set are applied with a few changes scale controls the scale for zoom (which isn't random), the cropping isn't random but we make sure to get the four corners of the image. Flipping isn't random but applied once on each of those corner images (so that makes 8 augmented versions total).

Gradient clipping


clip_grad(learn:Learner, clip:float=0.1) → Learner

Add gradient clipping of clip during training.

Mixed precision training


to_fp16(learn:Learner, loss_scale:float=512.0, flat_master:bool=False) → Learner

Put learn in FP16 precision mode.

Uses the MixedPrecision callback to train in mixed precision (i.e. forward and backward passes using fp16, with weight updates using fp32), using all NVIDIA recommendations for ensuring speed and accuracy.

Discriminative layer training

When fitting a model you can pass a list of learning rates (and/or weight decay amounts), which will apply a different rate to each layer group (i.e. the parameters of each module in self.layer_groups). See the Universal Language Model Fine-tuning for Text Classification paper for details and experimental results in NLP (we also frequently use them successfully in computer vision, but have not published a paper on this topic yet). When working with a Learner on which you've called split, you can set hyperparameters in four ways:

  1. param = [val1, val2 ..., valn] (n = number of layer groups)
  2. param = val
  3. param = slice(start,end)
  4. param = slice(end)

If we chose to set it in way 1, we must specify a number of values exactly equal to the number of layer groups. If we chose to set it in way 2, the chosen value will be repeated for all layer groups. See Learner.lr_range for an explanation of the slice syntax).

Here's an example of how to use discriminative learning rates (note that you don't actually need to manually call Learner.split in this case, since fastai uses this exact function as the default split for resnet18; this is just to show how to customize it):

# creates 3 layer groups
learn.split(lambda m: (m[0][6], m[1]))
# only randomly initialized head now trainable
Total time: 00:07

epoch train_loss valid_loss accuracy
1 0.091734 0.071656 0.976448
# all layers now trainable
# optionally, separate LR and WD for each group
learn.fit_one_cycle(1, max_lr=(1e-4, 1e-3, 1e-2), wd=(1e-4,1e-4,1e-1))
Total time: 00:10

epoch train_loss valid_loss accuracy
1 0.018156 0.007928 0.998528


lr_range(lr:Union[float, slice]) → ndarray

Build differential learning rates from lr.

Rather than manually setting an LR for every group, it's often easier to use Learner.lr_range. This is a convenience method that returns one learning rate for each layer group. If you pass slice(start,end) then the first group's learning rate is start, the last is end, and the remaining are evenly geometrically spaced.

If you pass just slice(end) then the last group's learning rate is end, and all the other groups are end/10. For instance (for our learner that has 3 layer groups):

learn.lr_range(slice(1e-5,1e-3)), learn.lr_range(slice(1e-3))
(array([1.e-05, 1.e-04, 1.e-03]), array([0.0001, 0.0001, 0.001 ]))



Unfreeze entire model.

Sets every layer group to trainable (i.e. requires_grad=True).



Freeze up to last layer.

Sets every layer group except the last to untrainable (i.e. requires_grad=False).



Freeze layers up to layer n.



Split the model at split_on.

A convenience method that sets layer_groups based on the result of split_model. If split_on is a function, it calls that function and passes the result to split_model (see above for example).

Saving and loading models

Simply call Learner.save and Learner.load to save and load models. Only the parameters are saved, not the actual architecture (so you'll need to create your model in the same way before loading weights back in). Models are saved to the path/model_dir directory.


load(name:PathOrStr, device:device=None, strict:bool=True, with_opt:bool=None)

Load model and optimizer state (if with_opt) name from self.model_dir using device.


save(name:PathOrStr, return_path:bool=False, with_opt:bool=True)

Save model and optimizer state (if with_opt) with name to self.model_dir.

Segmentation model


unet_learner(data:DataBunch, arch:Callable, pretrained:bool=True, blur_final:bool=True, norm_type:Optional[NormType]='NormType', split_on:Union[Callable, Collection[ModuleList], NoneType]=None, blur:bool=False, self_attention:bool=False, y_range:OptRange=None, last_cross:bool=True, bottle:bool=False, kwargs:Any)

Build Unet learner from data and arch.

Build a Learner for segmentation tasks from data, using arch that may be pretrained if that flag is True. split_on will overwrite the default way the layers are split for differential learning rates. The underlying model is a DynamicUnet with blur, blur_final, self_attention, sigmoid and last_cross. norm_type is passed to conv_layer, the kwargs are passed to the Learner constructor.

Other methods



Initializes all weights (except batchnorm) using function init, which will often be from PyTorch's nn.init module.


mixup(learn:Learner, alpha:float=0.4, stack_x:bool=False, stack_y:bool=True) → Learner

Add mixup https://arxiv.org/abs/1710.09412 to learn.



Pass item through the model and computes the gradient. Useful if backward_hooks are attached.


create_opt(lr:Floats, wd:Floats=0.0)

Create optimizer with lr learning rate and wd weight decay.

You generally won't need to call this yourself - it's used to create the optim optimizer before fitting the model.


dl(ds_type:DatasetType=<DatasetType.Valid: 2>)

Return DataLoader for DatasetType ds_type.

class Recorder[source]

Recorder(learn:Learner) :: LearnerCallback

A LearnerCallback that records epoch, loss, opt and metric data during training.

A Learner creates a Recorder object automatically - you do not need to explicitly pass to callback_fns - because other callbacks rely on it being available. It stores the smoothed loss, hyperparameter values, and metrics each batch, and provides plotting methods for each. Note that Learner automatically sets an attribute with the snake-cased name of each callback, so you can access this through Learner.recorder, as shown below.

Plotting methods


plot(skip_start:int=10, skip_end:int=5)

Plot learning rate and losses, trimmed between skip_start and skip_end.

This is mainly used with the learning rate finder, since it shows a scatterplot of loss vs learning rate.

learn = create_cnn(data, models.resnet18, metrics=accuracy)
LR Finder is complete, type {learner_name}.recorder.plot() to see the graph.



Plot training and validation losses.

Note that validation losses are only calculated once per epoch, whereas training losses are calculated after every batch.

Total time: 00:16

epoch train_loss valid_loss accuracy
1 0.176085 0.111174 0.962218
2 0.095665 0.071961 0.975957



Plot learning rate, show_moms to include momentum.




Plot metrics collected during training.

Note that metrics are only collected at the end of each epoch, so you'll need to train at least two epochs to have anything to show here.


Callback methods

You don't call these yourself - they're called by fastai's Callback system automatically to enable the class's functionality.


on_backward_begin(smooth_loss:Tensor, kwargs:Any)

Record the loss before any other callback has a chance to modify it.


on_batch_begin(train, kwargs:Any)

Record learning rate and momentum at beginning of batch.


on_epoch_end(epoch:int, num_batch:int, smooth_loss:Tensor, last_metrics='Collection', kwargs:Any) → bool

Save epoch info: num_batch, smooth_loss, metrics.


on_train_begin(pbar:PBar, metrics_names:StrList, kwargs:Any)

Initialize recording status at beginning of training.

Inner functions

The following functions are used along the way by the Recorder or can be called by other callbacks.



Add metrics to the inner stats.



Add names to the inner metric names.



Format stats before printing.

Module functions

Generally you'll want to use a Learner to train your model, since they provide a lot of functionality and make things easier. However, for ultimate flexibility, you can call the same underlying functions that Learner calls behind the scenes:


fit(epochs:int, model:Module, loss_func:LossFunction, opt:Optimizer, data:DataBunch, callbacks:Optional[Collection[Callback]]=None, metrics:OptMetrics=None)

Fit the model on data and learn using loss_func and opt.

Note that you have to create the Optimizer yourself if you call this function, whereas Learn.fit creates it for you automatically.


train_epoch(model:Module, dl:DataLoader, opt:Optimizer, loss_func:LossFunction)

Simple training of model for 1 epoch of dl using optim opt and loss function loss_func.

You won't generally need to call this yourself - it's what fit calls for each epoch.


validate(model:Module, dl:DataLoader, loss_func:OptLossFunc=None, cb_handler:Optional[CallbackHandler]=None, pbar:Union[MasterBar, ProgressBar, NoneType]=None, average=True, n_batch:Optional[int]=None) → Iterator[Tuple[IntOrTensor, Ellipsis]]

Calculate loss_func of model on dl in evaluation mode.

This is what fit calls after each epoch. You can call it if you want to run inference on a DataLoader manually.


get_preds(model:Module, dl:DataLoader, pbar:Union[MasterBar, ProgressBar, NoneType]=None, cb_handler:Optional[CallbackHandler]=None, activ:Module=None, loss_func:OptLossFunc=None, n_batch:Optional[int]=None) → List[Tensor]

Tuple of predictions and targets, and optional losses (if loss_func) using dl, max batches n_batch.


loss_batch(model:Module, xb:Tensor, yb:Tensor, loss_func:OptLossFunc=None, opt:OptOptimizer=None, cb_handler:Optional[CallbackHandler]=None) → Tuple[Union[Tensor, int, float, str]]

Calculate loss and metrics for a batch, call out to callbacks as necessary.

You won't generally need to call this yourself - it's what fit and validate call for each batch. It only does a backward pass if you set opt.

Other classes

class LearnerCallback[source]

LearnerCallback(learn:Learner) :: Callback

Base class for creating callbacks for a Learner.

class RecordOnCPU[source]

RecordOnCPU() :: Callback

Store the input and target going through the model on the CPU.