Customizing datasets in fastai¶

In this tutorial, we'll see how to create custom subclasses of ItemBase or ItemList while retaining everything the fastai library has to offer. To allow basic functions to work consistently across various applications, the fastai library delegates several tasks to one of those specific objects, and we'll see here which methods you have to implement to be able to have everything work properly. But first let's take a step back to see where you'll use your end result.

Links with the data block API¶

The data block API works by allowing you to pick a class that is responsible to get your items and another class that is charged with getting your targets. Combined together, they create a pytorch Dataset that is then wrapped inside a DataLoader. The training set, validation set and maybe test set are then all put in a DataBunch.

The data block API allows you to mix and match what class your inputs have, what class your targets have, how to do the split between train and validation set, then how to create the DataBunch, but if you have a very specific kind of input/target, the fastai classes might no be sufficient to you. This tutorial is there to explain what is needed to create a new class of items and what methods are important to implement or override.

It goes in two phases: first we focus on what you need to create a custom ItemBase class (which is the type of your inputs/targets) then on how to create your custom ItemList (which is basically a set of ItemBase) while highlighting which methods are called by the library.

Creating a custom ItemBase subclass¶

The fastai library contains three basic types of ItemBase that you might want to subclass:

• Image for vision applications
• Text for text applications
• TabularLine for tabular applications

Whether you decide to create your own item class or to subclass one of the above, here is what you need to implement:

Basic attributes¶

Those are the more important attributes your custom ItemBase needs as they're used everywhere in the fastai library:

• ItemBase.data is the thing that is passed to pytorch when you want to create a DataLoader. This is what needs to be fed to your model. Note that it might be different from the representation of your item since you might want something that is more understandable.
• __str__ representation: if applicable, this is what will be displayed when the fastai library has to show your item.

If we take the example of a MultiCategory object o for instance:

• o.data is a tensor where the tags are one-hot encoded
• str(o) returns the tags separated by ;

If you want to code the way data augmentation should be applied to your custom Item, you should write an apply_tfms method. This is what will be called if you apply a transform block in the data block API.

Example: ImageTuple¶

For cycleGANs, we need to create a custom type of items since we feed the model tuples of images. Let's look at how to code this. The basis is to code the data attribute that is what will be given to the model. Note that we still keep track of the initial object (usuall in an obj attrivute) to be able to show nice representations later on. Here the object is the tuple of images and the data their underlying tensors normalized between -1 and 1.

Then we want to apply data augmentation to our tuple of images. That's done by writing and apply_tfms method as we saw before. Here we just pass that call to the two underlying images then update the data.

We define a last method to stack the two images next ot each other, which we will use later for a customized show_batch/ show_results behavior.

This is all your need to create your custom ItemBase. You won't be able to use it until you have put it inside your custom ItemList though, so you should continue reading the next section.

Creating a custom ItemList subclass¶

This is the main class that allows you to group your inputs or your targets in the data block API. You can then use any of the splitting or labelling methods before creating a DataBunch. To make sure everything is properly working, here is what you need to know.

Class variables¶

Whether you're directly subclassing ItemList or one of the particular fastai ones, make sure to know the content of the following three variables as you may need to adjust them:

• _bunch contains the name of the class that will be used to create a DataBunch
• _processor contains a class (or a list of classes) of PreProcessor that will then be used as the default to create processor for this ItemList
• _label_cls contains the class that will be used to create the labels by default

_label_cls is the first to be used in the data block API, in the labelling function. If this variable is set to None, the label class will be set to CategoryList, MultiCategoryList or FloatList depending on the type of the first item. The default can be overridden by passing a label_cls in the kwargs of the labelling function.

_processor is the second to be used. The processors are called at the end of the labelling to apply some kind of function on your items. The default processor of the inputs can be overriden by passing a processor in the kwargs when creating the ItemList, the default processor of the targets can be overridden by passing a processor in the kwargs of the labelling function.

Processors are useful for pre-processing some data, but you also need to put in their state any variable you want to save for the call of data.export() before creating a Learner object for inference: the state of the ItemList isn't saved there, only their processors. For instance SegmentationProcessor's only reason to exist is to save the dataset classes, and during the process call, it doesn't do anything apart from setting the classes and c attributes to its dataset.

class SegmentationProcessor(PreProcessor):
    def __init__(self, ds:ItemList): self.classes = ds.classes
    def process(self, ds:ItemList):  ds.classes,ds.c = self.classes,len(self.classes)

_bunch is the last class variable usd in the data block. When you type the final databunch(), the data block API calls the _bunch.create method with the _bunch of the inputs.

Keeping __init__ arguments¶

If you pass additional arguments in your __init__ call that you save in the state of your ItemList, we have to make sure they are also passed along in the new method as this one is used to create your training and validation set when splitting. To do that, you just have to add their names in the copy_new argument of your custom ItemList, preferably during the __init__. Here we will need two collections of filenames (for the two type of images) so we make sure the second one is copied like this:

def __init__(self, items, itemsB=None, **kwargs):
    super().__init__(items, **kwargs)
    self.itemsB = itemsB
    self.copy_new.append('itemsB')

Be sure to keep the kwargs as is, as they contain all the additional stuff you can pass to an ItemList.

Important methods¶

- get¶

The most important method you have to implement is get: this one will enable your custom ItemList to generate an ItemBase from the thing stored in its items array. For instance an ImageList has the following get method:

def get(self, i):
    fn = super().get(i)
    res = self.open(fn)
    self.sizes[i] = res.size
    return res

The first line basically looks at self.items[i] (which is a filename). The second line opens it since the openmethod is just

def open(self, fn): return open_image(fn)

The third line is there for ImagePoints or ImageBBox targets that require the size of the input Image to be created. Note that if you are building a custom target class and you need the size of an image, you should call self.x.size[i].

- reconstruct¶

This is the method that is called in data.show_batch(), learn.predict() or learn.show_results() to transform a pytorch tensor back in an ItemBase. In a way, it does the opposite of calling ItemBase.data. It should take a tensor t and return the same king of thing as the get method.

In some situations (ImagePoints, ImageBBox for instance) you need to have a look at the corresponding input to rebuild your item. In this case, you should have a second argument called x (don't change that name). For instance, here is the reconstruct method of PointsItemList:

def reconstruct(self, t, x): return ImagePoints(FlowField(x.size, t), scale=False)

- analyze_pred¶

This is the method that is called in learn.predict() or learn.show_results() to transform predictions in an output tensor suitable for reconstruct. For instance we may need to take the maximum argument (for Category) or the predictions greater than a certain threshold (for MultiCategory). It should take a tensor, along with optional kwargs and return a tensor.

For instance, here is the analyze_pred method of MultiCategoryList:

def analyze_pred(self, pred, thresh:float=0.5): return (pred >= thresh).float()

thresh can then be passed as kwarg during the calls to learn.predict() or learn.show_results().

Advanced show methods¶

If you want to use methods such a data.show_batch() or learn.show_results() with a brand new kind of ItemBase you will need to implement two other methods. In both cases, the generic function will grab the tensors of inputs, targets and predictions (if applicable), reconstruct the corresponding ItemBase (as seen before) but it will delegate to the ItemList the way to display the results.

def show_xys(self, xs, ys, **kwargs)->None:

def show_xyzs(self, xs, ys, zs, **kwargs)->None:

In both cases xs and ys represent the inputs and the targets, in the second case zs represent the predictions. They are lists of the same length that depend on the rows argument you passed. The kwargs are passed from data.show_batch() / learn.show_results(). As an example, here is the source code of those methods in ImageList:

def show_xys(self, xs, ys, figsize:Tuple[int,int]=(9,10), **kwargs):
    "Show the `xs` and `ys` on a figure of `figsize`. `kwargs` are passed to the show method."
    rows = int(math.sqrt(len(xs)))
    fig, axs = plt.subplots(rows,rows,figsize=figsize)
    for i, ax in enumerate(axs.flatten() if rows > 1 else [axs]):
        xs[i].show(ax=ax, y=ys[i], **kwargs)
    plt.tight_layout()

def show_xyzs(self, xs, ys, zs, figsize:Tuple[int,int]=None, **kwargs):
    """Show `xs` (inputs), `ys` (targets) and `zs` (predictions) on a figure of `figsize`. 
    `kwargs` are passed to the show method."""
    figsize = ifnone(figsize, (6,3*len(xs)))
    fig,axs = plt.subplots(len(xs), 2, figsize=figsize)
    fig.suptitle('Ground truth / Predictions', weight='bold', size=14)
    for i,(x,y,z) in enumerate(zip(xs,ys,zs)):
        x.show(ax=axs[i,0], y=y, **kwargs)
        x.show(ax=axs[i,1], y=z, **kwargs)

Linked to this method is the class variable _show_square of an ItemList. It defaults to False but if it's True, the show_batch method will send rows * rows xs and ys to show_xys (so that it shows a square of inputs/targets), like here for images.

Example: ImageTupleList¶

Continuing our custom item example, we create a custom ItemList class that will wrap those ImageTuples properly. The first thing is to write a custom __init__ method (since we need a list of filenames here) which means we also have to change the new method.

We then specify how to get one item. Here we pass the image in the first list of items, and pick one randomly in the second list.

We also add a custom factory method to directly create an ImageTupleList from two folders.

Finally, we have to specify how to reconstruct the ImageTuple from tensors if we want show_batch to work. We recreate the images and denormalize.

There is no need to write a analyze_preds method since the default behavior (returning the output tensor) is what we need here. However show_results won't work properly unless the target (which we don't really care about here) has the right reconstruct method: the fastai library uses the reconstruct method of the target on the outputs. That's why we create another custom ItemList with just that reconstruct method. The first line is to reconstruct our dummy targets, and the second one is the same as in ImageTupleList.

To make sure our ImageTupleList uses that for labelling, we pass it in _label_cls and this is what the result looks like.

Lastly, we want to customize the behavior of show_batch and show_results. Remember the to_one method just puts the two images next to each other.