# Helper¶

This module provides helper functions and classes for the user. They make working with skorch easier but are not used by skorch itself.

## SliceDict¶

A SliceDict is a wrapper for Python dictionaries that makes them behave a little bit like numpy.ndarrays. That way, you can slice your dictionary across values, len() will show the length of the arrays and not the number of keys, and you get a shape attribute. This is useful because if your data is in a dict, you would normally not be able to use sklearn GridSearchCV and similar things; with SliceDict, this works.

## SliceDataset¶

A SliceDataset is a wrapper for PyTorch Datasets that makes them behave a little bit like numpy.ndarrays. That way, you can slice your dataset with lists and arrays, and you get a shape attribute. These properties are useful because if your data is in a dataset, you would normally not be able to use sklearn GridSearchCV and similar things; with SliceDataset, this works.

Note that SliceDataset can only ever return one of the values returned by the dataset. Typically, this will be either the X or the y value. Therefore, if you want to wrap both X and y, you should create two instances of SliceDataset, one for X (with argument idx=0, the default) and one for y (with argument idx=1):

ds = MyCustomDataset()
X_sl = SliceDataset(ds, idx=0)  # idx=0 is the default
y_sl = SliceDataset(ds, idx=1)
gs.fit(X_sl, y_sl)


## Command line interface helpers¶

Often you want to wrap up your experiments by writing a small script that allows others to reproduce your work. With the help of skorch and the fire library, it becomes very easy to write command line interfaces without boilerplate. All arguments pertaining to skorch or its PyTorch module are immediately available as command line arguments, without the need to write a custom parser. If docstrings in the numpydoc specification are available, there is also an comprehensive help for the user. Overall, this allows you to make your work reproducible without the usual hassle.

There is an example in the skorch repository that shows how to use the CLI tools. Below is a snippet that shows the output created by the help function without writing a single line of argument parsing:

\$ python examples/cli/train.py pipeline --help

<SelectKBest> options:
--select__score_func : callable
Function taking two arrays X and y, and returning a pair of arrays
(scores, pvalues) or a single array with scores.
--select__k : int or "all", optional, default=10
Number of top features to select.
The "all" option bypasses selection, for use in a parameter search.

...

<NeuralNetClassifier> options:
--net__module : torch module (class or instance)
A PyTorch :class:~torch.nn.Module. In general, the
uninstantiated class should be passed, although instantiated
modules will also work.
--net__criterion : torch criterion (class, default=torch.nn.NLLLoss)
Negative log likelihood loss. Note that the module should return
probabilities, the log is applied during get_loss.
--net__optimizer : torch optim (class, default=torch.optim.SGD)
The uninitialized optimizer (update rule) used to optimize the
module
--net__lr : float (default=0.01)
Learning rate passed to the optimizer. You may use lr instead
of using optimizer__lr, which would result in the same outcome.
--net__max_epochs : int (default=10)
The number of epochs to train for each fit call. Note that you
may keyboard-interrupt training at any time.
--net__batch_size : int (default=128)
...
--net__verbose : int (default=1)
Control the verbosity level.
--net__device : str, torch.device (default='cpu')
The compute device to be used. If set to 'cuda', data in torch
tensors will be pushed to cuda tensors before being sent to the
module.

<MLPClassifier> options:
--net__module__hidden_units : int (default=10)
Number of units in hidden layers.
--net__module__num_hidden : int (default=1)
Number of hidden layers.
--net__module__nonlin : torch.nn.Module instance (default=torch.nn.ReLU())
Non-linearity to apply after hidden layers.
--net__module__dropout : float (default=0)
Dropout rate. Dropout is applied between layers.


### Installation¶

To use this functionality, you need some further libraries that are not part of skorch, namely fire and numpydoc. You can install them thusly:

pip install fire numpydoc


### Usage¶

When you write your own script, only the following bits need to be added:

import fire
from skorch.helper import parse_args

# your model definition and data fetching code below
...

def main(**kwargs):
X, y = get_data()
my_model = get_model()

# important: wrap the model with the parsed arguments
parsed = parse_args(kwargs)
my_model = parsed(my_model)

my_model.fit(X, y)

if __name__ == '__main__':
fire.Fire(main)


This even works if your neural net is part of an sklearn pipeline, in which case the help extends to all other estimators of your pipeline.

In case you would like to change some defaults for the net (e.g. using a batch_size of 256 instead of 128), this is also possible. You should have a dictionary containing your new defaults and pass it as an additional argument to parse_args:

my_defaults = {'batch_size': 128, 'module__hidden_units': 30}

def main(**kwargs):
...
parsed = parse_args(kwargs, defaults=my_defaults)
my_model = parsed(my_model)


This will update the displayed help to your new defaults, as well as set the parameters on the net or pipeline for you. However, the arguments passed via the commandline have precedence. Thus, if you additionally pass --batch_size 512 to the script, batch size will be 512.

### Restrictions¶

Almost all arguments should work out of the box. Therefore, you get command line arguments for the number of epochs, learning rate, batch size, etc. for free. Moreover, you can access the module parameters with the double-underscore notation as usual with skorch (e.g. --module__num_units 100). This should cover almost all common cases.

Parsing command line arguments that are non-primitive Python objects is more difficult, though. skorch’s custom parsing should support normal Python types and simple custom objects, e.g. this works: --module__nonlin 'torch.nn.RReLU(0.1, upper=0.4)'. More complex parsing might not work. E.g., it is currently not possible to add new callbacks through the command line (but you can modify existing ones as usual).