Lightning CLI and config files¶
Another source of boilerplate code that Lightning can help to reduce is in the implementation of training command line
tools. Furthermore, it provides a standardized way to configure trainings using a single file that includes settings for
Trainer
and user extended
LightningModule
and
LightningDataModule
classes. The full configuration is automatically saved
in the log directory. This has the benefit of greatly simplifying the reproducibility of experiments.
The main requirement for user extended classes to be made configurable is that all relevant init arguments must have type hints. This is not a very demanding requirement since it is good practice to do anyway. As a bonus if the arguments are described in the docstrings, then the help of the training tool will display them.
Warning
LightningCLI
is in beta and subject to change.
LightningCLI¶
The implementation of training command line tools is done via the LightningCLI
class. The minimal installation of pytorch-lightning does not include this support. To enable it either install
lightning with the all
extras require or install the package jsonargparse[signatures]
.
The case in which the user’s LightningModule
class implements all required
*_dataloader
methods, a trainer.py
tool can be as simple as:
from pytorch_lightning.utilities.cli import LightningCLI
cli = LightningCLI(MyModel)
The help of the tool describing all configurable options and default values can be shown by running python
trainer.py --help
. Default options can be changed by providing individual command line arguments. However, it is better
practice to create a configuration file and provide this to the tool. A way to do this would be:
# Dump default configuration to have as reference
python trainer.py --print_config > default_config.yaml
# Create config including only options to modify
nano config.yaml
# Run training using created configuration
python trainer.py --config config.yaml
The instantiation of the LightningCLI
class takes care of parsing command line
and config file options, instantiating the classes, setting up a callback to save the config in the log directory and
finally running trainer.fit()
. The resulting object cli
can be used for instance to get the result of fit,
i.e., cli.fit_result
.
After multiple trainings with different configurations, each run will have in its respective log directory a
config.yaml
file. This file can be used for reference to know in detail all the settings that were used for each
particular run, and also could be used to trivially reproduce a training, e.g.:
python trainer.py --config lightning_logs/version_7/config.yaml
If a separate LightningDataModule
class is required, the trainer tool just
needs a small modification as follows:
from pytorch_lightning.utilities.cli import LightningCLI
cli = LightningCLI(MyModel, MyDataModule)
The start of a possible implementation of MyModel
including the recommended argument descriptions in the
docstring could be the one below. Note that by using type hints and docstrings there is no need to duplicate this
information to define its configurable arguments.
class MyModel(LightningModule):
def __init__(
self,
encoder_layers: int = 12,
decoder_layers: List[int] = [2, 4]
):
"""Example encoder-decoder model
Args:
encoder_layers: Number of layers for the encoder
decoder_layers: Number of layers for each decoder block
"""
super().__init__()
self.save_hyperparameters()
With this model class, the help of the trainer tool would look as follows:
$ python trainer.py --help
usage: trainer.py [-h] [--print_config] [--config CONFIG]
[--trainer.logger LOGGER]
...
pytorch-lightning trainer command line tool
optional arguments:
-h, --help show this help message and exit
--print_config print configuration and exit
--config CONFIG Path to a configuration file in json or yaml format.
(default: null)
Customize every aspect of training via flags:
...
--trainer.max_epochs MAX_EPOCHS
Stop training once this number of epochs is reached.
(type: int, default: 1000)
--trainer.min_epochs MIN_EPOCHS
Force training for at least these many epochs (type: int,
default: 1)
...
Example encoder-decoder model:
--model.encoder_layers ENCODER_LAYERS
Number of layers for the encoder (type: int, default: 12)
--model.decoder_layers DECODER_LAYERS
Number of layers for each decoder block (type: List[int],
default: [2, 4])
The default configuration that option --print_config
gives is in yaml format and for the example above would
look as follows:
$ python trainer.py --print_config
model:
decoder_layers:
- 2
- 4
encoder_layers: 12
trainer:
accelerator: null
accumulate_grad_batches: 1
amp_backend: native
amp_level: O2
...
Note that there is a section for each class (model and trainer) including all the init parameters of the class. This grouping is also used in the formatting of the help shown previously.
Trainer Callbacks and arguments with class type¶
A very important argument of the Trainer
class is the callbacks
. In
contrast to other more simple arguments which just require numbers or strings, callbacks
expects a list of
instances of subclasses of Callback
. To specify this kind of argument in a config
file, each callback must be given as a dictionary including a class_path
entry with an import path of the class,
and optionally an init_args
entry with arguments required to instantiate it. Therefore, a simple configuration
file example that defines a couple of callbacks is the following:
trainer:
callbacks:
- class_path: pytorch_lightning.callbacks.EarlyStopping
init_args:
patience: 5
- class_path: pytorch_lightning.callbacks.LearningRateMonitor
init_args:
...
Similar to the callbacks, any arguments in Trainer
and user extended
LightningModule
and
LightningDataModule
classes that have as type hint a class can be configured
the same way using class_path
and init_args
.
Multiple models and/or datasets¶
In the previous examples LightningCLI
works only for a single model and
datamodule class. However, there are many cases in which the objective is to easily be able to run many experiments for
multiple models and datasets. For these cases the tool can be configured such that a model and/or a datamodule is
specified by an import path and init arguments. For example, with a tool implemented as:
from pytorch_lightning.utilities.cli import LightningCLI
cli = LightningCLI(
MyModelBaseClass,
MyDataModuleBaseClass,
subclass_mode_model=True,
subclass_mode_data=True
)
A possible config file could be as follows:
model:
class_path: mycode.mymodels.MyModel
init_args:
decoder_layers:
- 2
- 4
encoder_layers: 12
data:
class_path: mycode.mydatamodules.MyDataModule
init_args:
...
trainer:
callbacks:
- class_path: pytorch_lightning.callbacks.EarlyStopping
init_args:
patience: 5
...
Only model classes that are a subclass of MyModelBaseClass
would be allowed, and similarly only subclasses of
MyDataModuleBaseClass
. If as base classes LightningModule
and
LightningDataModule
are given, then the tool would allow any lightning
module and data module.
Tip
Note that with the subclass modes the --help
option does not show information for a specific subclass. To
get help for a subclass the options --model.help
and --data.help
can be used, followed by the
desired class path. Similarly --print_config
does not include the settings for a particular subclass. To
include them the class path should be given before the --print_config
option. Examples for both help and
print config are:
$ python trainer.py --model.help mycode.mymodels.MyModel
$ python trainer.py --model mycode.mymodels.MyModel --print_config
Models with multiple submodules¶
Many use cases require to have several modules each with its own configurable options. One possible way to handle this
with LightningCLI is to implement a single module having as init parameters each of the submodules. Since the init
parameters have as type a class, then in the configuration these would be specified with class_path
and
init_args
entries. For instance a model could be implemented as:
class MyMainModel(LightningModule):
def __init__(
self,
encoder: EncoderBaseClass,
decoder: DecoderBaseClass
):
"""Example encoder-decoder submodules model
Args:
encoder: Instance of a module for encoding
decoder: Instance of a module for decoding
"""
super().__init__()
self.encoder = encoder
self.decoder = decoder
If the CLI is implemented as LightningCLI(MyMainModel)
the configuration would be as follows:
model:
encoder:
class_path: mycode.myencoders.MyEncoder
init_args:
...
decoder:
class_path: mycode.mydecoders.MyDecoder
init_args:
...
It is also possible to combine subclass_mode_model=True
and submodules, thereby having two levels of
class_path
.
Customizing LightningCLI¶
The init parameters of the LightningCLI
class can be used to customize some
things, namely: the description of the tool, enabling parsing of environment variables and additional arguments to
instantiate the trainer and configuration parser.
Nevertheless the init arguments are not enough for many use cases. For this reason the class is designed so that can be
extended to customize different parts of the command line tool. The argument parser class used by
LightningCLI
is
LightningArgumentParser
which is an extension of python’s argparse, thus
adding arguments can be done using the add_argument()
method. In contrast to argparse it has additional methods to
add arguments, for example add_class_arguments()
adds all arguments from the init of a class, though requiring
parameters to have type hints. For more details about this please refer to the respective documentation.
The LightningCLI
class has the
add_arguments_to_parser()
method which can be implemented to include
more arguments. After parsing, the configuration is stored in the config
attribute of the class instance. The
LightningCLI
class also has two methods that can be used to run code before
and after trainer.fit
is executed: before_fit()
and
after_fit()
. A realistic example for these would be to send an email
before and after the execution of fit. The code would be something like:
from pytorch_lightning.utilities.cli import LightningCLI
class MyLightningCLI(LightningCLI):
def add_arguments_to_parser(self, parser):
parser.add_argument('--notification_email', default='will@email.com')
def before_fit(self):
send_email(
address=self.config['notification_email'],
message='trainer.fit starting'
)
def after_fit(self):
send_email(
address=self.config['notification_email'],
message='trainer.fit finished'
)
cli = MyLightningCLI(MyModel)
Note that the config object self.config
is a dictionary whose keys are global options or groups of options. It
has the same structure as the yaml format described previously. This means for instance that the parameters used for
instantiating the trainer class can be found in self.config['trainer']
.
Another case in which it might be desired to extend LightningCLI
is that the
model and data module depend on a common parameter. For example in some cases both classes require to know the
batch_size
. It is a burden and error prone giving the same value twice in a config file. To avoid this the
parser can be configured so that a value is only given once and then propagated accordingly. With a tool implemented
like shown below, the batch_size
only has to be provided in the data
section of the config.
from pytorch_lightning.utilities.cli import LightningCLI
class MyLightningCLI(LightningCLI):
def add_arguments_to_parser(self, parser):
parser.link_arguments('data.batch_size', 'model.batch_size')
cli = MyLightningCLI(MyModel, MyDataModule)
The linking of arguments is observed in the help of the tool, which for this example would look like:
$ python trainer.py --help
...
--data.batch_size BATCH_SIZE
Number of samples in a batch (type: int, default: 8)
Linked arguments:
model.batch_size <-- data.batch_size
Number of samples in a batch (type: int)
Tip
The linking of arguments can be used for more complex cases. For example to derive a value via a function that takes multiple settings as input. For more details have a look at the API of link_arguments.
Tip
Have a look at the LightningCLI
class API reference to learn about other
methods that can be extended to customize a CLI.