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Instantiation only mode

The CLI is designed to start fitting with minimal code changes. On class instantiation, the CLI will automatically call the trainer function associated to the subcommand provided so you don’t have to do it. To avoid this, you can set the following argument:

cli = LightningCLI(MyModel, run=False)  # True by default
# you'll have to call fit yourself:
cli.trainer.fit(cli.model)

In this mode, there are subcommands added to the parser. This can be useful to implement custom logic without having to subclass the CLI, but still using the CLI’s instantiation and argument parsing capabilities.

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 parameter 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. If the package that defines a subclass is imported before the LightningCLI class is run, the name can be used instead of the full import path.

From command line the syntax is the following:

$ python ... \
    --trainer.callbacks+={CALLBACK_1_NAME} \
    --trainer.callbacks.{CALLBACK_1_ARGS_1}=... \
    --trainer.callbacks.{CALLBACK_1_ARGS_2}=... \
    ...
    --trainer.callbacks+={CALLBACK_N_NAME} \
    --trainer.callbacks.{CALLBACK_N_ARGS_1}=... \
    ...

Note the use of + to append a new callback to the list and that the init_args are applied to the previous callback appended. Here is an example:

$ python ... \
    --trainer.callbacks+=EarlyStopping \
    --trainer.callbacks.patience=5 \
    --trainer.callbacks+=LearningRateMonitor \
    --trainer.callbacks.logging_interval=epoch

Note

Serialized config files (e.g. --print_config or SaveConfigCallback) always have the full class_path’s, even when class name shorthand notation is used in command line or in input config files.

Multiple models and/or datasets

Additionally, 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:

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 fit --model.help mycode.mymodels.MyModel
$ python trainer.py fit --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: nn.Module, decoder: nn.Module):
        """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.

Class type defaults

The support for classes as type hints allows to try many possibilities with the same CLI. This is a useful feature, but it can make it tempting to use an instance of a class as a default. For example:

class MyMainModel(LightningModule):
    def __init__(
        self,
        backbone: torch.nn.Module = MyModel(encoder_layers=24),  # BAD PRACTICE!
    ):
        super().__init__()
        self.backbone = backbone

Normally classes are mutable as it is in this case. The instance of MyModel would be created the moment that the module that defines MyMainModel is first imported. This means that the default of backbone will be initialized before the CLI class runs seed_everything making it non-reproducible. Furthermore, if MyMainModel is used more than once in the same Python process and the backbone parameter is not overridden, the same instance would be used in multiple places which very likely is not what the developer intended. Having an instance as default also makes it impossible to generate the complete config file since for arbitrary classes it is not known which arguments were used to instantiate it.

A good solution to these problems is to not have a default or set the default to a special value (e.g. a string) which would be checked in the init and instantiated accordingly. If a class parameter has no default and the CLI is subclassed then a default can be set as follows:

default_backbone = {
    "class_path": "import.path.of.MyModel",
    "init_args": {
        "encoder_layers": 24,
    },
}


class MyLightningCLI(LightningCLI):
    def add_arguments_to_parser(self, parser):
        parser.set_defaults({"model.backbone": default_backbone})

A more compact version that avoids writing a dictionary would be:

from jsonargparse import lazy_instance


class MyLightningCLI(LightningCLI):
    def add_arguments_to_parser(self, parser):
        parser.set_defaults({"model.backbone": lazy_instance(MyModel, encoder_layers=24)})

Optimizers

If you will not be changing the class, you can manually add the arguments for specific optimizers and/or learning rate schedulers by subclassing the CLI. This has the advantage of providing the proper help message for those classes. The following code snippet shows how to implement it:

class MyLightningCLI(LightningCLI):
    def add_arguments_to_parser(self, parser):
        parser.add_optimizer_args(torch.optim.Adam)
        parser.add_lr_scheduler_args(torch.optim.lr_scheduler.ExponentialLR)

With this, in the config the optimizer and lr_scheduler groups would accept all of the options for the given classes, in this example Adam and ExponentialLR. Therefore, the config file would be structured like:

optimizer:
  lr: 0.01
lr_scheduler:
  gamma: 0.2
model:
  ...
trainer:
  ...

Where the arguments can be passed directly through command line without specifying the class. For example:

$ python trainer.py fit --optimizer.lr=0.01 --lr_scheduler.gamma=0.2

The automatic implementation of configure_optimizers can be disabled by linking the configuration group. An example can be when one wants to add support for multiple optimizers:

from pytorch_lightning.cli import instantiate_class


class MyModel(LightningModule):
    def __init__(self, optimizer1_init: dict, optimizer2_init: dict):
        super().__init__()
        self.optimizer1_init = optimizer1_init
        self.optimizer2_init = optimizer2_init

    def configure_optimizers(self):
        optimizer1 = instantiate_class(self.parameters(), self.optimizer1_init)
        optimizer2 = instantiate_class(self.parameters(), self.optimizer2_init)
        return [optimizer1, optimizer2]


class MyLightningCLI(LightningCLI):
    def add_arguments_to_parser(self, parser):
        parser.add_optimizer_args(nested_key="optimizer1", link_to="model.optimizer1_init")
        parser.add_optimizer_args(nested_key="optimizer2", link_to="model.optimizer2_init")


cli = MyLightningCLI(MyModel)

The value given to optimizer*_init will always be a dictionary including class_path and init_args entries. The function instantiate_class() takes care of importing the class defined in class_path and instantiating it using some positional arguments, in this case self.parameters(), and the init_args. Any number of optimizers and learning rate schedulers can be added when using link_to.

With shorthand notation:

$ python trainer.py fit \
    --optimizer1=Adam \
    --optimizer1.lr=0.01 \
    --optimizer2=AdamW \
    --optimizer2.lr=0.0001

You can also pass the class path directly, for example, if the optimizer hasn’t been imported:

$ python trainer.py fit \
    --optimizer1=torch.optim.Adam \
    --optimizer1.lr=0.01 \
    --optimizer2=torch.optim.AdamW \
    --optimizer2.lr=0.0001

Run from Python

Even though the LightningCLI class is designed to help in the implementation of command line tools, for some use cases it is desired to run directly from Python. To allow this there is the args parameter. An example could be to first implement a normal CLI script, but adding an args parameter with default None to the main function as follows:

from pytorch_lightning.cli import ArgsType, LightningCLI


def cli_main(args: ArgsType = None):
    cli = LightningCLI(MyModel, ..., args=args)
    ...


if __name__ == "__main__":
    cli_main()

Then it is possible to import the cli_main function to run it. Executing in a shell my_cli.py --trainer.max_epochs=100", "--model.encoder_layers=24 would be equivalent to:

from my_module.my_cli import cli_main

cli_main(["--trainer.max_epochs=100", "--model.encoder_layers=24"])

All the features that are supported from the command line can be used when giving args as a list of strings. It is also possible to provide a dict or jsonargparse.Namespace. For example in a jupyter notebook someone might do:

args = {
    "trainer": {
        "max_epochs": 100,
    },
    "model": {},
}

args["model"]["encoder_layers"] = 8
cli_main(args)
args["model"]["encoder_layers"] = 12
cli_main(args)
args["trainer"]["max_epochs"] = 200
cli_main(args)

Note

The args parameter must be None when running from command line so that sys.argv is used as arguments. Also, note that the purpose of trainer_defaults is different to args. It is okay to use trainer_defaults in the cli_main function to modify the defaults of some trainer parameters.

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