Base class for subcommands under


Subcommand(self, /, *args, **kwargs)

An abstract class representing subcommands for If you wanted to (for example) create your own custom special-evaluate command to use like

allennlp special-evaluate ...

you would create a Subcommand subclass and then pass it as an override to :func:~allennlp.commands.main .


    name: str,
    constructor: Optional[str] = None,
    exist_ok: bool = False,
) -> Callable[[Type[~T]], Type[~T]]

Register a class under a particular name.


  • name : str The name to register the class under.
  • constructor : str, optional (default=None) The name of the method to use on the class to construct the object. If this is given, we will use this method (which must be a @classmethod) instead of the default constructor.
  • exist_ok : bool, optional (default=False) If True, overwrites any existing models registered under name. Else, throws an error if a model is already registered under name.


To use this class, you would typically have a base class that inherits from Registrable:

class Vocabulary(Registrable):

Then, if you want to register a subclass, you decorate it like this:

class MyVocabulary(Vocabulary):
    def __init__(self, param1: int, param2: str):

Registering a class like this will let you instantiate a class from a config file, where you give "type": "my-vocabulary", and keys corresponding to the parameters of the __init__ method (note that for this to work, those parameters must have type annotations).

If you want to have the instantiation from a config file call a method other than the constructor, either because you have several different construction paths that could be taken for the same object (as we do in Vocabulary) or because you have logic you want to happen before you get to the constructor (as we do in Embedding), you can register a specific @classmethod as the constructor to use, like this:

@Vocabulary.register("my-vocabulary-from-instances", constructor="from_instances")
@Vocabulary.register("my-vocabulary-from-files", constructor="from_files")
class MyVocabulary(Vocabulary):
    def __init__(self, some_params):

    def from_instances(cls, some_other_params) -> MyVocabulary:
        ...  # construct some_params from instances
        return cls(some_params)

    def from_files(cls, still_other_params) -> MyVocabulary:
        ...  # construct some_params from files
        return cls(some_params)


dict() -> new empty dictionary dict(mapping) -> new dictionary initialized from a mapping object's (key, value) pairs dict(iterable) -> new dictionary initialized as if via: d = {} for k, v in iterable: d[k] = v dict(**kwargs) -> new dictionary initialized with the name=value pairs in the keyword argument list. For example: dict(one=1, two=2)