One of the design principles of AllenNLP is the use of a modular, declarative language (JSON) for defining experiments and models.
This is implemented by giving each AllenNLP class a method
.. code-block @classmethod def from_params(cls, params: Params, **extras) -> 'ClassName': ...
that contains the logic for instantiating a class instance from a JSON-like
Params object. Historically you had to implement your own
method on every class you wanted to instantiate this way, even though
most of the time you were simply popping off params and handing them to the
constructor (making sure that you popped them using the same default values
as in the constructor.)
It turns out that in those simple cases, we can generate a
method automatically. This implementation lives in the
Registrable subclass automatically gets it, and you can have your
Registrable classes subclass from it as well.
The inclusion of
extras allows for non-FromParams parameters to be passed
as well. For instance, all of our
Model subclasses require a
Vocabulary parameter. Accordingly, the
train command calls
model = Model.from_params(params=params.pop('model'), vocab=vocab)
As an AllenNLP user, you will probably never need to worry about this. However, if you do, note that the extra arguments must be called by keyword. Prior to this default implementation it was possible to call them positionally but this is no longer the case.
In some cases you might want the construction of class instances
to include more elaborate logic than "pop off params and hand them to the constructor".
In this case your class just needs to explicitly implement its own
takes_arg(obj, arg:str) -> bool
Checks whether the provided obj takes a certain arg. If it's a class, we're really checking whether its constructor does. If it's a function or method, we're checking the object itself. Otherwise, we raise an error.
takes_kwargs(obj) -> bool
Checks whether a provided object takes in any positional arguments. Similar to takes_arg, we do this for both the init function of the class or a function / method Otherwise, we raise an error
Optional[X] annotations are actually represented as Union[X, NoneType]. For our purposes, the "Optional" part is not interesting, so here we throw it away.
create_kwargs(constructor:Callable[..., ~T], cls:Type[~T], params:allennlp.common.params.Params, **extras) -> Dict[str, Any]
Given some class, a
Params object, and potentially other keyword arguments,
create a dict of keyword args suitable for passing to the class's constructor.
The function does this by finding the class's constructor, matching the constructor
arguments to entries in the
params object, and instantiating values for the parameters
using the type annotation and possibly a from_params method.
Any values that are provided in the
extras will just be used as is.
For instance, you might provide an existing
Vocabulary this way.
create_extras(cls:Type[~T], extras:Dict[str, Any]) -> Dict[str, Any]
Given a dictionary of extra arguments, returns a dictionary of kwargs that actually are a part of the signature of the cls.from_params (or cls) method.
construct_arg(class_name:str, param_name:str, annotation:Type, default:Any, params:allennlp.common.params.Params, **extras) -> Any
Does the work of actually constructing an individual argument for :func:
Here we're in the inner loop of iterating over the parameters to a particular constructor,
trying to construct just one of them. The information we get for that parameter is its name,
its type annotation, and its default value; we also get the full set of
constructing the object (which we may mutate), and any
extras that the constructor might
We take the type annotation and default value here separately, instead of using an
inspect.Parameter object directly, so that we can handle
Union types using recursion on
this method, trying the different annotation types in the union in turn.
FromParams(self, /, *args, **kwargs)
Mixin to give a from_params method to classes. We create a distinct base class for this because sometimes we want non-Registrable classes to be instantiatable from_params.
FromParams.from_params(params:allennlp.common.params.Params, constructor_to_call:Callable[..., ~T]=None, constructor_to_inspect:Callable[..., ~T]=None, **extras) -> ~T
This is the automatic implementation of
from_params. Any class that subclasses
Registrable, which itself subclasses
FromParams) gets this
implementation for free. If you want your class to be instantiated from params in the
"obvious" way -- pop off parameters and hand them to your constructor with the same names --
this provides that functionality.
If you need more complex logic in your from
from_params method, you'll have to implement
your own method that overrides this one.
constructor_to_inspect arguments deal with a bit of
redirection that we do. We allow you to register particular
@classmethods on a class as
the constructor to use for a registered name. This lets you, e.g., have a single
Vocabulary class that can be constructed in two different ways, with different names
registered to each constructor. In order to handle this, we need to know not just the class
we're trying to construct (
cls), but also what method we should inspect to find its
constructor_to_inspect), and what method to call when we're done constructing
constructor_to_call). These two methods are the same when you've used a
@classmethod as your constructor, but they are
different when you use the default
constructor (because you inspect
__init__, but call