from_params

[ allennlp.common.from_params ]

---

T

T = TypeVar("T", bound="FromParams")

takes_arg

def 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

def 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

can_construct_from_params

def can_construct_from_params(type_: Type) -> bool

is_base_registrable

def is_base_registrable(cls) -> bool

Checks whether this is a class that directly inherits from Registrable, or is a subclass of such a class.

remove_optional

def remove_optional(annotation: type)

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.

infer_params

def infer_params(
    cls: Type[T],
    constructor: Callable[..., T] = None
)

create_kwargs

def create_kwargs(
    constructor: Callable[..., T],
    cls: Type[T],
    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

def 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.

pop_and_construct_arg

def pop_and_construct_arg(
    class_name: str,
    argument_name: str,
    annotation: Type,
    default: Any,
    params: Params,
    **extras
) -> Any

Does the work of actually constructing an individual argument for create_kwargs.

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 Params for constructing the object (which we may mutate), and any extras that the constructor might need.

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.

construct_arg

def construct_arg(
    class_name: str,
    argument_name: str,
    popped_params: Params,
    annotation: Type,
    default: Any,
    **extras
) -> Any

The first two parameters here are only used for logging if we encounter an error.

FromParams Objects

class FromParams()

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.

from_params

 | @classmethod
 | def from_params(
 |     cls: Type[T],
 |     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 FromParams (or 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.

The constructor_to_call and 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 arguments (constructor_to_inspect), and what method to call when we're done constructing arguments (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 cls()).