Instantiating objects with Hydra

One of the best ways to drive different behavior in an application is to instantiate different implementations of an interface. The code using the instantiated object only knows the interface which remains constant, but the behavior is determined by the actual object instance.

Hydra provides hydra.utils.call() (and its alias hydra.utils.instantiate()) for instantiating objects and calling functions. Prefer instantiate for creating objects and call for invoking functions.

Call/instantiate supports:

• Class names : Call the __init__ method
• Callables like functions, static functions, class methods and objects

def call(config: Any, *args: Any, **kwargs: Any) -> Any:
    """
    :param config: An object describing what to call and what params to use.
                   Must have a _target_ field.
    :param args: optional positional parameters pass-through
    :param kwargs: optional named parameters pass-through
    :return: the return value from the specified class or method
    """
    ...

# Alias for call
instantiate = call

The config passed to these functions must have a key called _target_, with the value of a fully qualified class name, class method, static method or callable.
Any additional parameters are passed as keyword arguments to the target.

For example, your application may have a User class that looks like this:

user.py

class User:
  name: str
  age : int
  
  def __init__(self, name: str, age: int):
    self.name = name
    self.age = age

Config

bond:
  _target_: user.User
  name: Bond
  age: 7

Instantiation

user : User = instantiate(cfg.bond)
assert isinstance(user, user.User)
assert user.name == "Bond"
assert user.age == 7

For convenience, instantiate/call returns None when receiving None as input.

assert instantiate(None) is None