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Version: 1.1

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

Call/instantiate supports:

  • Constructing an object by calling the __init__ method
  • Calling functions, static functions, class methods and other callable global objects
Instantiate API (Expand for details)
def instantiate(config: Any, *args: Any, **kwargs: Any) -> Any:
:param config: An config object describing what to call and what params to use.
In addition to the parameters, the config must contain:
_target_ : target class or callable name (str)
And may contain:
_args_: List-like of positional arguments to pass to the target
_recursive_: Construct nested objects as well (bool).
True by default.
may be overridden via a _recursive_ key in
the kwargs
_convert_: Conversion strategy
none : Passed objects are DictConfig and ListConfig, default
partial : Passed objects are converted to dict and list, with
the exception of Structured Configs (and their fields).
all : Passed objects are dicts, lists and primitives without
a trace of OmegaConf containers
:param args: Optional positional parameters pass-through
:param kwargs: Optional named parameters to override
parameters in the config object. Parameters not present
in the config objects are being passed as is to the target.
IMPORTANT: dataclasses instances in kwargs are interpreted as config
and cannot be used as passthrough
:return: if _target_ is a class name: the instantiated object
if _target_ is a callable: the return value of the call
# Alias for instantiate
call = instantiate

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. For convenience, None config results in a None object.

Named arguments : Config fields (except reserved fields like _target_) are passed as named arguments to the target. Named arguments in the config can be overridden by passing named argument with the same name in the instantiate() call-site.

Positional arguments : The config may contain a _args_ field representing positional arguments to pass to the target. The positional arguments can be overridden together by passing positional arguments in the instantiate() call-site.

Simple usage#

Your application might have an Optimizer class:

Example class
class Optimizer:
algo: str
lr: float
def __init__(self, algo: str, lr: float) -> None:
self.algo = algo = lr
_target_: my_app.Optimizer
algo: SGD
lr: 0.01
opt = instantiate(cfg.optimizer)
# Optimizer(algo=SGD,lr=0.01)
# override parameters on the call-site
opt = instantiate(cfg.optimizer, lr=0.2)
# Optimizer(algo=SGD,lr=0.2)

Recursive instantiation#

Let's add a Dataset and a Trainer class. The trainer holds a Dataset and an Optimizer instances.

Additional classes
class Dataset:
name: str
path: str
def __init__(self, name: str, path: str) -> None: = name
self.path = path
class Trainer:
def __init__(self, optimizer: Optimizer, dataset: Dataset) -> None:
self.optimizer = optimizer
self.dataset = dataset

With the following config, you can instantiate the whole thing with a single call:

Example config
_target_: my_app.Trainer
_target_: my_app.Optimizer
algo: SGD
lr: 0.01
_target_: my_app.Dataset
name: Imagenet
path: /datasets/imagenet

Hydra will instantiate nested objects recursively by default.

trainer = instantiate(cfg.trainer)
# Trainer(
# optimizer=Optimizer(algo=SGD,lr=0.01),
# dataset=Dataset(name=Imagenet, path=/datasets/imagenet)
# )

You can override parameters for nested objects:

trainer = instantiate(
optimizer={"lr": 0.3},
dataset={"name": "cifar10", "path": "/datasets/cifar10"},
# Trainer(
# optimizer=Optimizer(algo=SGD,lr=0.3),
# dataset=Dataset(name=cifar10, path=/datasets/cifar10)
# )

Similarly, positional arguments of nested objects can be overridden:

obj = instantiate(
# pass 1 and 2 as positional arguments to the target object
1, 2,
# pass 3 and 4 as positional arguments to a nested child object
child={"_args_": [3, 4]},

Disable recursive instantiation#

You can disable recursive instantiation by setting _recursive_ to False in the config node or in the call-site In that case the Trainer object will receive an OmegaConf DictConfig for nested dataset and optimizer instead of the instantiated objects.

optimizer = instantiate(cfg.trainer, _recursive_=False)


'_target_': 'my_app.Optimizer', 'algo': 'SGD', 'lr': 0.01
'_target_': 'my_app.Dataset', 'name': 'Imagenet', 'path': '/datasets/imagenet'

Parameter conversion strategies#

By default, the parameters passed to the target are either primitives (int, float, bool etc) or
OmegaConf containers (DictConfig, ListConfig). OmegaConf containers have many advantages over primitive dicts and lists but in some cases it's desired to pass a real dicts and lists (for example, for performance reasons).

You can change the parameter conversion strategy using the _convert_ parameter (in your config or the call-site). Supported values are:

  • none : Default behavior, Use OmegaConf containers
  • partial : Convert OmegaConf containers to dict and list, except Structured Configs.
  • all : Convert everything to primitive containers

Note that the conversion strategy applies to all the parameters passed to the target.

Last updated on by Jasha10