The Answer

Simply use builds(<target>, zen_partial=True, [...]) or just(functools.partial(<target>, [...])) to create a partial config for <target>.

Partially Configure an Object#

It is often the case that our configuration of a target-object ought to only partially describe its input parameters; while some of the target’s parameters can be specified via our app’s interface, other parameters may need to be specified only after our app has launched.

In this How-To we will write a config that only partially configures a target object. We will

  1. Define toy examples of a class and a function, respectively.

  2. Use builds(<target>, zen_partial=True) and just(functools.partial(<target>)) to create their partial configurations.

  3. Check that instantiate() only partially-instantiates the targets of these two configs.

  4. Examine the YAML-config for one of these partial configs.

Let’s define toy examples of a class and a function; we’ll create partial configs for both of these.

1: Defining toy examples.#
class Optimizer:
    def __init__(self, model_weights, learning_rate):
        self.weights = model_weights
        self.lr = learning_rate

def logger(message: str, format_spec: str) -> str:
    return format_spec.format(message)

Suppose that the model_weights parameter in Optimizer, and the message parameter in logger() ought not be configured by our app’s interface. builds() provides the zen_partial feature, which makes it trivial to partially-configure a target. We can also apply just() to a instance of functools.partial Let’s create configs for our toy class and toy function.

2: Creating partial configs.#
from functools import partial
from hydra_zen import builds, instantiate, just

OptimConf = builds(Optimizer, learning_rate=0.1, zen_partial=True)

partial_logger = partial(logger, format_spec='{0:>8s}')
LogConf = just(partial_logger)

Instantiating these configs will apply functools.partial() to the config’s target.

3: Instantiating the configs#
>>> partiald_optim = instantiate(OptimConf)
>>> partiald_optim
functools.partial(<class '__main__.Optimizer'>, learning_rate=0.1)

>>> partiald_optim(model_weights=[1, 2, 3])
<__main__.Optimizer at 0x189991fbb50>

>>> partiald_logger = instantiate(LogConf)
>>> partiald_logger
functools.partial(<function logger at 0x0000018998D1E040>, format_spec='{0:>8s}')

>>> partiald_logger("hello")
'   hello'
>>> partiald_logger("goodbye")
' goodbye'

Lastly, let’s inspect the YAML-serialized config for OptimConf and LongConf.

4: Examining a YAML-serialized partial config.#
>>> from hydra_zen import to_yaml

>>> print(to_yaml(OptimConf))
_target_: __main__.Optimizer
_partial_: true
learning_rate: 0.1

>>> print(to_yaml(LogConf))
_target_: __main__.logger
_partial_: true
format_spec: '{0:>8s}'
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