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Field Metadata

All metadata you set either with to.field or added by Treeo by default will be available in the field_metadata: Mapping[str, FieldMetadata] property.

class Person(to.Tree):
    height: float = to.field(node=True)
    name: str = to.field(node=False, opaque=True)

mike = Person(height=1.5, name='Mike')

# no quite true, but you get the idea
assert mike.field_metadata == {
    'height': FieldMetadata(
    'name': FieldMetadata(

Changing field metadata

If at anypoint you want to change the metadata of any field you can do so by using the update_field_metadata method. For example, imagine we have this definition of BatchNorm:

class BatchNorm(to.Tree):
    # nodes
    mean: jnp.ndarray = to.field(node=True, kind=BatchStat)
    # static
    features_in: int
    momentum: float

model = BatchNorm(features_in=32, momentum=0.9)
The momentum hyperparameter field here is a float that you wish to make diffentiable in e.g. a meta-gradient setting. However, the original author of the class didn't consider this field a node, but you can get around this by updating its metadata:

class DifferentiableHyperParam:

model = model.update_field_metadata(
    'momentum', node=True, kind=DifferentiableHyperParam