Python traitlets: example from documentation

parity.py

# Example from traitlets documentation

from traitlets import Bool, HasTraits, Int, TraitError, observe, validate


class Parity(HasTraits):
    value = Int()
    parity = Int()

    @validate("value")
    def _valid_value(self, proposal):
        if proposal["value"] % 2 != self.parity:
            raise TraitError("value and parity should be consistent")
        return proposal["value"]

    @validate("parity")
    def _valid_parity(self, proposal):
        parity = proposal["value"]
        if parity not in [0, 1]:
            raise TraitError("parity should be 0 or 1")
        if self.value % 2 != parity:
            raise TraitError("value and parity should be consistent")
        return proposal["value"]

    @observe("parity")
    def _observe_value(self, change):
        print(
            f"observing changes on { change['name'] }: { change['old'] } ->  { change['new'] }"
        )


parity_check = Parity(value=2)

# Changing required parity and value together while holding cross validation.
# Hold the custom cross validation until the context manager is released.
with parity_check.hold_trait_notifications():
    parity_check.value = 1
    parity_check.parity = 1