Declare an ontology with Python classes again

[paul-paliychuk]

set_ontology takes lists of EntityType and EdgeType, so building an ontology by hand means repeating every property's name, type and description as data. Earlier SDK versions let you declare it once as Pydantic classes and derived the payload; that layer targeted the old types and was dropped when this branch moved to generated output.

Usage

from zep_cloud.ontology import EdgeModel, EntityModel, EntityText, build_ontology
from zep_cloud.types import EdgeSourceTarget

class Traveler(EntityModel):
    """Someone who takes trips."""
    home_city: EntityText = None

class TraveledTo(EdgeModel):
    """A traveler visiting a destination."""
    purpose: EntityText = Field(default=None, description="Why they went")

entity_types, edge_types = build_ontology(
    entities={"Traveler": Traveler},
    edges={"TRAVELED_TO": (TraveledTo, [EdgeSourceTarget(
        source_entity_type="Traveler", target_entity_type="Destination")])},
)
client.graph.set_ontology(graph_uuid, entity_types=entity_types, edge_types=edge_types)

The same output goes to client.project.set_ontology for the project default. The class docstring becomes the type description; a field's description becomes the property description.

Two deliberate differences from the old layer

A function, not a client subclass. The generated clients expose sub-clients as read-only properties and already define set_ontology, so subclassing collided with both. A function also means one hand-written file to preserve instead of five.

The property type comes off an annotation marker, not a JSON-schema round trip. Shorter, and it keeps the wire spelling in a single place — which matters because the generated EntityPropertyType is a Literal union rather than an enum, so there are no members to reference.

Behavior worth noting

An unannotated field raises, naming the field:

ValueError: Bad.oops is not an ontology property: annotate it with
EntityText, EntityInt, EntityFloat or EntityBoolean

Silently dropping it would ship an ontology missing a property the caller declared.

Verification

mypy clean across 168 files; 7 new tests covering type derivation, each annotation's wire value, field descriptions, source targets present and absent, the unannotated-field error, and empty input.

Adds src/zep_cloud/ontology.py and tests/ontology/ to .fernignore — everything it builds on (EntityType, EdgeType, EntityProperty) stays generated. Also drops two stale .fern/replay entries whose file no longer exists.