Momoa

Generate typed, self-validating Python classes from JSON Schema at runtime - no code generation.

Basic Usage

StathamEngine (default)

from datetime import datetime
from momoa import Schema
from momoa.engines.statham import UNDEFINED

schema = Schema.from_uri("file://path/to/schema.json")
PersonModel = schema.model

person = PersonModel(firstName="Boris", lastName="Harrison")

# Unset optional fields return UNDEFINED
assert person.age is UNDEFINED

# Fields can be set after instantiation
person.age = 53
person.birthday = datetime(1969, 11, 23)

assert person.age == 53
assert person.serialize()["birthday"] == "1969-11-23"

PydanticEngine

from momoa import Schema
from momoa.engines.pydantic import PydanticEngine

schema = Schema.from_uri("file://path/to/schema.json", engine=PydanticEngine())
PersonModel = schema.model  # a pydantic.BaseModel subclass

person = PersonModel(firstName="Boris", lastName="Harrison", age=53)

# Unset optional fields return None
assert person.birthday is None

# Serialize omits unset fields
serialized = person.serialize()
assert serialized["firstName"] == "Boris"
assert "birthday" not in serialized

# Pydantic's native API is also available
person.model_dump(exclude_unset=True)
PersonModel.model_validate({"firstName": "Alice", "lastName": "Smith"})

Engine Selection

Momoa compiles schemas through a pluggable engine. The default is StathamEngine. To use Pydantic v2 BaseModel subclasses instead, pass engine= explicitly:

from momoa import Schema
from momoa.engines.pydantic import PydanticEngine

schema = Schema.from_file("path/to/schema.json", engine=PydanticEngine())
person = schema.deserialize({"firstName": "Alice", "lastName": "Smith"})
print(person.model_dump())  # standard Pydantic API also available

The active engine can also be set via the MOMOA_DEFAULT_ENGINE environment variable (statham or pydantic), which is useful for switching without changing code.

Compatibility

Momoa supports two engines with different JSON Schema draft coverage:

Engine	Backend	JSON Schema drafts
StathamEngine (default)	Statham	Draft 6
PydanticEngine	datamodel-code-generator + Pydantic v2	Draft 4 – 2020-12